Predicting transonic stability?

[mao0720]

Is there any way to predict how a specific bullet will be affected when it drops into transonic speed? Are there any features to bullets that tend to show better stability as they move into the transonic zone? Thanks,
Mason

 

[Bryan Litz Ballistics]

This is a complex subject matter, which is precisely the sort of thing this forum was created to discuss.

If there is a general statement to be made about bullet design and transonic stability, it would be that: shorter, fatter bullets with small, shallow BT's or no BT tend to remain most stable at transonic speed. Going up in altitude also results in more stability in transonic, so some bullets which may tumble at transonic at sea level could fly thru fine at 5,000 feet altitude.

The obvious problem with this general statement is that LR shooters don't typically want to shoot short fat bullets, we like the long pointy bullets.

So when selecting a bullet for Extreme Long Range (ELR) where bullets will encounter transonic speed, you have to decide if you want to push transonic far downrange with a long pointy bullet that will be questionable at transonic speed, or select a low BC bullet that will hit transonic at a shorter range, but remain stable as it flies at this speed.

Most ELR shooters favor the high BC option, and there is a lot of grey area as to how these bullets will fly thru transonic. Most will experience some degree of yaw as they fly thru transonic. The technical term is called 'limit cycle yaw' which is a fancy name for 'trim'. In a place where stability is challenged, as in transonic flight, the bullet isn't stable in perfect point forward flight, but can 'trim' at a small yaw angle which puts the bullet in equilibrium for that condition. The yaw isn't a steady pointing to one side, rather a dynamic coning motion around the bullets flight path.

Limit cycle yaw causes 'induced drag' on the bullet, which is proportional to the magnitude of limit cycle yaw angle. Many bullets can fly straight and remain accurate thru transonic even though their drag is +10% or greater than it's 'zero yaw' drag. You can suppress the magnitude of the limit cycle yaw angle by using faster twist rates. Doing so allows the bullet to fly thru transonic with lower angles, and therefore lower drag.

This is why transonic drag and trajectory prediction can be such a grey area; drag can literally be affected by twist rate.
As to what bullet features contribute to limit cycle yaw, it's difficult to say because the entire stability condition is determined by a complex interaction of the bullets mass and aerodynamic properties.

One feature of bullet design that's been identified as having an affect is the camfer at the base of the bullet. At the very base, or heel of a bullet, there is a 0.010" to 0.020" camfer, or radius. The presence of this radius actually causes the bullet to fly with greater limit cycle yaw angles. I first became aware of this by reading an AIAA paper (http://arc.aiaa.org/doi/abs/10.2514/6.2009-3851). I then repeated the experiment by taking some typical bullets that had about 0.020" heal, and cutting that heal off in a lathe. Note, just the small heal was cut off, not the entire boat tail. It amounted to less than 4 grains of material. These bullets I called 'square heal bullets'. I then performed the following comparison. I measured the BC of the un-altered bullets from a 1:12" and 1:8" twist barrel thru transonic speed. For these unaltered bullets, there was noticably (~7%) more drag for the 1:12" twist compared to the 1:8" twist. I then fired the square heal bullets thru both barrels under the same conditions, and observed that there was no difference in transonic drag for these bullets fired from the 1:12" and 1:8". The square heal bullets exhibited the same drag from both barrels that the un-altered bullets had from the 1:8".

So my results matched with the AIAA paper; removing the heal allows the bullet to fly with less limit cycle yaw.

This is a long answer to your question, but it goes to show that sometimes even minor features of a bullets shape can have a substantial effect on it's transonic flight. I don't think any generalizations can be made, which is why I'm working on testing bullets thru various twist rate barrels to determine their transonic flight quality. The end objective for me is to experimentally determine transonic effects for bullets, so it can be described in models used in ballistics software to predict more accurate trajectories thru transonic.

BTW, if you're interested in reading more about the square base experiment and my other tests on how stability affects BC (supersonic and transonic), how twist rate affects MV, precision, etc. it's all published in: 'Modern Advancements in Long Range Shooting'.

-Bryan

 

[dmoran]

^^^^ Very informative !.!.!

Again, Great Stuff..... Thanks Bryan
Donovan

 

[mao0720]

Brian, you're an awesome resource to have around. Thank you very much for the detailed response. Could this be the new big thing is bullet prepping? Squaring off of the heel of bullets. I must say that if it meant better transonic stability I would definitely consider. That's a big deal, since I just started neck turning my brass (literally just did my first 100 cases this week) and generally hate case/ bullet prep.

