Duh, 1/7.
Why would you go any other way?
Why would you go any other way?
1/7, 1/8, 1/9 which for a combat carbine?
- Thread starter Matt Landfair
- Start date
I'd be fine with 1/7 or 1/8; either handles 77gr fine, and 1/8 should handle lighter bullets (like most available 5.56) slightly better.
- Jake
- Jake
regdudedrtyjob
Regular Member
My BCM Standard Barrel is 1/7, it seemed to really like the 62 gr. Soft Point ammo I get for work. I wouldn't be opposed to 1/8, but more for a 3-gun type of set-up.
A lot of people are going to 1:8 twist as this, and a proper chamber, allows you to shoot everything from the 42gr frange to the 87gr tungsten without a significant loss in accuracy. The 1:7 twist is awesome if you want to shoot heavy bullets all the time - but the 55gr accuracy will suffer.
Bourneshooter
Blue Line Sheepdog
The 1/8 handling 55gr better than 1/7 is not something I've considered before. I plink/train with 55gr, but carry a 75gr in personal guns (1/7) and a 64gr in duty gun(1/9).
Something worth considering for sure.
Something worth considering for sure.
When we were working on the MK1 series at PWS we did a lot of testing with different chambers and different twist rates and the .223 Wylde Chamber and a 1:8 twist is a winner! Eats everything and shoots everything!
Bourneshooter
Blue Line Sheepdog
Interesting Todd. Thanks for the info!
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Chris Taylor
Random Factor of the K Power
1- Lighter bullets tend to not shoot as well in faster twist guns. I'm of the opinion that they "over-stabilize" due to the rotational forces. Just like heavy (longer) bullets don't stabilze as well in slower twist guns (they're spinning too slow for proper gyroscopic stability), my thought is the inverse is happening, the lighter (shorter) bullets are spinning too fast for proper stability. Haven't done any testing on it yet, so it's still a WAG at this point, but it makes sense to me. 
2- Something of a hold over from using light jacketed, lightly constructed bullets for varmint hunting, where too fast of a twist will cause the bullets to vaporize occasionally due to rotational forces being to much for the bullet's integrity.
Edit: lighter bullets tend to shoot better in fast twist guns than heavy bullets do in slow twist ones, so it's better to twist a bbl for the heaviest pill you plan on shooting.
2- Something of a hold over from using light jacketed, lightly constructed bullets for varmint hunting, where too fast of a twist will cause the bullets to vaporize occasionally due to rotational forces being to much for the bullet's integrity.
Edit: lighter bullets tend to shoot better in fast twist guns than heavy bullets do in slow twist ones, so it's better to twist a bbl for the heaviest pill you plan on shooting.
Of interest this is taken from a 1986 study commissioned by the US Army of the M16A2. Elsewhere in the study, it states that independent testing done by the NRA showed the M16A1 with M193 ammo to be more accurate at 200mtrs then the A2/M855 combo. The Army was primarily concerned with accuracy to 300mtrs.
It is almost 30 y/o data, and I'd be interested in knowing if any of it has been superseded or disproved by scientific testing.
"While any alternative must be tested, previous firing tests have confirmed that a 1:9 twist will provide for stability of a bullet similar to the new ball round. Reducing barrel twist to 1:9 will result in less stress on the bullet, barrel life will be improved, and barrel fouling will be reduced. While this twist may not fully stabilize the tracer round to maximum range, the rifle tracer is normally used as a marking round at extended ranges and precise accuracy is not required.
The M16A1 has one twist (rifling) for each 12 inches of barrel length and it has very effective terminal ballistics against personnel targets. It is generally accepted that less bullet stability will enhance terminal ballistics. Therefore, the increased twist of the M16A2, one twist in seven inches, should be tested against a one in nine twist barrel, which would probably produce better terminal ballistics against personnel targets.
A very important consideration is that reducing twist to 1:9 will probably improve accuracy at all ranges, particularly at 25 meters and in the primary range band out to 300 meters. A general rule is that minimum twist should be used to stablilize the round. Any additional twist will increase variability, causing the bullet to move in a corkscrew-type pattern at closer ranges. With the Army dependence on 25-meter ranges, this factor is much more important to the Army than the Marine Corps. An example of appropriate twist may be seen in the Army Marksmanship Unit rifles which have a 1:10 twist for firing at ranges up to 1000 yards. When the same type rifle and ammunition is used for 300-meter competition, the twist is reduced from 1:10 to 1:14 to obtain greater accuracy. From reviewing available firing data and giving consideration to terminal ballistics, employment ranges, barrel fouling, accuracy, and compatibility with M193 ammunition, a 1:9 twist appears optimum for the Army rifle."
It is almost 30 y/o data, and I'd be interested in knowing if any of it has been superseded or disproved by scientific testing.
