Does a bullet go through any adverse effects by being over stabilized. Is there a point where this becomes a problem? I just entered my info into Berger's stability calculator and got 1.98 as a stability factor.
Effects of over stability
- Thread starter jsthntn247
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Bryan Litz Ballistics
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There's really no such thing as 'over stability', at least not any practical concern.
If you had a bullet that was on the verge of failure, like a 40 grain bullet running 3800 fps out of a 220 swift and you went from a 1:14" to a 1:7", that might tip the scales and blow the bullet up. In that case, the problem would be structural integrity, not over stability.
Also, if you have bullets that are poorly balanced, then your precision (group size) will be harmed more by faster than necessary twist. Again, it's not the stability level causing a problem, it's poor bullets. These days, it's hard to find very badly balanced bullets. In other words, you might go from .4" to .5" groups by increasing spin rate by a couple inches.
In all practical reality, there's very little downside to spinning bullets faster than 'conventional'. One consequence of fast twist that's actually related to stability is spin drift. Higher stability levels produce more spin drift, but that's the same for every shot and can be accounted for so it's not really a problem.
The thing about bullets being too stable to 'trace' with the trajectory, and going 'nose high' on the downrange side of the trajectory does not apply to small arms. It can be a problem for artillery shells that weight 100's of lb's, go 1000's of feet up in altitude at angles exceeding 35 degrees above line of sight. For small arms where bullet weight is measured in grains and firing angles are typically under 1 degree (60 MOA), failure to trace is not an issue.
I routinely shoot bullets with stability factors in excess of 3 with no ill effect. Some of the best groups we've seen have been with twist rates that would be considered 'fast' for the bullet.
-Bryan
If you had a bullet that was on the verge of failure, like a 40 grain bullet running 3800 fps out of a 220 swift and you went from a 1:14" to a 1:7", that might tip the scales and blow the bullet up. In that case, the problem would be structural integrity, not over stability.
Also, if you have bullets that are poorly balanced, then your precision (group size) will be harmed more by faster than necessary twist. Again, it's not the stability level causing a problem, it's poor bullets. These days, it's hard to find very badly balanced bullets. In other words, you might go from .4" to .5" groups by increasing spin rate by a couple inches.
In all practical reality, there's very little downside to spinning bullets faster than 'conventional'. One consequence of fast twist that's actually related to stability is spin drift. Higher stability levels produce more spin drift, but that's the same for every shot and can be accounted for so it's not really a problem.
The thing about bullets being too stable to 'trace' with the trajectory, and going 'nose high' on the downrange side of the trajectory does not apply to small arms. It can be a problem for artillery shells that weight 100's of lb's, go 1000's of feet up in altitude at angles exceeding 35 degrees above line of sight. For small arms where bullet weight is measured in grains and firing angles are typically under 1 degree (60 MOA), failure to trace is not an issue.
I routinely shoot bullets with stability factors in excess of 3 with no ill effect. Some of the best groups we've seen have been with twist rates that would be considered 'fast' for the bullet.
-Bryan
We still have a fair few people here (UK) who think in terms of the slowest twist that apparently works, a carryover from the days of British Commonwealth 'Target Rifle' using abysmal 7.62 Nato ammunition loaded with what were often non-concentric imbalanced bullets. Same with most of those with a short-range benchrest shooting background. 1-14" twist was very common on 7.62 TR rifles when I shot in this discipline in the 80s and 90s and examples of older rifles still turn up with the twist rate.
When FTR started up going on the advice of friends, one of whom built my first proper FTR gun, three of us jointly bought Bartlein barrel blanks in a 1-13.5" twist rate to shoot the 155gn Lapua Scenar. Sg value 1.10 !! They shot not too badly except at 1,000 where elevations started to get really rough especially in colder weather - my first European F-Class championship outing was OK at 800 and 900 but very disappointing at 1,000 on low lying Bisley in early November. With hindsight, I'm convinced that I lost so much BC through the slow twist that bullets were barely supersonic at this distance in the conditions and I was getting all sorts of transonic turbulence and maybe a mixture of super and subsonic bullets at the target. That twist rate would just shoot the 185gn Juggernaut at Diggle Ranges ~900ft ASL (1.07 Sg) until winter arrived. The first long-range match in colder weather on a day that was somewhere around the high 30s F started really disastrously - luckily I anticipated this possibility and took 155gn ammunition along too. Nowadays I use a 10" twist in 308 irrespective of the bullets I'm likely to use, 2.2 Sg with the 155.5gn Fullbore in typical conditions, but I just went onto the Berger Ballistics site's twist rate stability calculator to see what they were running at in Raton back in August 2013 in the 90s and 8,000 ft up - 3.16. Any bad shots (and there were many!) were down to me, the rifle and bullet performance was really good throughout my US experience right through to the last morning when we shot at the steel buffalo behind the silhouttes range to use up all remaining ammunition before the flight home. 1,200 and something yards? Too easy and Des Parr with his 7WSM and I ended up shooting at ever smaller rocks on the ground around the 'buffalo', and hitting most of them too on a still morning without much mirage. Fantastic air clarity / visibility in New Mexico!