 

[Bryan Litz Ballistics]

I thought of that (designing a tool to cut a square base on bullets). But it would really only affect a minority of shots, and the improvement isn't night and day. Conventional bullets with the camfered base still fly accurately and predictably (once tested in a given twist). It's one of those things that is difficult to see the benefit unless you're specifically looking for it.

I enjoyed the experience because it was demonstrating an obscure effect that was described in an academic paper. Always enjoyable to verify things like that with live fire.

-Bryan

 

[Laurie]

Of course, the other approach is to avoid transonic velocities. Ideally we'd stay above 1.2 MACH or ~1,350 fps and there are a surprising number of cartridges and disciplines that see that at 1,000 yards. Nothing competitive in F-Open will be as slow at that at 1,000 these days - a relatively modest 2,800 fps MV sees the 180gn 7mm Hybrid at over 1,600 fps in standard ballistic conditions and the older VLD is just short of that speed.

It's more an issue with the smaller sixes and .308 Win in Palma, Target Rifle and FTR. In the latter, the MVs are such now that the 155.5gn Berger Fullbore is 'there' and my favourite, the 168gn Hybrid has no trouble either. The 6BR with a 105gn VLD at 2,800 fps is 100 fps 'short' at 1,000, but the Dasher's extra couple of hundred fps MV puts it in the right place. So, people who're shooting at distances beyond 1,000 yards aside, the problem is mostly the TR / Palma people shooting in matches where the rules limit them to 'less than 156gn'. Much TR shooting in the UK uses the old Sierra 155gn MK (#2155) in RWS ammo loaded to a nominal 2,925 fps (30-inch 'tight bore' barrel). With Bryan's average 0.214 G7 BC that's right on the speed of sound in likely Bisley 'Imperial Meeting' match temperatures of 68-deg F or thereabouts. Perhaps it's as well that it's a 2-MOA 'Bull' and 1-MOA 'V'-ring, but even so, the scores and V-counts being put in these days by the 'Queen's 100' finalists at 900 and 1,000 yards are staggering if the conditions are settled and one wonders how long the target size will last before further reductions take place. (The use of organiser issued RWS 155 Sierra cartridges is mandatory in the GB 'Imperial Meeting' TR matches.)

I assume the fact that the old Sierra #2155 is such a relatively short bullet by modern long-range bullet standards with a short tail section has much to do with this? It also makes one wonder how the higher BC but longer overall new 'Palma MatchKing' (#2156) stacks up at MVs around 2,925-2,950 fps?

Somewhere in German Salazar's Rilfeman's Journal Blogspot, there is a series of articles by Frederick Salberta on 7.62mm NATO Long Range cartridges as loaded by US government arsenals, and there is a reference there to the old 173gn FMJBT .30-06 and early 7.62 match loads with the comment that the US Army had found in range-testing that the key retained velocity with this bullet was ~1,225 fps, or in round figures 100 fps above the speed of sound. Drop below that and the dispersion increased substantially and also individual wind strength / direction changes produced a much increased effect on POI. So, the bottom line was that the higher MV .30-06 match round just got away with it ballistically with this not exactly over-admired bullet in the match role, but the 7.62 version was too slow. Having demilled some pre-WW2 standard military ball 173gn .30-06-M1 rounds a while back, I was struck by the length of the boat-tail section and its steep angle. (I also wondered why the US Army had taken the Swiss 174gn bullet, a proven superb long-range performer, as its starting point after WW1 but given it a much steeper tail angle when they wanted it in a 4-5,000 yard capable machine-gun cartridge).

http://riflemansjournal.blogspot.co.uk/2012/02/cartridges-762-nato-long-range-match.html

So, the layman's (and likely erroneous) conclusion is that the much shorter Sierra 2155 and its short tail can hover around the speed of sound and still perform well whilst the old M1 based match bullet needed an extra 100 retained fps.

 

[Bart B.]

