barrel twist and tumble

[Watchman]

Greetings,

I just read a thread in another forum where the user claims keyhole issues with 55gr 1:7 twist rifle barrel at 50 and 100 yard targets. And not as bad from the same twist in a carbine barrel...

I was about to claim he made back asswards response but I'm no uber expert on the subject. I thought slower twists (not enough spin) caused bullets to tumble. Can a too fast a twist cause the same thing?

Thanks.

[5pins]

Sounds odd to me also.

I have read post where someone claims that 55gr bullets will tumble in a 1/7 barrel but I have never seen it. However if the bullet is tumbling I would be surprised it is hitting the target at 100yds.

[Todd.K]

The only way to get 55gr to tumble in a 1/7 twist is to shoot out the rifling.

[ForTehNguyen]

never had a problem shooting M193 out of my 1:7s

[6933]

Bullets don't tumble by design from an AR. There is a problem with the gun if the bullet tumbles.

[Sundo]

The most credible explanation I've read is that 55 gr bullets spin too fast out of a 1:7 barrel, so the jacket deforms and disturbs the flight of the bullet, leading to reduced accuracy.

The issue is not the weight itself, but the fact that the lighter bullet has a much higher muzzle velocity (3200 fps for 55 gr vs 2700 fps for 77 gr). For the same barrel twist rate, higher muzzle velocity translates directly into a faster bullet spin rate. There is a limit on spin rate where the centrifugal forces will be enough to deform the bullet.

One back-of-the-envelope check on this theory is to estimate the the bullet spin rate for ideally matched bullet weights and rifle twists. From what I've read, 55 gr bullets are best with 1:12, 62 gr best with 1:9, 69 gr best with 1:8, and 77 gr best with 1:7. If you take the differences in muzzle velocities into account and calculate the bullet spin rates for these combinations, I expect they will be fairly close together in spin rates. OTOH, match the velocity of the 55 gr bullet to a 1:7 rifling twist rate and I expect the bullet spin rate will come out much higher.

Anybody care to calculate the numbers?

[Todd.K]

Tumbling and coming apart from twist are totally different.

People who talk about how poorly 55gr bullets shoot in a 1/7" twist are generally interested in justifying their 1/9" twist barrel because if they had a 1/7" they would know that 55gr shoots just fine.

We had to push the 36gr VG hard in a 26" 1/6.5" twist to get bullets to come apart. Maybe 1 out of 5 didn't make it to the target, those that did were about 1 MOA.

[Stickman]

The only way to get 55gr to tumble in a 1/7 twist is to shoot out the rifling.


Or he is shooting through something. I've seen it happen in training where guys have shot through plywood barricades and ended up with keyholes on their targets.

[UVvis]

Could also be a badly damaged crown, really screwed up compensator, or a bad lot of bullets.

[Sundo]

...
We had to push the 36gr VG hard in a 26" 1/6.5" twist to get bullets to come apart. Maybe 1 out of 5 didn't make it to the target, those that did were about 1 MOA.

36 gr VG bullets are premium Barnes bullets that specifically advertise the ability to remain intact at high velocities. Would a run-of-the-mill, low-cost 55 gr bullet (like the ones found in PMC Bronze cartridges) hold up as well as the Barnes VG?

Also, Noveske has a reputation for producing exceptionally accurate AR rifles. I suspect the 1 MOA achieved with a Noveske might open up to 1.5 or 2 MOA on a run-of-the-mill AR.

The ability of 36 gr Barnes bullets to maintain 1 MOA accuracy out of a Noveske rifle might say more about the respective bullet and rifle manufacturers than about the general performance of light bullets in fast twist barrels.

[Cameron]

Last week I shot a clay bird at 200 yards with Federal 50gr HP out of a 1 in 7" twist 16" barrel.

It will easily shoot sub MOA groups with 50gr rounds at 100 yards.

Cameron

[Todd.K]

Most 55gr FMJ will have a thicker jacket than a 36gr varmint bullet. The velocity and hence RPM are much greater on the 36gr bullet as well.

I have shot thousands of 55gr FMJ in a 1/7" twist, never had a problem getting decent accuracy relative to the quality of the ammo.

[AMMOTECH]

I ran a 100+ rounds of 55gr UMC thru a 14.5" 1/7 BCM this morning with targets out to 100m with zero issues. All holes were as round as could be.

.

[akxx]

Sounds really odd--something is likely wrong with the barrel.

I've got 40-grain Nosler BT's and 50-grain Sierra Blitzkings that shoot sub-moa out of of 1:7 barrels ranging from 12.5" to 20".

[Alpha Sierra]

The most credible explanation I've read is that 55 gr bullets spin too fast out of a 1:7 barrel, so the jacket deforms and disturbs the flight of the bullet, leading to reduced accuracy.
That is possible, but if the jacket deforms enough to disturb the stability of the bullet, it will most likely also rupture and the bullet will fly apart in flight.

I have had that happen with a target rifle chambered in 260 Remington with a 1/8 twist (fast for caliber) 30" barrel, shooting handloads with Hornady 95 grain VMAX (thin skinned varmint bullets). Got a white/gray puff of dust about 50 yards downrange and a miss (no hit) on the target.

I have never had a bullet fly apart in any AR I have had. None of my ARs have had a rifling rate slower than 1/8 and none have ever shredded any bullet including the 45 grain WWB varmint loads that I used to use for 200 yard training.

[akxx]

A 55-grain FMJ has too thick a jacket to separate and break up due to rotational forces out of a standard AR-type rifle, regardless of barrel twist.

It is not just barrel twist that factors into bullet breakup in flight:
An extremely thin-walled varmint jacket (think Hornady SPSX) in 1) light weights and 2) high twist (1:7) AND 3) high velocities may break up in flight, which will be evident by the bullet never reaching the target. This is not an uncommon occurrence and can be replicated by anyone meeting the 3 conditions above. Just ask any 22-250 shooter using .224 bullets driven at very high velocities (they can achieve bullet breakup without super fast-twist barrels because they are launching bullets at very high velocities).

Polymer tipped bullets like Vmax, NBT and such are generally made to withstand higher rotational forces since they rely on their tips to initiate rapid fragmentation/expansion, therefore they can be shot out of 1:7 barrels without breaking up.

[5pins]

I had an interesting outing at the range today. I shot some handloads with some 50gr Remington HP bullets and notice some tumbling. The target was about 30 or so yards away. I’m pretty sure it was a bullet issue because the 50gr Nosler BT shot really well.

This was out of a DD hammer forge lightweight barrel with a 1/7 twist.

[goodoleboy]

I've shot 55 gr from my 1:7 twist 16 inch Colt 6920 with no keyholes.

[gcp]

Me too, no keyholes from my 16" 1in7 LMT, shoots the Fiocchis FMJ 55s just fine!