I first went for the 10 to shoot heavier bullets in 308, but despite some people telling me I was mad, decided it was the way to go for the lot. So, the first edition of Applied Ballistics for Long Range was a great comfort when it appeared. Most FTR people here now take the 10 twist as normal for the cartridge, although I've still got friends who are highly dubious. Anyway, a chore for this afternoon is to get my latest FTR project - an old 7.62 Paramount TR rifle that has been given the original 'Farky' treatment with scope and bi-pod - out of its cabinet and see what the rifling twist rate is. It's possible it'll be a 14 which will be very limiting, so it's fingers crossed for 13", most unlikely to be anything faster.
As an aside Bryan, do you know why the US DoD settled on 1-11.25 as the standard 7.62/308 twist rate for marksman and sniper rifles?
When FTR started up going on the advice of friends, one of whom built my first proper FTR gun, three of us jointly bought Bartlein barrel blanks in a 1-13.5" twist rate to shoot the 155gn Lapua Scenar. Sg value 1.10 !! They shot not too badly except at 1,000 where elevations started to get really rough especially in colder weather - my first European F-Class championship outing was OK at 800 and 900 but very disappointing at 1,000 on low lying Bisley in early November. With hindsight, I'm convinced that I lost so much BC through the slow twist that bullets were barely supersonic at this distance in the conditions and I was getting all sorts of transonic turbulence and maybe a mixture of super and subsonic bullets at the target. That twist rate would just shoot the 185gn Juggernaut at Diggle Ranges ~900ft ASL (1.07 Sg) until winter arrived. The first long-range match in colder weather on a day that was somewhere around the high 30s F started really disastrously - luckily I anticipated this possibility and took 155gn ammunition along too. Nowadays I use a 10" twist in 308 irrespective of the bullets I'm likely to use, 2.2 Sg with the 155.5gn Fullbore in typical conditions, but I just went onto the Berger Ballistics site's twist rate stability calculator to see what they were running at in Raton back in August 2013 in the 90s and 8,000 ft up - 3.16. Any bad shots (and there were many!) were down to me, the rifle and bullet performance was really good throughout my US experience right through to the last morning when we shot at the steel buffalo behind the silhouttes range to use up all remaining ammunition before the flight home. 1,200 and something yards? Too easy and Des Parr with his 7WSM and I ended up shooting at ever smaller rocks on the ground around the 'buffalo', and hitting most of them too on a still morning without much mirage. Fantastic air clarity / visibility in New Mexico!
I first went for the 10 to shoot heavier bullets in 308, but despite some people telling me I was mad, decided it was the way to go for the lot. So, the first edition of Applied Ballistics for Long Range was a great comfort when it appeared. Most FTR people here now take the 10 twist as normal for the cartridge, although I've still got friends who are highly dubious. Anyway, a chore for this afternoon is to get my latest FTR project - an old 7.62 Paramount TR rifle that has been given the original 'Farky' treatment with scope and bi-pod - out of its cabinet and see what the rifling twist rate is. It's possible it'll be a 14 which will be very limiting, so it's fingers crossed for 13", most unlikely to be anything faster.
As an aside Bryan, do you know why the US DoD settled on 1-11.25 as the standard 7.62/308 twist rate for marksman and sniper rifles?
I think I recall reading something in the Hornady manual about how the bullet spins around the geometric center while it's in the bore and then spins about the center of gravity while in flight. During the transition from the bore to the air, there may be some perturbation if the center of gravity is too different from the geometric center. It also seems like a wobbly bullet (not concentric) may have more drag in flight.