I was one of a few who worked up loads for Sierra's first prototype lot of their 30 caliber 155-gr. Palma bullet back in early 1991. Several top ranked long range shooters from around the planet shot that bullet in a 4 day match at the NRA range near Raton, New Mexico, later that year. Altitude there is 6600 feet. Ammo was loaded on two Dillon 1050 progressives with 45.3 +/- .15 grains (.3 grain spread) of IMR4895, excellent Winchester cases unprepped except for uniforming mouth diameters, Federal 210M primers and Sierra's new bullet that had runout up to near .004". 20 rounds randomly picked tested 2.7" at 600 yards with a Palma rifle clamped in a machine rest; it's group is in a fall 1991 issue of Handloader magazine. Top ranked competitors felt it easily shot under 4 inches at 600 yards across their barrels with different inside dimensions, under 8 inches at 1000. I had the high combined aggregate across 2 long range and 3 Palma matches and an Aussie from down under was second. I shot 200-14X at 600, he had 200-16X. Asked him what he thought of that ammo, to which he replied "I wish the Commonwealth would let us hand load ammo; the best arsenal or commercial issued stuff is nowhere as good as what you Yanks put together. A few weeks later, the International Palma Committee dubbed that Sierra bullet "the" one for official competition internationally, or something like that.

People shooting it in 24 to 26 inch barrels oft times had big groups at 1000 yards. 600 yard ones were great. Those with 30 inch barrels were OK. At the Nationals on Camp Perry's ranges at 600 feet altitude, short barrels had worse problems with that bullet; thicker air slowed them down more.

The Brits had to use 30 inch barrels with 7.62 NATO M80 ball ammo for their fullbore matches to keep them supersonic through 1000 yards. But the best thing was that ammo had more uniform muzzle velocities which produced good groups at 300 to 1000 yards. Unlike their SMLE's with cordite charged ammo shooting heavy 174-gr bullets at great velocity spreads. Their SMLE's mid range accuracy wasn't quite as good as long range as positive compensation of the barrel's vertical whip let the slower bullets leave later in the muzzle axis up swing so their greater drop at target range was mitigated.

Lots of folks use tuning weights on their barrel muzzles to change the frequency the bore axis there whips at; they want slower bullets to leave at a slightly higher angle than faster ones. A British man proved this compensation stuff over a century ago. But that compensation is often hard to do at short ranges through 300 yards with centerfire, although some do. Rimfire 22's have tuners and some folks use one setting for 50 yards and another for 100. It works.

 

[Bart B.]

s there any way to predict how a specific bullet will be affected when it drops into transonic speed? Are there any features to bullets that tend to show better stability as they move into the transonic zone?

In the late 1800's when long range matches were shot with 45 caliber 500+ grain bullets, they left at 1300 to near 1400 fps. They went transonic soon after leaving the barrel. Target dimensions at 1000 yards had a 36" square black bullseye. Their ammo would test about 20 inches for groups.

That target was made with round bullseye the same diameter when the 30-40 Krag and 30-03 and 30-06 cartridges were used. It stayed the same size until 1972 when it changed to a 10" X ring and 20" 10 ring; the 308 Win and newer 30 caliber magnums' better accuracy pushed the venerable .30-06 and 300 H&H Mag cartridges off the firing lines.

 

[Bart B.]

So, the bottom line was that the higher MV .30-06 match round just got away with it ballistically with this not exactly over-admired bullet in the match role, but the 7.62 version was too slow.

The least known fact about that military 173 grain match bullet is each lot of match ammo had them from 3 or 4 different bullet making machines; one machine made the best ones but the others were not so good. Military teams replaced them with Sierra 168's or 180's and the best 7.62 service rifles shot them about 2/3 MOA at 600 yards properly tested. Best arsenal match ammo relegated to the Nationals shot near 2 MOA with a mean radius of 1.9", extreme spread over 10 inches.

M118 and M852 7.62 match ammo stayed supersonic through 1000 yards in 24" Garand barrels. I and others have shot enough in cold weather in thick air at low altitudes to see it did so. M14's and M1A's 22" barrels did well also; if their groove diameter was small enough to shoot them fast enough, as well as newer with minimal throat erosion that slowly lowers muzzle velocity for a given lot of ammo.

The army decided to use Sierra's 175's to replace the 168's in M852 because that bullet's boattail was not good for long range use across all barrels. That problem raised its ugly head again with the Sierra 180's military teams stuffed in M118 ammo. The first Sierra 180 gr. 30 caliber match bullets had boattails like their 190 and heavier ones; long 9 degree ones instead of the 168's 13 degree short ones. They stayed supersonic through 1000 shot from 7.62 ammo rebulleted. Then, in a cost cutting plan, Sierra changed the 180's boattail to that of their 168. No longer did the military like Sierra 180's for 1000 yards; 600 was still OK. The US Army AMU contracted Sierra to make special lots of 180 HPMK's using the original die shape to make their heels shaped like the original ones. That solved the problem.

