Importance of staying supersonic at the target?
- Thread starter thefitter
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Alex Wheeler
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I am sure an intelligent person will reply, until then.... Modern boat tail bullets don't seem to like to go transonic like a round rose flat base 45-70 bullet for example. They get unstable.
Some very interesting photos of aircraft being passed by a 'sonic' wave as they transition back to subsonic from super sonic are quite extraordinary. The influences of this wave is quite clear on the aircraft. There may be photos of a bullet experiencing the same influences.
As said above, hopefully someone smarter than me will reply, but here is what I've seen.
When a bullet transends(sp)into subsonic it starts to wobble and the bullet spinning and the wobble combined makes the bullet spin like a top as it is slowing down.
I shoot a lot of smallbore. With supersonic rounds from a rifle, they usually go subsonic somewhere around 85yds. They will shoot a tiny group at 75m and a 4"+(sometimes more depending on the bullet) group at 100m. A subsonic bullet from the same rifle will shoot small groups at 100m.
You can see the effects of the sonic wave on military jets on youtube.
When a bullet transends(sp)into subsonic it starts to wobble and the bullet spinning and the wobble combined makes the bullet spin like a top as it is slowing down.
I shoot a lot of smallbore. With supersonic rounds from a rifle, they usually go subsonic somewhere around 85yds. They will shoot a tiny group at 75m and a 4"+(sometimes more depending on the bullet) group at 100m. A subsonic bullet from the same rifle will shoot small groups at 100m.
You can see the effects of the sonic wave on military jets on youtube.
There is no simple answer. Some bullets manage OK, some not so well, some fail entirely, and I've never seen a guide as to which models do and which don't. That's other than the 'boat-tail angle rule', anyway. Bryan Litz says the ideal boat-tail angle is 7-9 deg. Go much above 10 and it's too steep for the air to follow the bullet sides around to the base. This seems to manifest itself as much increased drag and turbulence leading to instability in transonic flight.
It's this effect that has led to the common advice of don't use 30-cal 168s at 1,000yd. That is misleading advice as it came out of the 168gn Sierra 'International' aka MatchKing with its 13-deg BT angle, a 300-metre specialised design originally and the various near copies on the market from Speer, Hornady and Nosler. Berger 168s are designed as long-range bullets with 8.9, 8.5 and a really nice 7-deg angle on the BT, VLD and Hybrid respectively. Hornady A-Max 30s other than the 208 fall into this enforced shorter range bracket too thanks to their BTs. The 155 is a massive 13.5-deg, 168 12.8-deg, and 178 12.6-deg.
Even this 'rule' doesn't always seem to apply. Many older L-R Service Rifle shooters talk about good results at 1,000 with some batches of 7.62mm match ammo in their 20-inch barrel M14s using the 168gn SMK.
You can tell it is a transonic issue as it's also velocity dependent, and we usually think '308 Winchester / 7.62mm military' here. I've successfully used Hornady and Sierra 168s at 1,000yd in 30-cal magnums which are driving them fast enough to just keep out of trouble at this distance. Still not recommended of course thanks to their low BCs compared to better long-range speciality bullets.
I was much exercised by this worry in the early days of F Class, shooting a .223 and 80-grainers at 2,800 fps MV or even a bit less. Even the optimistic G1 ballistic charts of the time said they'd be subsonic at 1,000. (Bryan Litz's Point Mass Ballistic Solver 2.0's program says 1,078 fps at 1,000yd at 2,800 fps MV in standard conditions for the SMK; below 1.2 MACH beyond a point somewhere around 780 yards.) In fact they shot fine in a large range of conditions apart from needing around 60% more windage allowance than the 6.5s that were appearing. The biggest problem apart from my wind-reading skills was constantly getting out of the rhythm to call to have the target pulled as the butts crew didn't hear the subsonic bullets and had trouble seeing their little holes.