"While any alternative must be tested, previous firing tests have confirmed that a 1:9 twist will provide for stability of a bullet similar to the new ball round. Reducing barrel twist to 1:9 will result in less stress on the bullet, barrel life will be improved, and barrel fouling will be reduced. While this twist may not fully stabilize the tracer round to maximum range, the rifle tracer is normally used as a marking round at extended ranges and precise accuracy is not required.
The M16A1 has one twist (rifling) for each 12 inches of barrel length and it has very effective terminal ballistics against personnel targets. It is generally accepted that less bullet stability will enhance terminal ballistics. Therefore, the increased twist of the M16A2, one twist in seven inches, should be tested against a one in nine twist barrel, which would probably produce better terminal ballistics against personnel targets.
A very important consideration is that reducing twist to 1:9 will probably improve accuracy at all ranges, particularly at 25 meters and in the primary range band out to 300 meters. A general rule is that minimum twist should be used to stablilize the round. Any additional twist will increase variability, causing the bullet to move in a corkscrew-type pattern at closer ranges. With the Army dependence on 25-meter ranges, this factor is much more important to the Army than the Marine Corps. An example of appropriate twist may be seen in the Army Marksmanship Unit rifles which have a 1:10 twist for firing at ranges up to 1000 yards. When the same type rifle and ammunition is used for 300-meter competition, the twist is reduced from 1:10 to 1:14 to obtain greater accuracy. From reviewing available firing data and giving consideration to terminal ballistics, employment ranges, barrel fouling, accuracy, and compatibility with M193 ammunition, a 1:9 twist appears optimum for the Army rifle."
Chris Taylor
Random Factor of the K Power
Heavier bullets shoot better in fast twist guns than lighter bullets do. That is something I've seen with my own eyes enough to be pretty positive about saying. As for the second half of that point, I admitted that it was a WAG, and haven't done any testing on it. If I'm not sure of something, I say so. Granted, "over-stabilized" may be a poor word choice, but "sub-optimal twist to achieve the desired gyroscopic stability for maximum performance" is a little long.
I base that WAG on occasions where a gun will shoot better at longer ranges, 1moa at 100 and 3/4moa at 200 as one example I've witnessed. Due to slight misalignments that can happen between a bullet and the bore, it takes a certain amount of time for a bullet to achieve rotational stability (the dreaded "bullet going to 'sleep'" phenomenon to use another questionable theory ). If a bullet is spun faster than it optimally needs to achieve that stability, it takes longer to stabilize. It can happen especially with lighter (shorter) bullets in guns set up for heavier pills (longer throats or more freebore), since the extra length of the throat allows more chance for the short bullet to get a little sideways. This one reason why match ammo is typically loaded just off the lands, to decrease the potential for misalignment.
Are there other factors that may contribute to, or cause, weird shit like that to happen? Absolutely. Hopefully I'll get a chance to do some legitimate testing in the future to get some hard data on it.
I may be wrong, like I said, it's a WAG, but there is some logic behind the guess, not voodoo.
As for the other, meh, shit happens, and is outside the purview of this discussion. I threw it out strictly as one possible rationale.
I base that WAG on occasions where a gun will shoot better at longer ranges, 1moa at 100 and 3/4moa at 200 as one example I've witnessed. Due to slight misalignments that can happen between a bullet and the bore, it takes a certain amount of time for a bullet to achieve rotational stability (the dreaded "bullet going to 'sleep'" phenomenon to use another questionable theory
). If a bullet is spun faster than it optimally needs to achieve that stability, it takes longer to stabilize. It can happen especially with lighter (shorter) bullets in guns set up for heavier pills (longer throats or more freebore), since the extra length of the throat allows more chance for the short bullet to get a little sideways. This one reason why match ammo is typically loaded just off the lands, to decrease the potential for misalignment.Are there other factors that may contribute to, or cause, weird shit like that to happen? Absolutely. Hopefully I'll get a chance to do some legitimate testing in the future to get some hard data on it.
I may be wrong, like I said, it's a WAG, but there is some logic behind the guess, not voodoo.
As for the other, meh, shit happens, and is outside the purview of this discussion. I threw it out strictly as one possible rationale.
Chris Taylor
Random Factor of the K Power
Bob posted as I was typing, and apparently Big Army saw a similar phenomonon back in '86. Maybe there's something to it after all?
This main focus is a hard use combat carbine- with that in mind, what bullet weights would be used? I would guess at least a 62gr
Chris Taylor
Random Factor of the K Power
@Riafdnal , with the popularity of cheap 55gr ammo out there, probably need to go that low. 62 and up would be preferable. Next up we can do an ammo chart.
@TerminalEffect , not really arguing dude. I tend to twist the shit out of my guns, since I tend to believe in mo'weight is mo'bettah, and always twist a bbl for the heaviest pill that someone may shoot. However, I have anecdotal evidence, since nothing (yet) has been subjected to any sort of testing protocols, that lighter weight bullets don't shoot as well in fast twist guns, with some statements from Shilen and articles from from BR pubs to backstop my belief. Granted, the differences have been minor, but they've been there.
