AR15 ammo question...

[blkbrd666]

I ordered some .223 ammo and they sent me the wrong stuff. I ordered 55gr FMJ and I received 75gr HP. My question for all the AR pros out there...would you shoot the 75gr ammo in an AR with a 1/9 twist rate???

 

[Pure Kustom]

I think there is a thread talking about this subject in the rifle section. It was very detailed.

 

[average_shooter]

I believe, though I could be wrong, that the 1-in-9 twist ratio is supposed to adequately stabilize the widest range of bullet weights.

I think as a general rule slower twists for lighter weights, faster for heavier.

 

[cbrgator]

I think as a general rule slower twists for lighter weights, faster for heavier.

That's correct. The heavier the bullet, the faster it needs to spin to be stable.

 

[blkbrd666]

I keep hearing the 75gr might be too heavy for a 1/9 twist, but my rifle does have a 20" barrel which might help. I think??? I'm just debating on whether to send a half case of ammo back to ammoman and get what I ordered, or just keep it. I can't go test it in time to make the decision...and of course, if I bust open the pack then I can't send it back. I was hoping someone had shot 75gr in a 20" Match HBAR and would tell me it shoots perfect at 100yds in most cases.

 

[Neener Neener]

If they charged you the 55 grain price, sell the 75 grain yourself, because you should make some money on it. Then get yourself some 55 grain.

 

[Bartkowski]

If they charged you the 55 grain price, sell the 75 grain yourself, because you should make some money on it. Then get yourself some 55 grain.

Exactly, unless they sent you steel case ammo when you ordered brass. Though you could just shoot the 75gr. and see what happens. It won't hurt the gun in any way, just might not produce very tight groups. Then again it might shoot very accurately.

 

[usmcpmi]

Guys, The bullet weight/twist ratio you stated, I believe is backward.....My AR has a rate of 1/9" for the 55gr. bullet. My 50 BMG at 750 gr has a rate of 1/24". If I followed your twist formula, my 50 BMG should have something like 1/3"... MG

 

[rust collector]

The rate of twist rule relates to a specific bore size. Cross platforms, and there are too many other variable introduced. Heavier bullets generally have better ballistic coefficients and downrange performance, so are currently receiving a lot of attention in the AR arena.

If it were me, I'd want to try em out. After all, engineers have proven that bees can't fly, but fortunately they haven't convinced the bees.

 

[cliffy]

A 1-in-12" Twist will stabilize 60 grainers well, so . . .

a 1-in-9" should spin 75 grain bullets adequately. BUT ordering one thing and receiving another is WRONG! Try MIDSOUTH SHOOTER SUPPLY, because they DON'T send the WRONG thing. cliffy

 

[jrou111]

The obvious answer is to buy another AR-15 with a 1:7

It's for the children.

 

[Rubber_Duck]

Just shoot it.

 

[taliv]

cliffy, mistakes happen

 

[average_shooter]

Guys, The bullet weight/twist ratio you stated, I believe is backward.....My AR has a rate of 1/9" for the 55gr. bullet. My 50 BMG at 750 gr has a rate of 1/24". If I followed your twist formula, my 50 BMG should have something like 1/3"... MG

Wow... talk about apples and oranges. We're talking .223 Remington in an AR. The heaviest bullet being, what...77 grains? Your talking about .50 BMG's with bullets weighing 750 grains, so a factor of ten... of course ballistics are going to be different.

In an AR it's often stated that lighter bullets (like 45 grains) will tend to want to fly apart if spun too quickly, which would happen with a twist rate of something like 1 twist in 7 inches, or 1/7 twist ratio. Lighter bullets tend to prefer slower twist ratios, maybe closer to 1/12.

So, heavier bullets (in .223 Rem at any rate...) prefer faster twist rates to maintain stability over distance. Hence for the heavier bullets, like 75 grains or so, 1/7 is preferred by some, perhaps many. The compromise was 1/9 which seems to work pretty slick with 55 grain bullets and does OK with other weights.

 

[RockyMtnTactical]

75gr OTM

Works fine in all of my 1/9's.

 

[Swampy]

My question for all the AR pros out there...would you shoot the 75gr ammo in an AR with a 1/9 twist rate???

Borderline.....

Many guys report good results... many report instability with bullets that heavy in a 1-9" 20" AR.

Won't know till you try it, but at that point you are stuck with it since you've already opened it.

Best of luck,
Swampy

Garands forever