 

[Laurie]

The Brits had to use 30 inch barrels with 7.62 NATO M80 ball ammo for their fullbore matches to keep them supersonic through 1000 yards. But the best thing was that ammo had more uniform muzzle velocities which produced good groups at 300 to 1000 yards. Unlike their SMLE's with cordite charged ammo shooting heavy 174-gr bullets at great velocity spreads. Their SMLE's mid range accuracy wasn't quite as good as long range as positive compensation of the barrel's vertical whip let the slower bullets leave later in the muzzle axis up swing so their greater drop at target range was mitigated.

The stuff that was used in the UK in the days of 7.62 arsenal ammunition and pre 155gn bullets was more often than not Radway Green RG 'Green Spot', a so-called sniper round. (Well ... it was a 'sniper round' in that unfortunate British Army snipers were issued with it and expected to make one-shot hits at 800 metres.) It used what was supposed to be selected batches of standard 144gn FMJBT bullets with the concave bases filled flush making them nominally 146gn. It wasn't the extra 2gn weight that was important, it was the more consistent and flush bases, as the standard 7.62 'Black Spot' round was known to suffer variable deflections from the gasses exiting the muzzle around it.

One thing the Green Spot certainly didn't do was remain supersonic at 1,000 yards in all Bisley conditions. Jacket and core concentricity and consistency left a lot to be desired too, and there were enormous performance variations between manufacturing lots, the poor ones being dire with complaints that shooting matches with this issued ammunition was 'a lottery'. The GB NRA would try and ensure that the annual July 'Imperial Meeting' had a 'good batch' reserved for it, but didn't always succeed, one year in particular causing uproar particularly amongst the visiting shooters who had paid a great deal of money and time to fly to the UK from Canada, Australia etc and for two weeks accommodation and entry fees. The usual rifling twist rate on GB TR rifles of that era was 1 turn in 14", the slowest that would stabilise this bullet and the reduced rate of spin reduced the amount of dispersion of these non-concentric projectiles.

Later issued RG ammunition used an RG manufactured 155gn FMJBT bullet that was much better, but not as good as the original 155gn Sierra MK.

There were major complaints from the Army in the post-conflict 'learn the lessons' exercises after the Falklands war about the L42A1 (Number 4 based) rifle, the WW2 era scope sights that had originally been made for the No.4(T) predecessor model, and its RG ammunition's performance. The army decided to replace the rifle shortly afterwards with both Parker-Hale with its M85 model and the Accuracy International rifle obtaining acceptance, but the latter getting nearly all the orders. As part of the procurement process, a selection of rifles and ammunition in use by other NATO partners was obtained and shot alongside existing L42A1s which were used as a 'control' to judge the performance of those rifles put forward in the tenders. The word is that the UK Ministry of Defence officials and senior officers were shocked by the poor performance of the existing rifle / ammunition combo against that of other nations' equivalents.

 

[Bart B.]

Laurie, nice words well organized.

I'm aware of most of that stuff. Thanks for filling in the blanks.

The Brit's a century ago decided that the best way to see who was the best shot/marksman was to issue the same make and model of rifle to all and shoot ammo from the same lot. That "leveled the playing field" or so to say. I've mentioned the 1:14 twist the Brits used with that stuff and people don't believe it.

Of course, those keeping records of what rifle shot best never made sure their buddies got one of them when they issued them to the competitors.

 

[Bart B.]

The British take on positive compensation was documented over a century ago. Read about it in

https://ia600701.us.archive.org/6/items/philtrans05900167/05900167.pdf

 

[Laurie]

Laurie, nice words well organized.

I'm aware of most of that stuff. Thanks for filling in the blanks.

The Brit's a century ago decided that the best way to see who was the best shot/marksman was to issue the same make and model of rifle to all and shoot ammo from the same lot. That "leveled the playing field" or so to say. I've mentioned the 1:14 twist the Brits used with that stuff and people don't believe it.

Of course, those keeping records of what rifle shot best never made sure their buddies got one of them when they issued them to the competitors.

In theory, issued ammunition 'leveled the playing field'. In practice, as you note, money still gave an advantage in that some wealthy competitors had several TR rifles each with slightly different bore / groove dimensions. Whenever a new lot of RG appeared, they'd try it in each rifle to see which barrel shot best.