I have never seen any hard data either way. If you have, or can point me to, data that disproves my beliefs, please send it to me, or post it in another thread so we quit derailing Matt's topic. I will gladly change my tune, since I want to have and pass on *correct* info.
As I said, I may be wrong. It has been known to happen occasionally. If I am, I want someone to correct me. But I want data and proof .
I have a buddy at the AMU looking to see if they have any hard data one way or the other. Hopefully they do. If so, I'll post it in another thread.
@TerminalEffect , not really arguing dude. I tend to twist the shit out of my guns, since I tend to believe in mo'weight is mo'bettah, and always twist a bbl for the heaviest pill that someone may shoot. However, I have anecdotal evidence, since nothing (yet) has been subjected to any sort of testing protocols, that lighter weight bullets don't shoot as well in fast twist guns, with some statements from Shilen and articles from from BR pubs to backstop my belief. Granted, the differences have been minor, but they've been there.
I have never seen any hard data either way. If you have, or can point me to, data that disproves my beliefs, please send it to me, or post it in another thread so we quit derailing Matt's topic. I will gladly change my tune, since I want to have and pass on *correct* info.
As I said, I may be wrong. It has been known to happen occasionally.
If I am, I want someone to correct me. But I want data and proof .I have a buddy at the AMU looking to see if they have any hard data one way or the other. Hopefully they do. If so, I'll post it in another thread.
Chris Taylor
Random Factor of the K Power
Zero arguments on the ammo, and I would appreciate any info you can pass along bud.
rob_s
Member
While I am not a ballistician, I was under the impression that the issue was projectile length,and not weight per se. Yes, given a restricted diameter by bore size and an increase in weight, the projectile can only grow in length, But there are also changes in material density to take into account.
Personally, I don't think it all matters much outside super-precision shooting. I bet most shooters/optics/barrels/ammo/triggers can't hold tighter than the deviation across 1:7 and 1:8 But, I think I've arrived at the same "logic" as much of the industry which is that if 1:9 is bad because it won't stabilize >70 grain loads, and 1:7 is bad because it won't stabilize <55 grain loads, then 1:8 must be the solution. I'm not sure that science backs that up, but it *sounds* reasonable. However, I'm not convinced this isn't an "emperor's new clothes" scenario where nobody wants to be wrong, even though everyone might actually be wrong.
My solution was always to shoot 62 grain Brown Bear, but that's probably heretical too.
Personally, I don't think it all matters much outside super-precision shooting. I bet most shooters/optics/barrels/ammo/triggers can't hold tighter than the deviation across 1:7 and 1:8 But, I think I've arrived at the same "logic" as much of the industry which is that if 1:9 is bad because it won't stabilize >70 grain loads, and 1:7 is bad because it won't stabilize <55 grain loads, then 1:8 must be the solution. I'm not sure that science backs that up, but it *sounds* reasonable. However, I'm not convinced this isn't an "emperor's new clothes" scenario where nobody wants to be wrong, even though everyone might actually be wrong.
My solution was always to shoot 62 grain Brown Bear, but that's probably heretical too.
Chris Taylor
Random Factor of the K Power
Chris Taylor
Random Factor of the K Power
Ok... I think I see where the disconnect, if there was one, is.
1- seems like you, and Mr Hodnett, are talking specifically about high end projos, where I was talking in general, with regular old OTS boolits.
2- I wasn't disagreeing that we should twist the hell outta our bbls, I was giving a possible rationale why people would go with a slower twist. Like I said, I twist the shit outta bbls, but I've done mostly hunting guns, where tolerances can be, and usually are, a little looser than out on the ragged edge.
1- seems like you, and Mr Hodnett, are talking specifically about high end projos, where I was talking in general, with regular old OTS boolits.
2- I wasn't disagreeing that we should twist the hell outta our bbls, I was giving a possible rationale why people would go with a slower twist. Like I said, I twist the shit outta bbls, but I've done mostly hunting guns, where tolerances can be, and usually are, a little looser than out on the ragged edge.
TerminalEffect is right on--in most cases you don't loose anything going with a faster twist, but may gain quite a bit both in external and terminal ballistics.
Back when they brought out the M16A2 with 1/7 twist in the late 1980's, there was still lots of M193 in inventory which we routinely shot out to 500 on quals without problems.
The only issue with a fast twist can be with thin jacketed bullets which cannot tolerate the faster rotational velocity--we often saw such bullets (ex. 40 gr Blitz) break apart in flight and fail to hit targets, even at close range.
Back when they brought out the M16A2 with 1/7 twist in the late 1980's, there was still lots of M193 in inventory which we routinely shot out to 500 on quals without problems.
The only issue with a fast twist can be with thin jacketed bullets which cannot tolerate the faster rotational velocity--we often saw such bullets (ex. 40 gr Blitz) break apart in flight and fail to hit targets, even at close range.