"Stability" of 80 gr. bullet with 1:9 twist

[David Sinko]

I recently bought a box of 80 gr. A-Max to shoot out of my DPMS carbine which has a 1:9 twist. By all accounts this isn't supposed to work very well, as the twist is too slow. To get the rounds to cycle through the magazine I cut off the plastic tips of the bullets and loaded them so they barely fit. I used four different powders and the best results were clearly with Varget. I got a 4" group that shot right where the sights were looking (for all intents and purposes about 1" lower than my 55 gr. load) and every load produced nice round holes. One group with a different powder was about 12", but even that gave me nice round holes. I know that 4" groups are nothing to brag about, but I was expecting the holes to be out of round or even sideways. So, just how "stable" are these bullets? Obviously they are hitting point first, but what happens after that? What happens when they hit meat? Will they tumble immediately on impact? Or do they still penetrate in a straight line, the only result being that they are not spinning fast enough to achieve tight grouping?

On the other hand, I shot some 35 gr. bullets out of a 1:7 twist and got excellent accuracy. I didn't expect that either. Just how "stable" were these bullets? When referring to "stability" and rifling twist, do we mean in terms of accuracy or terminal ballistics?

Dave Sinko

[MustangGreg66]

Disclaimer: I'm by no means an expert and just passing along what I've read or come to understand.

I'm pretty sure that when they talk about stability they're talking about in-flight stability, which leads to accuracy. If a bullet isn't spun fast enough, it will wobble and yaw and produce the more irregular or keyhole shapes on target. Usually with a faster 1:9 or 1:7 twist you can still shoot the lighter bullets, you'll just end up spinning them really really fast which can cause some thin jacketed bullets to come apart in flight. Imagine firing a round off and seeing no impact at all downrange.

I'd like to ask what range you're getting your groups at. I just cringe when you mention cutting the tips off your A-max bullets. It's that pointed tip that gives the bullet it's great BC. I can't imagine what that does to the BC of the bullet. I understand that most .223 bullets will tumble on impact, but that may not be so with the larger, longer, heavier bullets. I don't know.

I have heard that specific barrels will stabilize certain weight bullets better or worse. For instance I've read about people having trouble stabilizing 69gr bullets in 1:9 twist barrels and others having no trouble. Could be other factors at work too, but I've read it on here. Your particular barrel may not mind the 80gr A-Max bullets so much.

It'd be interesting if you loaded some with the tips still on (single load of course) and see if they produce a tighter group yet.

[David Sinko]

All my shooting so far is at only 100 yards. I cut the tips off the bullets out of necessity so that they'd fit into the magazine. I don't do any long range target shooting so the idea of single loading extremely long cartridges has no appeal to me. Actually, I did load one bullet with the tip intact and seated deep enough to fit the magazine. That was the only bullet that made a hole that appeared to be slightly oblong. I realize this effort may have little value other than letting me know that my rifle can shoot an 80 gr. bullet in a pinch with fair short range accuracy if nothing else is available.

Dave Sinko

[gm iprod]

Cutting the tips off the bullets makes them shorter (obviously) shorter bullets can be stabilized in slower twist rates. Be aware you have also reduced the weight of the bullet a little and reduced it's BC significantly. I suspect that cutting the bullet down will reduce the BC to below that of an unmodified 75gr or 77gr projectile.

If you want heavy bullets for the AR then look at the Sierra 77gr or Hornady 75gr, BC is down on an unmodified 80gr Balistic Tip type projectile which wont fit, both of which were specifically designed to be run at the OAL required to fit in a magazine long Boat Tail / Short Ogive. Check you twist to see if they will go well or not.

I have a Remington SPS 223 with a 1:9 twist 20" barrel. It does not shoot well with any 80gr at any range. The 75gr Hornady and the 77gr Sierra are also crap accuracy wise in my rifle. Now the 1:9 was developed as a good alround twist to suit 55gr to 69gr. It sometimes in certain rifles if you can generate enough velocity shoot the 77gr very well. My rifle just does not seem to want to do that. I also have access to a Savage LE2 26" Tactical Rifle and it produces 3025fps with 25gr Varget with the 77gr Sierra. That works very well to the 300M we have tried it to. Great on goats too! But with the 69gr MK and the 65gr GK that both those rifles just sing. The ammo that goes through my Remington also has to go through my bosses Bushmaster. Both are 1:9 so I load to magazine OAL.

Consult the maker to see of the bullet you wish to use is suitable for your twist.

https://www.sierrabullets.com/index.cfm?section=store&page=item&stock_num=1477

[Graham Smith]

I cut off the plastic tips of the bullets and loaded them so they barely fit.

I'm not an expert and I don't play one on TV, but doing that seems a lot like buying a Maine lobster and throwing away the claws - it just makes no sense.

If you have a 1:9 twist, you need to be using a lighter round. If you are only shooting to 100yds then a 55gr is fine. If you are going to be routinely shooting at 200 and beyond, you might want to consider Sierra 69gr Match King. That's pretty much the heaviest recommended for that twist rate.

[AustinT]

The only way to shoot 80 gr Amax out of an AR is to single load them. Anything else is a waste of time, money, and good equipment.

Furthermore, with a 4" group they must be on the bare edge of stabilization. I bet if you moved back to 200 yards you wouldn't get round holes anymore.

[RufDog]

55s treat me fine, even past 400