Re the 14-inch twist, yes hard to imagine it was so nowadays! I shoot Berger 155.5s out of a 10-twist Broughton in FTR these days and it shoots much better than I can. However, I do have an old Paramount ex-TR rifle I bought as a project vehicle maybe four, five years ago which had a 30-inch Lothar Walther 1 in 14" barrel on it and a LOT of throat erosion but which still shot the commercial 308 ammunition with 155gn SMK #2155 that the GB NRA now buys in remarkably small short-range groups. (I never chronographed them unfortunately with that barrel on nor shot it at even mid-range.) Using today's 308 Fullbore / Palma bullets and running them through the Berger Bullets / Miller Twist Rules spreadsheets, this slow twist is very, very limiting these days even applying the old 1.40 Sg value. The only half-decent match bullet you can use is the old Sierra 2155, the newer 2156 giving a very low Sg value, likewise all of the Berger 155s and Lapua Scenar. Used as a knockabout 100-yard range plinker for a very elderly friend, it'd shoot the first gen 168s fine at this distance despite Sg values in the 1.2s IIRC. (It's now a .30BR with a 10-twist barrel - that's what was knocking around spare - shooting 115s to 125s, so that is going from one stability / rotational extreme to the other!)

 

[Laurie]

The British take on positive compensation was documented over a century ago. Read about it in

https://ia600701.us.archive.org/6/items/philtrans05900167/05900167.pdf

In theory, it only kicked in beyond 600 yards, but I've seen it work at shorter distances. I once shot alongside a very competent rifleman using a .303 Number 4 rifle and good quality Mk7 milspec ammunition in a 300 / 500 / 600 yards range day. His elevations noticeably decreased as we moved back and changed from pretty poor at 300 to excellent at 600. Not scientific, and maybe he was just getting his 'eye in' as the day progressed, but I'm sure positive compensation played a part.

When I started TR shooting, many keen GB competitors still had two 7.62 TR rifles for use at different distances - a Mauser system short-distance (up to 600 yards) rifle and an Enfield No.4 action long-distance rifle for 800-1,000 stages. It was the arrival of the very stiff 4-lug Swing / Paramount / RPA series that finally ended this practice. I bought a friend's No.4 that had filled this role for him when he had been a keen county shooter and regular 'Imperial' competitor long after he had stopped using it. Not only did the heavy Enfield Lock manufactured forged barrel have the slow twist, but it was TIGHT, like really tight compared to a modern barrel - just pushing a patch through it let you know that. This was because European milspec 7.62 bullets were also undersized compared to both contemporary US M180 equivalents and even more so commercial match bullets such as the 150 and 155gn SMKs. As I handloaded everything, this mismatch didn't work too well, so I never used the rifle that much and sold it after a few years.

What you do find with these old 'tight' barrels is that they really push pressures up compared to a .300/.3080 model. I'd forgotten that when I acquired the old Paramount with its Lothar Walther and making up some handloads with 168s and Viht N150 that would be mild in a modern rifle saw pressure problems even at the bottom of my charge weight range, so they had to be pulled and reloaded with yet lower charges. This was explored in a detail test series run at Bisley in the UK many years ago and which can still be found somewhere on the US Rifle Teams Long-Range Forum.

 

[Bart B.]

I'm going to put these words about the Brit's 303's positive compensation based on Mallock's century old calculations on a plaque then hang it on the wall in my shop:

In theory, it only kicked in beyond 600 yards, but I've seen it work at shorter distances. I once shot alongside a very competent rifleman using a .303 Number 4 rifle and good quality Mk7 milspec ammunition in a 300 / 500 / 600 yards range day. His elevations noticeably decreased as we moved back and changed from pretty poor at 300 to excellent at 600. Not scientific, and maybe he was just getting his 'eye in' as the day progressed, but I'm sure positive compensation played a part.

