AR-15, twist and what ammo?

[RW2111]

Right I understand that, I was just checking the ammo oracle, because that is where the most comprehensive data on this topic is compiled on the Internet, that I know of, and I can't remember or find anything about length of barrel being a variable when talking bullet stability . I know very light rounds spun very fast at high velocities can come apart, but other than that not sure what length of barrel has to do with it.

Ok I understand your question now. No barrel length does not effect. A 1:9 10inch will have the same Results as a 1:9 20inch. Now the amount of un-burnt powder will be significant in these two. But that is getting into velocity not stability. I'm no expert either so anyone feel free to correct me.

 

[Forest]

A 1:9 10inch will have the same Results as a 1:9 20inch..

That is not true.

Stability of a bullet depends on how fast it's spinning.

Go faster, even with the same twist rate and the bullet will spin faster.

Both will spin 1 turn every 9"; but the longer barrel (faster MV) will allow it's bullet to have a higher RPM (because it's moving through those 9" segments at a faster pace).

It's why some 1:9 SBRs can't stabilize Hornady 75gr OTMs, while just about every 20" 1:9 I've shot can. Specifically look at Greenhill's forumula on bullet stability you'll see velocity is a factor.

 

[RW2111]

It's all rellavent

 

[annihilation-time]

Right I understand that, I was just checking the ammo oracle, because that is where the most comprehensive data on this topic is compiled on the Internet, that I know of, and I can't remember or find anything about length of barrel being a variable when talking bullet stability . I know very light rounds spun very fast at high velocities can come apart, but other than that not sure what length of barrel has to do with it.

It's not talked about often enough, but it is certainly worth discussing.

Here's a thread on the Spike's Tactical forum with the manufacturer breaking things down in a very general manner.

http://www.spikestactical.com/new/z/forum/viewtopic.php?f=8&t=646

 

[annihilation-time]

Both will spin 1 turn every 9"; but the longer barrel (faster MV) will allow it's bullet to have a higher RPM (because it's moving through those 9" segments at a faster pace).

It's why some 1:9 SBRs can't stabilize Hornady 75gr OTMs, while just about every 20" 1:9 I've shot can. Specifically look at Greenhill's forumula on bullet stability you'll see velocity is a factor.

Great post.

 

[TxAggie]

Right now, whatever you can find.

+1

 

[hitman1415]

It's not talked about often enough, but it is certainly worth discussing.

Here's a thread on the Spike's Tactical forum with the manufacturer breaking things down in a very general manner.

http://www.spikestactical.com/new/z/forum/viewtopic.php?f=8&t=646

Ok, I'll look at it. I understand the longer barrels will create higher velocities and it makes sense... just honestly never seen barrel length mentioned in stabalization conversations.

 

[Jerry M]

Interesting chart, but my 1-9 shoots 77 gr SMK better than 75 OTM at 600 yards. I also made hits on prairie dogs out past 500 yards with 53 gr SMK in South Dakota last June.

Bullet quality is very important.

Good luck

Jerry

 

[BradMacc82]

Past 3 AR's in 5.56 (all 1:9 twist) have safely (and surprisingly accurately) handled weight's from varmint grenades to 75gr, one did well with 77gr - other 2 were so-so.

Another factor to consider is not all 1:9 or 1:7 are cut the same, there are times where they can be closer to 1:8 in reality.

Just get whatever you can find, and if you have a variety to try, shoot them and see what works best for YOUR rifle.

 

[Turbo2Point4]

weight suggestion for 1:7?

 

[Bolts Rock]

weight suggestion for 1:7?

Optimum is 75-90. That said my 7 twist WOP shoots 52 A-Max just fine, 55 FMJBT, 69 SMK but it really likes 75 BTHP match and 80 A-Max.

 

[Forest]

Interesting chart, but my 1-9 shoots 77 gr SMK better than 75 OTM at 600 yards. I also made hits on prairie dogs out past 500 yards with 53 gr SMK in South Dakota last June.

Bullet quality is very important.
Good luck

Jerry

That part it red is critical. The length difference between a 75 & 77gr isn't enough to worry about (indeed I've shot 77gr that were shorter than the Hornady 75gr OTMs).

The 77gr SMK generally will outshoot the 75gr OTM. The 75gr outperforms the lighter bullets in accuracy, but for many of us the important thing is it outperforms the lighter bullets AND the 77gr SMK when it comes to terminal performance.