In the early days of F/TR I used a 24" barrel factory tactical rifle that was billed as F/TR ready - it wasn't! The much touted 175gn Sierra MK as used in the US military M118LR sniper round was allegedly good at 1,000yd at 308 velocities - it wasn't! It would group OK in stable conditions, but any significant change would cause a much greater deflection on the target than the ballistic charts predicted, so transonic flight was obviously making it barely stable. I also suspect conditions on the day had a big effect as Litz's program says it's just subsonic at 2,650 fps MV at 1,000 in standard conditions. Throw in MV spread and there was a risk of some being supersonic, some not and get warmer / colder air moving onto the range under some conditions might change things for the better / worse. I used the combination on Scotland's notorious Blair Atholl range at 1,000 in one competition in a day of cold headwinds from the north and frequent rain squalls. The temperatures plummeted during the squalls (and the wind went mad too!) and what was an 'interesting group pattern' outside of squall conditions changed to seeing me do well to just stay on the target frame at all - and they are BIG frames at Blair!
On ranges other than Blair (which is electronic, so no butts crew), target markers reported they heard faint supersonic 'crack' and saw round holes on the paper, so they appeared to remain stable and just supersonic in summer shooting conditions. Confirmation of this transonic performance phenomenon has since come from USMC snipers who say the M118LR's performance 'falls off a cliff' beyond 800 metres (900yd), which is just what I found with the bullet at slightly higher than M118LR MVs. A move to the 190gn Sierra MK with Viht N550 keeping the MVs reasonable gave a vast improvement in 1,000yd performance.
Incidentally, the old long-range 30-cal SMKs with their extra length boat-tail sections have a superb reputation for stable transonic / subsonic flight. They were used by GB and British Commonwealth 'Match Rifle' shooters (.308W shot at 1,000, 1,100 and 1,200yd) for many years before the current bunch of 210gn and up VLDs and Hybrids appeared - that's the venerable 190, 200 and 220gn MK bullets.
The 'easy' / better answer to all this is to use a design such as the 30-cal 185gn Berger LRBT with a reputation for good long range performance and to load it to achieve or exceed 1,350 fps at 1,000yd. If I can get the combination I'm using to be predicted to hold 1,400 fps at this range in a G7 based program calculation, I'm happier still.
It's this effect that has led to the common advice of don't use 30-cal 168s at 1,000yd. That is misleading advice as it came out of the 168gn Sierra 'International' aka MatchKing with its 13-deg BT angle, a 300-metre specialised design originally and the various near copies on the market from Speer, Hornady and Nosler. Berger 168s are designed as long-range bullets with 8.9, 8.5 and a really nice 7-deg angle on the BT, VLD and Hybrid respectively. Hornady A-Max 30s other than the 208 fall into this enforced shorter range bracket too thanks to their BTs. The 155 is a massive 13.5-deg, 168 12.8-deg, and 178 12.6-deg.
Even this 'rule' doesn't always seem to apply. Many older L-R Service Rifle shooters talk about good results at 1,000 with some batches of 7.62mm match ammo in their 20-inch barrel M14s using the 168gn SMK.
You can tell it is a transonic issue as it's also velocity dependent, and we usually think '308 Winchester / 7.62mm military' here. I've successfully used Hornady and Sierra 168s at 1,000yd in 30-cal magnums which are driving them fast enough to just keep out of trouble at this distance. Still not recommended of course thanks to their low BCs compared to better long-range speciality bullets.
I was much exercised by this worry in the early days of F Class, shooting a .223 and 80-grainers at 2,800 fps MV or even a bit less. Even the optimistic G1 ballistic charts of the time said they'd be subsonic at 1,000. (Bryan Litz's Point Mass Ballistic Solver 2.0's program says 1,078 fps at 1,000yd at 2,800 fps MV in standard conditions for the SMK; below 1.2 MACH beyond a point somewhere around 780 yards.) In fact they shot fine in a large range of conditions apart from needing around 60% more windage allowance than the 6.5s that were appearing. The biggest problem apart from my wind-reading skills was constantly getting out of the rhythm to call to have the target pulled as the butts crew didn't hear the subsonic bullets and had trouble seeing their little holes.