When I started TR shooting, many keen GB competitors still had two 7.62 TR rifles for use at different distances - a Mauser system short-distance (up to 600 yards) rifle and an Enfield No.4 action long-distance rifle for 800-1,000 stages. It was the arrival of the very stiff 4-lug Swing / Paramount / RPA series that finally ended this practice. I bought a friend's No.4 that had filled this role for him when he had been a keen county shooter and regular 'Imperial' competitor long after he had stopped using it. Not only did the heavy Enfield Lock manufactured forged barrel have the slow twist, but it was TIGHT, like really tight compared to a modern barrel - just pushing a patch through it let you know that. This was because European milspec 7.62 bullets were also undersized compared to both contemporary US M180 equivalents and even more so commercial match bullets such as the 150 and 155gn SMKs. As I handloaded everything, this mismatch didn't work too well, so I never used the rifle that much and sold it after a few years.

I wonder what Brian Litz would say about that. I think it kicked in sooner as several benchrest folks shooting up to 300 yards use tuners; but their rifles are typically a tiny bit more accurate at short ranges than those used at 300 yards and further.

And yes, Messers Swenson and King made their "Sw-ing" 4-lug action the solution to out of square case heads arsenal ammo oft times had shoot very accurate. I used a Swenson rear sight on a Palma rifle to win the Colorado State Long Range Match some years ago; thrice across the Palma course. The Brits have had many good ideas about long range precision shooting. Like making barrels with .3065" groove diameter for those .3070" diameter arsenal bullets. USA arsenal bullets were typically .3075" or a tad bigger; arsenal 173-gr. FMJBT match bullets were .3084" to .3086" diameter. Sierra's 155's mic'd .3084" compared to all other Sierra 30 caliber match bullets at .3082" as I measured them.

I flew to South Africa to shoot their fullbore Nationals twice in the late 1990's with the USA team. The General DeWet range near Bloomfontaine is great. As were all those Afrikaners we mingled with; great people indeed. We were allowed to bring Sierra 155's and primed cases then use their powder to load our ammo in 1997. The load I chose enabled me to shoot the second day's high aggregate score across 300 to 900 meters. Used my Paramount action rifle with a Kreiger 32" barrel.

In 1998, their rules had changed. Their arsenal was able to get Sierra 155's from a Canadian wholesale dealer as USA laws didn't allow our bullets be shipped directly to South Africa. Those 155's were loaded in their arsenal ammo and accuracy varied quite a bit across the USA Team's 16 rifles. Mine shot poorly. One USA shooter had their Nationals winning score in hand shooting a clean on the last target when he was scored a miss on his next to last shot. He challenged but lost. His scorekeeper saw the bullet trace go in center. The referee said the pit decision stands. Our teammate shot center on his last shot to place third overall by a few points. A local shooting his Musgrave was declared the winner. Go figure that out.

 

[Laurie]

arsenal 173-gr. FMJBT match bullets were .3084" to .3086" diameter. [Bart B]

Some years back I did a local gun dealer a favour by pulling maybe 500/600 old American manufactured .30-06 military rounds that were in very poor condition. (1943-45 dated Winchester examples with ball powder remained apparently good internally with no sign of powder breakdown; many loaded with extruded powders had the charge turn into a corrosive gelatinous mess that weakened the case so much so that several pulled apart in the press using a collet type puller.) Around one in five rounds was pre-war (can't remember the factory now) loaded with the inter-war M1 173gn FMJBT bullet. Although everything else in these rounds was junk, the bullets were still serviceable and being a Scottish born and bred cheapskate I thought I'd try them in handloads for a 30-06 BSA CF2 sporting rifle I owned at the time.

Talk about pressure! I blew the primer and could hardly open the bolt on my first starting load test round, so back to the drawing board! I hadn't miked them so your 0.3086" was no doubt at least something to do with this. Years afterwards, in an email correspondence with .30-06 aficionado and expert German Salazar, I mentioned this episode and he said that these bullets had very hard cores and thick jackets and yes, they invariably produce much higher pressures than an equivalent modern bullet such as the 175gn Sierra MK.

As an aside, although all allegedly .30-06 rounds which the dealer had acquired many years before as part of a mixed gun auction lot, I discovered half a dozen examples of old and still live Kynoch manufactured .318 Westley Richards Rimless Nitro Express cartridges. You could understand how they ended up with 30-06 ammo as the case is very similar, Westley Richards obviously having necked the 30-06 up to take a .330" diameter bullet and changed little else. They were loaded with long 250gn RN bullets, three each SP and FMJ and are now in my inert ammo collection. This was a fine general purpose Africa cartridge for Mauser type bolt guns and I'm told the 318 (actually a .33 in today's naming practices) is still in use in that continent when people can get ammunition - the last time I looked, Kynamco Ltd. in Suffolk, England loads it again.