It may be that modern bullets have corrected this by ensuring all bullets have centers of mass perfectly concentric with the center of gravity (down to 0.000001" or whatever). However, given the dimensional differences and weight differences that have been reported between different bullets in a box or between different lots, I guess I would remain open to the possibility that a void in the lead core or a void between the lead core and jacket might significantly displace the center of gravity of some bullets from the geometric center.
The existence of boxes of bullets where the effect is insignificant may not imply that there are not boxes of bullets where the effect is significant. Perhaps the concern is that a faster twist will throw a noncentric bullet further from the group than a slower twist. I think I'll still favor barrels that provide the slowest twist that will ensure the full BC of a bullet.
It may be that modern bullets have corrected this by ensuring all bullets have centers of mass perfectly concentric with the center of gravity (down to 0.000001" or whatever). However, given the dimensional differences and weight differences that have been reported between different bullets in a box or between different lots, I guess I would remain open to the possibility that a void in the lead core or a void between the lead core and jacket might significantly displace the center of gravity of some bullets from the geometric center.
The existence of boxes of bullets where the effect is insignificant may not imply that there are not boxes of bullets where the effect is significant. Perhaps the concern is that a faster twist will throw a noncentric bullet further from the group than a slower twist. I think I'll still favor barrels that provide the slowest twist that will ensure the full BC of a bullet.
liseo
Silver $$ Contributor
My question is
Two bullets with the same weight, same speed and with the same ability to group the 100 m, but one of them fired a barrel 1 / 8.5 twist and another with a 1 / 6.5 twist (example only)
Considering a crosswind of equal intensity for both the projectile , the one with faster twist could deflect least its trajectory caused by wind, due to its higher angular velocity?
Two bullets with the same weight, same speed and with the same ability to group the 100 m, but one of them fired a barrel 1 / 8.5 twist and another with a 1 / 6.5 twist (example only)
Considering a crosswind of equal intensity for both the projectile , the one with faster twist could deflect least its trajectory caused by wind, due to its higher angular velocity?
dcali
Bullet Maker
The wind deflection is less for a faster spin because marginally stable bullets yaw more, increasing drag. Once you get fully spun up (which is faster than stability requires), the difference is tiny or zero. Bryan may have more info on that and at what point it happens. I would imagine it's bullet-dependent.
The bottom line is that for point blank benchrest, you want the slowest twist you can get away with. For long range, you want to push the twist a little more to wring out every last drop of BC, knowing that you are giving up a little in intrinsic group size. At distance, that's a good trade. Up close, not so much.
The bottom line is that for point blank benchrest, you want the slowest twist you can get away with. For long range, you want to push the twist a little more to wring out every last drop of BC, knowing that you are giving up a little in intrinsic group size. At distance, that's a good trade. Up close, not so much.
I'm sure somebody can do the torque calculations, but subjectively I've never noticed any difference with the 155.5 at 3,050 fps in a 1-12 twist barreled rifle and 1-10" even when shooting them back to back.
Move from 155 to 210 in a heavy load and you DO notice an increase in torque. Most people are likely unaware of these effects unless the cant lock on a bipod slips, but I do most of my shooting off turf firing points on a range that sees a lot of rain so the going is soft underfoot. In the days when I used ski-foot tracking bi-pods (I use the FLEX-Pod now which has its feet fixed) you'd finish a 20-round match with two parallel three-inch long tracks from the feet if you got the rifle handling right. It was normal to see one track visibly deeper than the other through more pressure having been put onto that foot.
Move from 155 to 210 in a heavy load and you DO notice an increase in torque. Most people are likely unaware of these effects unless the cant lock on a bipod slips, but I do most of my shooting off turf firing points on a range that sees a lot of rain so the going is soft underfoot. In the days when I used ski-foot tracking bi-pods (I use the FLEX-Pod now which has its feet fixed) you'd finish a 20-round match with two parallel three-inch long tracks from the feet if you got the rifle handling right. It was normal to see one track visibly deeper than the other through more pressure having been put onto that foot.
laurie--the m-24 (US Army) and m110 or mk 11 rifles (Army, USMC and Navy) feature the 11.25" twist barrels. the m-40 series rifles (USMC) have a 12" twist. why the USMC is going with the slower twist is beyond me--probably because that is what they always had for twist. I would think a 10" twist in their rifles and shooting a 185 grain bullet would be the ticket and ballistically superior to the 175 loads currently in use (mk 318????)