In the early days of F/TR I used a 24" barrel factory tactical rifle that was billed as F/TR ready - it wasn't! The much touted 175gn Sierra MK as used in the US military M118LR sniper round was allegedly good at 1,000yd at 308 velocities - it wasn't! It would group OK in stable conditions, but any significant change would cause a much greater deflection on the target than the ballistic charts predicted, so transonic flight was obviously making it barely stable. I also suspect conditions on the day had a big effect as Litz's program says it's just subsonic at 2,650 fps MV at 1,000 in standard conditions. Throw in MV spread and there was a risk of some being supersonic, some not and get warmer / colder air moving onto the range under some conditions might change things for the better / worse. I used the combination on Scotland's notorious Blair Atholl range at 1,000 in one competition in a day of cold headwinds from the north and frequent rain squalls. The temperatures plummeted during the squalls (and the wind went mad too!) and what was an 'interesting group pattern' outside of squall conditions changed to seeing me do well to just stay on the target frame at all - and they are BIG frames at Blair!
On ranges other than Blair (which is electronic, so no butts crew), target markers reported they heard faint supersonic 'crack' and saw round holes on the paper, so they appeared to remain stable and just supersonic in summer shooting conditions. Confirmation of this transonic performance phenomenon has since come from USMC snipers who say the M118LR's performance 'falls off a cliff' beyond 800 metres (900yd), which is just what I found with the bullet at slightly higher than M118LR MVs. A move to the 190gn Sierra MK with Viht N550 keeping the MVs reasonable gave a vast improvement in 1,000yd performance.
Incidentally, the old long-range 30-cal SMKs with their extra length boat-tail sections have a superb reputation for stable transonic / subsonic flight. They were used by GB and British Commonwealth 'Match Rifle' shooters (.308W shot at 1,000, 1,100 and 1,200yd) for many years before the current bunch of 210gn and up VLDs and Hybrids appeared - that's the venerable 190, 200 and 220gn MK bullets.
The 'easy' / better answer to all this is to use a design such as the 30-cal 185gn Berger LRBT with a reputation for good long range performance and to load it to achieve or exceed 1,350 fps at 1,000yd. If I can get the combination I'm using to be predicted to hold 1,400 fps at this range in a G7 based program calculation, I'm happier still.
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Great explanation and in-the-field info, Laurie.
Thanks for posting!
Thanks for posting!
Never mentioned that's important is that poor performance in transonic isn't a matter of gyroscopic stability in itself, but of dynamic instability seriously challenging normal gyroscopic stability.
It's important because this can't really be predicted.
It's discovered through testing.
Why military style competitions choose components and ranges that bring this issue to bear, has never made any sense to me. Why a 308Win or 223Rem -at 1kyds? They're not even good, much less ideal 1Kyd cartridges..
If the rest of the world simply moved on with a better plan, then the subject would remain only in very old books.
It's important because this can't really be predicted.
It's discovered through testing.
Why military style competitions choose components and ranges that bring this issue to bear, has never made any sense to me. Why a 308Win or 223Rem -at 1kyds? They're not even good, much less ideal 1Kyd cartridges..
If the rest of the world simply moved on with a better plan, then the subject would remain only in very old books.
mikecr said:
memilanuk
Gold $$ Contributor
You can see those scores now, today, and have been able to for the last umpteen years.
Its fairly common (at least around here) to have a Palma course (unlimited sighters + 15 for record @ 800, then 2 sighters + 15 for record @ 900 & 1000) followed by another 1000yd stage consisting of unlimited sighters + 20 shots for record. The latter stage is usually either 'Any Rifle Any Sight' or 'Any Rifle Iron Sight' in this neck of the woods (PNW). Caliber and gun can be pretty much anything you can shoulder and fire from position for 30 minutes and however many rounds. Some people shoot their Match Rifle (another NRA classification), commonly in calibers such as 6XC, 6CM, .260 Rem, etc., while others shoot dedicated Prone guns in 6.5-284, 7mm RSAUM, .300 WM, etc. Some just leave well enough alone, and shoot their Palma Rifle, iron sights, .308 Win, etc. as is.