 

[Forest]

weight suggestion for 1:7?

Just about anything you want. They shoot 55gr very well, so if you want to save money just buy some 55gr FMJ.

But when you want to shoot the match bullets - it's still good-to-go!

 

[stu929]

That part it red is critical. The length difference between a 75 & 77gr isn't enough to worry about (indeed I've shot 77gr that were shorter than the Hornady 75gr OTMs).

The 77gr SMK generally will outshoot the 75gr OTM. The 75gr outperforms the lighter bullets in accuracy, but for many of us the important thing is it outperforms the lighter bullets AND the 77gr SMK when it comes to terminal performance.

While this is true for most they wont shoot in everyone's rifle

 

[Forest]

While this is true for most they wont shoot in everyone's rifle

This is correct. Not all 1:9s are created equal.

Which is why I prefer 1:7 twist barrels for my 5.56 ARs.

 

[Forest]

Ok, I'll look at it. I understand the longer barrels will create higher velocities and it makes sense... just honestly never seen barrel length mentioned in stabalization conversations.

Because most people use "rule of thumb" like the Miller equation (which assumes a velocity). If you're into the physics use the Greenhill equation as it makes no velocity assumptions.

That chart posted earlier? Those yellow ranges get shifted a bit based on bullet RPM (which relates. Directly to twist and bullet velocity) .

 

[SigMatt]

Ok I understand your question now. No barrel length does not effect. A 1:9 10inch will have the same Results as a 1:9 20inch. Now the amount of un-burnt powder will be significant in these two. But that is getting into velocity not stability. I'm no expert either so anyone feel free to correct me.

Incorrect. Barrel length affects velocity. If the rounds come out too slowly, they can't spin up fast enough and stabilize. It doesn't have as much impact as an alteration in twist rate but it does matter.

Bullets need minimum rpms to stabilize. Spin too fast and they tear apart. Too slow and they tumble or fall off. Fast twists are intended for heavier rounds. That's what the chart at the start of the thread shows.

Formula for rpm: MV in feet x 720 / twist rate in inches

I had a fellow shooting 55gr rounds out of a 1:7 twist 20 inch barrel at a 100 yard target and seeing no holes or keyholing. The rounds were literally spinning themselves to pieces on the way downrange. Moved up to 75gr and he was getting round holes. It was recommended he move back to a 1:9 twist for service rifle or a 1:8 if he was shooting high power on heavier rounds. He was pushing velocity too but the formula showed backing the loads off wouldn't significantly affect his issue. A 1 inch slower twist has far more impact than a decrease of 100fps in velocity. His numbers showed he was spinning up 55gr rounds to 324,000 rpm. Seems the little rounds didn't like that too much.

Short version: Heavier rounds are less likely to spin themselves apart. If you want more accuracy, go heavier, up the velocity and twist rate. Just understand a 1:7 rifle tuned for 80gr or 90gr rounds will likely rip a 40gr or 55gr to pieces.

Matt

 

[Forest]

I had a fellow shooting 55gr rounds out of a 1:7 twist 20 inch barrel at a 100 yard target and seeing no holes or keyholing. The rounds were literally spinning themselves to pieces on the way downrange.

That's a problem when you use bullets with too thin a jacket and your rifling is sharp enough to crease the jacket to the lead. Its unusual enough a phenomena when shooting 40gr thin skinned varmint loads - but 55gr pills?

 

[SigMatt]

That's a problem when you use bullets with too thin a jacket and your rifling is sharp enough to crease the jacket to the lead. Its unusual enough a phenomena when shooting 40gr thin skinned varmint loads - but 55gr pills?

He was pushing them fast too. Brand matters and the make of the bullet is what was suspected to cause the disintegration. 62gr was holding up better but were showing some instability in staying together as well. These were handloads at maximum charges. But 1:7 is a fast twist in general and it was affecting his shooting. A slower twist worked better. I wouldn't be shooting 1:7 on light rounds. At least 62gr or heavier. I'm looking at a possible competition rifle on a 1:7 but firing 80gr and 90gr rounds.

Matt

 

[hitman1415]

We have shot hundreds of thousands, if not millions of 55gr m or xm193 out of our 1/7 Colts at our agency and have never had one come apart. Those must of been some very poorly constructed bullets.