Barrel Twist Voodoo?

[curisu]

Hey there,

I'm running a .223 18" bull barrel with 1:9 twist (Les Baer if that makes a difference to anyone, i.e., match grade).

On a good day, i can dime-hole, 1/2 MOA with Federal Gold Medal Match 69gr. Benchrested, using a bipod in front and bags on the stock.

On a whim, I bought some 77gr FGMM. Last night I ran 10 rounds through with a warm barrel (50ish rounds shot before) and a fairly warm evening (65ish). I got a fairly tight 1.25 MOA group, with 1 flyer.

Curious, as i've read that a 1:9 twist isn't fast enough to stabilize above 69gr. Is 1.25 MOA a result of unstable flight? (I know that i've gotten groups as large as that with the 69gr version, blame resting squarely on my shoulders)

Any thoughts?

Thanks,
chris

[NiteQwill]

69 is probably optimal for 1:9 twist. Of course, your rifle will tell you.

It is much too slow to stabilize 75 gr bullets though.

I shoot 69 SMK and Nosler CC bullets through my 1:9 223.

[SixPointEight]

A 1: 9 will stabilize a 75/77 out to 100 more often than not because muzzle velocity is so high. Past about 250 you'll probably see stability issues.

Also there's a lot of tolerance in twist rates. My 1: 9 is actually 1: 8.7

It stabilizes 75hpbt at 100 just fine. But a 75amax gives me 6" groups and oblong holes

[Spdjunkie]

Twist Rate & stabilization = Bullet Length.

Longer the Bullet = Faster the Twist Rate
* Fact is Majority of heavier bullets are longer than lighter bullets in the same caliber.

.223/5.56: 1:9 = I don't shoot anything past 69 grain SMK. Not to say it wont stabilize anything heavier, it may but usually a lighter bullet will perform better in a slower twist rate barrel.

.223/5.56: 1:7 = Majority of the time 77 grain is the longest/heaviest you can go in a magazine feed semi-auto, AR15 platform. 55-77 work fine, 55-62 being mid range, 69-77 further out.

Selection between 69 & 77:
* Many shooters test 69 & 77 side by side @ 100, which is fine, but testing needs to be done out to 300+ yards to see the benefit between the two for a particular rifle. Some rifles will print/group a 69 grain load well at 100, but at distance will open up compared to a 77 grain load. The 77 grain load may shoot slightly larger at 100, but out perform the 69 grain load at distance, 300-400 yards.

imho: I really like the 69 SMK & 69 Nosler Comp., Great HPBT Match Bullet. Great balance of case capacity to bullet length to achieve good FPS & Accuracy. Never an issue with accuracy out to 600 yards with a 16"/18"/20" AR15.
If I shot out to 700-800+ majority of the time, I'd opt for the 77 SMK. . . . . BUT, I'd probably be shooting a .308Win with a 168/175 then.

fwiw: 75 AMAX is longer than the 77 SMK & near impossible to load at proper fps in a mag feed AR15. Single Load is possible. 75 AMAX in a fast twist .223/.223AI bolt gun is FTW!

[Whiterabbit]

Hey there,

I'm running a .223 18" bull barrel with 1:9 twist (Les Baer if that makes a difference to anyone, i.e., match grade).

On a good day, i can dime-hole, 1/2 MOA with Federal Gold Medal Match 69gr. Benchrested, using a bipod in front and bags on the stock.

On a whim, I bought some 77gr FGMM. Last night I ran 10 rounds through with a warm barrel (50ish rounds shot before) and a fairly warm evening (65ish). I got a fairly tight 1.25 MOA group, with 1 flyer.

Curious, as i've read that a 1:9 twist isn't fast enough to stabilize above 69gr. Is 1.25 MOA a result of unstable flight? (I know that i've gotten groups as large as that with the 69gr version, blame resting squarely on my shoulders)

Any thoughts?

Thanks,
chris

you can stabilize a 130 grain bullet if you can shoot the sucker fast enough. You might not have the case capacity to do it in a 223 .