Is an interesting question and I ask myself the same question as to why Accuracy International also choose the 12" twist for their 308 barrels in my AI AT. I think there is a specific reason in there, just that we don't know. FWIW, my gun has no problem stabilizing the Berger 185 grain Juggs.cmillard said:
I'm surprised by questions about the 185 Juggernaut in a 12 inch twist. Until the revision of Sg values up from 1.4 to 1.5 solely to guarantee 100% of the theoretical BC value is delivered, this was regarded and moreover officially advised as the optimum for this bullet at achievable 308 Win MVs.
In fact the 13 inch twist rate was regarded as sub-optimal but acceptable from a solely stability point of view for the bullet. One B. Litz of Michigan had a very successful period sling shooting the then new 185 in Palma Rifle, US any bullet weight rules with a rifle set up for 155s and with this twist rate, and only changed to 12 later. Whether that improved his scores noticeably - well, he'll have to comment on that.
I'd still stick with the ten inch rate for this and other bullets, but there are many, many Fullbore and FTR shooters getting excellent 1,000 yard results with the 185 LRBT in 12 inch twist barrels. With an MV of 2,800 fps, the Sg is computed as 1.41 in standard ballistic conditions, but this rises to the optimum 1.5 on any range 1,000 ft or more ASL and in temperatures of 80F or so. Moreover, the Miller Rules formula is a little pessimistic on HPBT type bullet's values depending on the jacket length up front and how much airspace is in the nose / tip section. To take an extreme example, the 155gn Lapua Scenar at 3,000-3,100 fps was popular on the GB FTR scene when the discipline got going. With considerable internal airspace up-front, this is a long bullet for its weight at ~1.29". GB 'Target Rifle' (sling) shooters had used this bullet for years at 2,950 fps or so out to 1,100 yards in all weathers (we shoot year round in the UK) down to freezing air with 13 and even 14-inch twist rate barrels. Run these values through the Berger Ballistics program and 14 computes 1.01 Sg at 59-deg and 13 gives 1.17 at the same temperature with this bullet at 2,950 fps, so there were apparently a LOT of complete fluke 1-MOA V-Bulls (Xs) being shot at long ranges. According to the program, one loses 10% of the BC value in a 13 inch twist at these velocities, and 15% in the 14 inch rate, so bullets would (rather, should) have been transonic even in 800 yard matches at these MVs and certainly subsonic at 1,000. (More often than not, they weren't and still aren't from 13 inch twist rate rifles as anybody marking targets for TR matches will tell you!)
Russell Simmonds, former world FTR champion and now number 2 after Raton in 2013, started out with a 14 inch twist barrel and it didn't stop him winning the inaugural GB FTR league championship in a series predominately comprising 1,000 yard matches.
As I wrote in earlier post, I'm not advocating the use of slow twists with today's bullets - quite the reverse - but I'm saying that this isn't as black and white an issue as sometimes portrayed and as always don't let the technical trivia override everything else. The people who win matches use equipment that works but once they have that they get their scores by seeing what the wind is doing and keeping an eye on their long-range elevations during a match and adjusting sight settings accordingly. Constantly changing equipment and ammunition specifications to achieve some theoretical optimum is a distraction which these shooters wisely avoid.
In fact the 13 inch twist rate was regarded as sub-optimal but acceptable from a solely stability point of view for the bullet. One B. Litz of Michigan had a very successful period sling shooting the then new 185 in Palma Rifle, US any bullet weight rules with a rifle set up for 155s and with this twist rate, and only changed to 12 later. Whether that improved his scores noticeably - well, he'll have to comment on that.
I'd still stick with the ten inch rate for this and other bullets, but there are many, many Fullbore and FTR shooters getting excellent 1,000 yard results with the 185 LRBT in 12 inch twist barrels. With an MV of 2,800 fps, the Sg is computed as 1.41 in standard ballistic conditions, but this rises to the optimum 1.5 on any range 1,000 ft or more ASL and in temperatures of 80F or so. Moreover, the Miller Rules formula is a little pessimistic on HPBT type bullet's values depending on the jacket length up front and how much airspace is in the nose / tip section. To take an extreme example, the 155gn Lapua Scenar at 3,000-3,100 fps was popular on the GB FTR scene when the discipline got going. With considerable internal airspace up-front, this is a long bullet for its weight at ~1.29". GB 'Target Rifle' (sling) shooters had used this bullet for years at 2,950 fps or so out to 1,100 yards in all weathers (we shoot year round in the UK) down to freezing air with 13 and even 14-inch twist rate barrels. Run these values through the Berger Ballistics program and 14 computes 1.01 Sg at 59-deg and 13 gives 1.17 at the same temperature with this bullet at 2,950 fps, so there were apparently a LOT of complete fluke 1-MOA V-Bulls (Xs) being shot at long ranges. According to the program, one loses 10% of the BC value in a 13 inch twist at these velocities, and 15% in the 14 inch rate, so bullets would (rather, should) have been transonic even in 800 yard matches at these MVs and certainly subsonic at 1,000. (More often than not, they weren't and still aren't from 13 inch twist rate rifles as anybody marking targets for TR matches will tell you!)