Similar matches take place probably every weekend across the country @ mid-range and long range Prone matches. I've seen 'great' Palma shooters (as in: on the US or Canadian teams) win with either setup. Thats the one thing about this sport, its a whole lot more about the Indian than the arrow. I will admit... a really good Prone shooter with a 6 BR @ mid-range, or a 6.5-284 or similar @ long-range... will probably tend to edge out another really good Prone shooter running a .308 Win over the course of an Agg as the little things add up.
Its fairly common (at least around here) to have a Palma course (unlimited sighters + 15 for record @ 800, then 2 sighters + 15 for record @ 900 & 1000) followed by another 1000yd stage consisting of unlimited sighters + 20 shots for record. The latter stage is usually either 'Any Rifle Any Sight' or 'Any Rifle Iron Sight' in this neck of the woods (PNW). Caliber and gun can be pretty much anything you can shoulder and fire from position for 30 minutes and however many rounds. Some people shoot their Match Rifle (another NRA classification), commonly in calibers such as 6XC, 6CM, .260 Rem, etc., while others shoot dedicated Prone guns in 6.5-284, 7mm RSAUM, .300 WM, etc. Some just leave well enough alone, and shoot their Palma Rifle, iron sights, .308 Win, etc. as is.
Similar matches take place probably every weekend across the country @ mid-range and long range Prone matches. I've seen 'great' Palma shooters (as in: on the US or Canadian teams) win with either setup. Thats the one thing about this sport, its a whole lot more about the Indian than the arrow. I will admit... a really good Prone shooter with a 6 BR @ mid-range, or a 6.5-284 or similar @ long-range... will probably tend to edge out another really good Prone shooter running a .308 Win over the course of an Agg as the little things add up.
I added this because their format challenges them beyond cartridge capability anyway.mikecr said:
And this IMO removes any basis out of their shooting that a 308win is a good choice for 1kyd shooting.
Put a 308win in IBS events, short range or long, and it will represent only a disability in the long run.
It's a military cartridge, effective to tactical ranges, and not a competitive design.
I like the 308 & love the 223. But something like a 6Dasher(a competitive design) could ballistically stomp either at any range in competition, and with no worries of dynamic instability from light-for-bore, wannabe competitive bullets.
The path that leads to today's competitive cartridges starts with bullets.
Then it goes to capacity to drive them to advantage, given available powders.
Then it goes to case/chamber availability/design that supports advantage in the long run.
None of this leads to a 308win or a 223Rem..
This is why I perhaps misunderstand the efforts to push these cartridges beyond their design(1kyd).
Mike, it might make sense to you if you realize that military style competition began with the objective of seeing how military folks could shoot military rifles and military ammunition. Modern day centerfire rifle matches in the USA and Great Britian started over a hundred year ago with M1903's and .30-06 on our side of the pond. The Brits started the same time with their LM's and SMLE's using the .303. The civilians just copied their stuff for the most part.mikecr said:
Whether or not any cartridge is "good" for long range competition depends on what the orgainzing folks's objectives are in using it. There's better cars for oval track racing than what NASCAR specs for their discipline. All this aside, as far as accuracy goes, the .308 Win. will shoot just as accurate as any other cartridge at 1000 yards. It will drift more or less than others when the wind blows, but such is true with all cartridges. Therefore, the .308 Win. is "very good" for those choosing to see how folks do with just that cartridge.
There's better cartridges for 50 yards, meters and 100 yards matches than the .22 long rifle rimfire round, too. But that's what the IOC specs these days for biathlon and free rifle matches. The IOC used to have 300 metre free rifle and biathlon events using centerfire cartridges in the Olympics. In 1976, that was the last year they did so. .22's were spec'd so other country's ranges could be more easily built with less real estate 'cause of the few-mile safety zone required far down range for centerfire ammunition.
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