[Whiterabbit]

Here's some math:

Miller stability for SMK's (aka FGMM) for 69 and 77 grain bullets. Lengths are .9 and .994 inches.

I'm guessing your velocity is 2800 and 2600 based on what CTD says about the ammo and your short barrel.

Miller stability is 1.65 and 1.35, respectively. so it IS less. Is that bad?

Assuming the military says 1.3 to 2.0 is good for stability, you are just fine with both. It might even imply that the 77's will be better as you stretch out (due to I assume a better BC)

-----------------

You can take all that with a grain of salt. I've done miller stability till I was blue in the face and it didnt explain why I was getting oblong holes and huge groups are VERY short distances. Much less long. But in this case, it's telling me there isnt a whole lot of difference between each of those federal loads. Combined with your claim that the holes are round, I'd say you are good to go.

[GeoffLinder]

Never seen a 1:9 that was very good at stabilizing 75gr or heavier. You really need 1:8 or 1:7 twist to do this properly. Even if a 1:9 manages to stabilize the heavies, it's won't do optimum job. Stick with 45-70gr in your 1:9

[curisu]

i appreciate all your feedback. much to think about.

i see mention of oblong holes as indicators of unstable flight; are there others I should be aware of?

attached is the peel'n'stick with 10rds through it. Holes look uniform and round.

[curisu]

Here's some math:

Miller stability for SMK's (aka FGMM) for 69 and 77 grain bullets. Lengths are .9 and .994 inches.

I'm guessing your velocity is 2800 and 2600 based on what CTD says about the ammo and your short barrel.

Miller stability is 1.65 and 1.35, respectively. so it IS less. Is that bad?

Assuming the military says 1.3 to 2.0 is good for stability, you are just fine with both. It might even imply that the 77's will be better as you stretch out (due to I assume a better BC)

-----------------

You can take all that with a grain of salt. I've done miller stability till I was blue in the face and it didnt explain why I was getting oblong holes and huge groups are VERY short distances. Much less long. But in this case, it's telling me there isnt a whole lot of difference between each of those federal loads. Combined with your claim that the holes are round, I'd say you are good to go.

Interesting - thanks for looking that up. I did chrono a magazine of 69gr FGMM. Except for 1 outlier, they were all with 30fps of 2700fps. Pretty consistent, and the load i'm trying to match.

next time i'm up at Richmond Rod & Gun, i'll run a mag of 77gr and see if it matches up with your CTD numbers.

thanks,
chris

[Quinc]

I shot some 77gr SMK's this weekend out of my 16" 1-9 twist by mistake and they had no problem hitting soda cans at 100.

[Whiterabbit]

my CTD numbers are just based on what Cheaper Than Dirt says, with a SWAG adjustment cause I assume they measure that with a 24 inch barrel instead of 18. So basically, I pulled those numbers out of my arse. take them with a grain of salt. I tried to be harsh so they'd err on the side of destabilization if that's the way it was gonna swing.

As for other signs of destabilization, I don't know. I AM TOLD another sign is a trumpet cone of fire. For example, my pistol is 1-2" at 50 yards and 6" at 100y. I was told 100y groups could be due to destabilization from not pushing the bullet hard enough. I'm not sure I believe that. As a bullet slows down, the speed slows faster than the rotation slows, so it should get MORE stable as time goes on. On the other hand, I put more powder in the case and saw the group shrink.

I'm no expert. I just try stuff.

[GeoffLinder]

I shot some 77gr SMK's this weekend out of my 16" 1-9 twist by mistake and they had no problem hitting soda cans at 100.

The 77's are meant for far downrange performance (300 plus). 100 yards is nowhere near enuff distance to evaluate the performance or stabilization level imparted by barrel twist. It is very typical for the 77's to not even fully stabilize out of 1:7 and 1:8 barrels till they are at or past 200 yards.

The main reason the military uses them in some loadings for the M4 is the extra bullet mass (read stopping power and penetration here) they bring to the table in comparison to 55gr pills.