Russell Simmonds, former world FTR champion and now number 2 after Raton in 2013, started out with a 14 inch twist barrel and it didn't stop him winning the inaugural GB FTR league championship in a series predominately comprising 1,000 yard matches.
As I wrote in earlier post, I'm not advocating the use of slow twists with today's bullets - quite the reverse - but I'm saying that this isn't as black and white an issue as sometimes portrayed and as always don't let the technical trivia override everything else. The people who win matches use equipment that works but once they have that they get their scores by seeing what the wind is doing and keeping an eye on their long-range elevations during a match and adjusting sight settings accordingly. Constantly changing equipment and ammunition specifications to achieve some theoretical optimum is a distraction which these shooters wisely avoid.
the USMC's M40 series rifles us a 25" Schneider barrel 6 groove 1-12" twist. from what I have read (take that for what it's worth) about shooting at extended range overseas by USMC scout snipers, there is erratic bullet flight--could be instability as it goes through the transonic stage??? would more twist help solve this? switching to say the 185 and more twist would obviously extend the range.
Most likely the combination of a relatively low BC 175 and 24-inch barrel with the resulting nominal 2,550 fps MV with the M118LR cartridge. My first FTR rifle was a factory FN Special Police Rifle shooting the self-same 175gn Sierra MK in a load with a bit over 2,600 fps. 800 yard performance was acceptable, 900 dropping off noticeably and 1,000 terrible unless conditions were very calm. With its 0.243 G7 BC it is computed to run at ~1,120 fps at 1,000 so the odds were it had a mixture of sub and barely supersonic bullets at the target
This is a transonic performance issue, and the 175 0.308" SMK is by no means a bad trans/subsonic performer in relative terms. Changing to a faster twist rate would be unikely to help as the Sg is already at 1.66 with 2,550 fps and the 12-inch twist under standard ballistic conditions.
In my case I switched to the old 190gn Sierra MK and a hottish load of Viht N550 as a short term palliative until I got a proper competition rifle. Despite a lower Sg value, it performed much better beyond 800 simply because it got the 1,000 yard velocity up to somewhere around 1,200 fps, still much slower than desirable but with less distance travelled at transonic speeds and everything guaranteed to be at least supersonic. (The old Sierra 190/200/220gn MKs had a reputation as good transonic bullets too. Before the modern breed of 210gn LR and VLD bullets appeared they were favourites among British and Commonwealth 'Match Rifle' competitors who are also limited to .308 Win but whose match stages are 1,000 / 1,100 / 1,200 yards and longer where the rfile range allows.)
This is a transonic performance issue, and the 175 0.308" SMK is by no means a bad trans/subsonic performer in relative terms. Changing to a faster twist rate would be unikely to help as the Sg is already at 1.66 with 2,550 fps and the 12-inch twist under standard ballistic conditions.
In my case I switched to the old 190gn Sierra MK and a hottish load of Viht N550 as a short term palliative until I got a proper competition rifle. Despite a lower Sg value, it performed much better beyond 800 simply because it got the 1,000 yard velocity up to somewhere around 1,200 fps, still much slower than desirable but with less distance travelled at transonic speeds and everything guaranteed to be at least supersonic. (The old Sierra 190/200/220gn MKs had a reputation as good transonic bullets too. Before the modern breed of 210gn LR and VLD bullets appeared they were favourites among British and Commonwealth 'Match Rifle' competitors who are also limited to .308 Win but whose match stages are 1,000 / 1,100 / 1,200 yards and longer where the rfile range allows.)
Keith Glasscock
Gold $$ Contributor
Bobr1 said:
I just made that change on my 284. Torque change is imperceptible in the bags.
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