 

[RGRobinett]

Thank you, Bryan - yes, "ball-tail" bullets suck! RG

 

[sparker]

This is a complex subject matter, which is precisely the sort of thing this forum was created to discuss.

If there is a general statement to be made about bullet design and transonic stability, it would be that: shorter, fatter bullets with small, shallow BT's or no BT tend to remain most stable at transonic speed. Going up in altitude also results in more stability in transonic, so some bullets which may tumble at transonic at sea level could fly thru fine at 5,000 feet altitude.

The obvious problem with this general statement is that LR shooters don't typically want to shoot short fat bullets, we like the long pointy bullets.

So when selecting a bullet for Extreme Long Range (ELR) where bullets will encounter transonic speed, you have to decide if you want to push transonic far downrange with a long pointy bullet that will be questionable at transonic speed, or select a low BC bullet that will hit transonic at a shorter range, but remain stable as it flies at this speed.

Most ELR shooters favor the high BC option, and there is a lot of grey area as to how these bullets will fly thru transonic. Most will experience some degree of yaw as they fly thru transonic. The technical term is called 'limit cycle yaw' which is a fancy name for 'trim'. In a place where stability is challenged, as in transonic flight, the bullet isn't stable in perfect point forward flight, but can 'trim' at a small yaw angle which puts the bullet in equilibrium for that condition. The yaw isn't a steady pointing to one side, rather a dynamic coning motion around the bullets flight path.

Limit cycle yaw causes 'induced drag' on the bullet, which is proportional to the magnitude of limit cycle yaw angle. Many bullets can fly straight and remain accurate thru transonic even though their drag is +10% or greater than it's 'zero yaw' drag. You can suppress the magnitude of the limit cycle yaw angle by using faster twist rates. Doing so allows the bullet to fly thru transonic with lower angles, and therefore lower drag.

This is why transonic drag and trajectory prediction can be such a grey area; drag can literally be affected by twist rate.
As to what bullet features contribute to limit cycle yaw, it's difficult to say because the entire stability condition is determined by a complex interaction of the bullets mass and aerodynamic properties.

One feature of bullet design that's been identified as having an affect is the camfer at the base of the bullet. At the very base, or heel of a bullet, there is a 0.010" to 0.020" camfer, or radius. The presence of this radius actually causes the bullet to fly with greater limit cycle yaw angles. I first became aware of this by reading an AIAA paper (http://arc.aiaa.org/doi/abs/10.2514/6.2009-3851). I then repeated the experiment by taking some typical bullets that had about 0.020" heal, and cutting that heal off in a lathe. Note, just the small heal was cut off, not the entire boat tail. It amounted to less than 4 grains of material. These bullets I called 'square heal bullets'. I then performed the following comparison. I measured the BC of the un-altered bullets from a 1:12" and 1:8" twist barrel thru transonic speed. For these unaltered bullets, there was noticably (~7%) more drag for the 1:12" twist compared to the 1:8" twist. I then fired the square heal bullets thru both barrels under the same conditions, and observed that there was no difference in transonic drag for these bullets fired from the 1:12" and 1:8". The square heal bullets exhibited the same drag from both barrels that the un-altered bullets had from the 1:8".

So my results matched with the AIAA paper; removing the heal allows the bullet to fly with less limit cycle yaw.

This is a long answer to your question, but it goes to show that sometimes even minor features of a bullets shape can have a substantial effect on it's transonic flight. I don't think any generalizations can be made, which is why I'm working on testing bullets thru various twist rate barrels to determine their transonic flight quality. The end objective for me is to experimentally determine transonic effects for bullets, so it can be described in models used in ballistics software to predict more accurate trajectories thru transonic.

BTW, if you're interested in reading more about the square base experiment and my other tests on how stability affects BC (supersonic and transonic), how twist rate affects MV, precision, etc. it's all published in: 'Modern Advancements in Long Range Shooting'.

-Bryan

Fascinating!

 

[Bart B.]

This thread is fun to read. In spite of his misspelling bullet's heel.

Haven't had this much fun since watching a crocodile eat my mean older sister back when I was a 6-year old.

Maybe I was dreaming.

 

[dmoran]

This thread is fun to read.

Haven't had this much fun since watching a crocodile eat my mean older sister back when I was a 6-year old.

Maybe I was dreaming.

Yep..... a 2-1/2 year old thread (maybe you missed that aspect).