Newbie AR Q&A

[Gunner3456]

Well I guess what I was trying to say is, 55gr groups acceptably. If you attach the rifle to a bench rest it should be ~1.5-2moa with a 1:7 barrel and 55gr bullets. Anything less will be very overstabilized though. I just wanted to show with my target that you aren't going to overstabilize the 55gr much if at all. You're right that 1:9 isn't silly but if you try to shoot 75gr+ stuff in it they will keyhole. The coolest home defense and target ammunition is 75gr and 77gr so that's why I chose it. If I recall, all 5.56x45 bullets use fragmentation as the main wounding mechanism, including the high grain boat tail hollowpoints. There are some expanding .223 JHP and JSP though.

That's the point. Fragmentation. It's too bad you're getting your info from a blogger. A 55 or 62 gr bullet which is designed to fragment (that's all military and most civilian fmj) is in real danger of fragmenting in the barrel or just after leaving the barrel which has a 1:7 twist! Link to Guntests

"The Crucial Twist Rate Choice"

"In these various applications, rifling twist rates will vary a great deal. Most hunting rifles will have a 1-in-12 (or 1-in-14 in some early rifles) twist, while competition rifles are often as fast as a 1-in-7 twist. Some, such as the Colt HBAR Sporter, split the difference with a 1-in-9 twist. The twist rates are a prime consideration in reloading the round. A twist of 1-in-14 will not stabilize a 55-grain bullet very well, and at least a 1-in-12 rate is needed for bullets of this weight. The heavy bullets used for long-range target shooting, such as the 80-grain Sierra projectile, will require a 1-in-7 twist to stabilize. Any bullet of more than 60 grains will likely shoot better with at least a 1-in-9 twist.

Conversely, the 1-7 twist is too fast for most varmint-weight bullets and those designed for fragmentation on contact with the target. When fired at full velocity, the Speer TNT, Hornady SX or V-Max, or the Sierra Blitz will tear apart in flight from the centrifugal force of the too-fast rotational velocity. I have fired Hornady 40-grain V-Max factory loads in a Colt HBAR with a 1-in-9 twist with excellent results. This twist should provide a good compromise for shooters looking to use the long 60- to 70-grain bullets while still providing excellent results with bullets as light as 40 grains."

That's a very interesting thing about the bullet length, I haven't actually heard people talk about that before. Usually when people describe what type of ammunition works well they only talk about bullet weights and give a range like "55gr-80gr." When I think about it, length makes a lot of sense though. Also, is there any 55gr steel core or is that stuff all 62gr m855?

Bullet length is the only thing.

I don't know anyone who fires a lot of .223/.556 in an AR-15 who doesn't wind up shooting a lot of 55 gr rounds simply because of price and availability. They are also accurate and fragment on an animal ('water born') target.

They can also fly apart in a 1:7 twist causing danger to you and the gun on the next shot. The longer heavier bullets are also made with a thicker copper jacket.

I've said it. I hope you don't plan on shooting common lightweight bullets much, and have access to plenty of longer heavy bullets for the good (lifespan) of your barrel and yourself.

 

[cameronhu]

I think it all boils down to what kind of ammo you want to shoot regularly, and how accurate you want to be with it.

 

[Gunner3456]

I think it all boils down to what kind of ammo you want to shoot regularly, and how accurate you want to be with it.

+1

It also depends on whether you want the "terminal ballistics" bullet performance to be reliable on contact.

I never buy cheap brand or no-brand bullets because the thickness of the jacket is critical. Longer, heavier bullets need a heavier jacket to withstand the sudden and abrupt rotation of a 1:7 barrel. They are still heavy enough to fragment on impact. Lighter shorter (55 gr ?) bullets can use a thinner jacket in the barrel, and will still fragment on impact with the thinner jacket. They might tear apart in too fast a twist barrel though.

I search gunbroker for bullets, and often find myself buying from the bullet farm. They seem to come up with brand name bullets regularly, and the price is often right depending on how the auction goes. Here's one: Link to auction

I also shoot 55 gr because it's enough to do SHTF business, it's cheap to practice with, and I don't like re-centering my rifle just because I changed bullet weights.

 

[spengo]

I've never had 55gr stuff tear apart on me. I might see issues if I try super lightweight varminting rounds though.

The only problem I've had with the 1:7 twist rate is with those .22lr conversion kits. Little 36-40gr .22lr goes nuts. The difference in bore size PLUS the horribly fast twist rate makes the grouping look like it came from a shotgun rather than a rifle. They never fragmented apart in the air (not going fast enough with .22lr pressures I guess) but they certainly did not fly straight either. It was like a 5" group at 15yds. Now I *know* I don't shoot that badly...

 

[Gunner3456]

I'm standing my ground.

The .223/556 is optimized for 55 - 62 gr bullets. Those bullets do best in 1:9 - 1:12 twist barrels.

If one is truly planning to shoot heavier bullets and that's his intention when buying the gun, he should get a larger caliber (which is readily available in AR-15.)

Our military fights today in the mountains of Afghanistan with 55 and 62 gr fmj .556 bullets. Who knows better than they?

I'm outta here. I've done all the damage I can do to one thread.

 

[the4thshake]

There is alot of misinformation going on here.

A 1-9 twist may understabilize bullets over 70 grains causing a loss of accuracy. Or it may do just fine.

A 1-7 twist may overstabilize really light bullets. The lighter they are, and the faster they go, the more the chances of this happening. Overstabilization causes the bullet to literaly spin apart. It blows up mid air. If light bullets are making their way to the target in one piece using a 1-7 twist they aren't overstabilized. Overstabilization is less of an issue with the shorter barrels used in many AR rifles as the velocity of the bullet is less. Saying lighter bullets are inaccurate in a 1-7 twist just isn't true. A certain gun may not like a certain bullet but overstabilization is not the problem. I have personaly fired a large quantity of 55 grain bullets out of various 1-7 twist ARs. Not a single one blew up mid air. These guns group well with the 55 grain as well.

Most of the heavier bullets available are of an open tip "match" design. Their jackets are thinner then the ones used in full metal jackets. The thinner jackets are more likely to fragment at a lower velocity. The heavier bullets retain more of their energy down range. A lighter bullet will loose energy much quicker and thus the ability to fragment.

The special forces use the Mk 262 77 grain bullet due to its superior terminal ballistics. Our millitary hasn't used a 55 grain bullet in quite some time. They have never used a 1-9 twist AR.

 

[Gunner3456]

There is alot of misinformation going on here.

A 1-9 twist may understabilize bullets over 70 grains causing a loss of accuracy. Or it may do just fine.

-snip-

Well, I said I was outta here but...

Bullet stabilization is a matter of rpm. The longer the projectile, the greater the rpms needed.

Now: The faster the bullet in fps, the greater the rpm with any given twist rate. Think of it theoretically this way. If a bullet comes out of a barrel at a measly 1 fps and the twist rate is 1:12, the bullet will be spinning once per foot of travel and once per second. The rpm is 60.

However, if the bullet comes out of the same barrel at 2 fps, it will be spinning twice per second, or 2 rpm. And so it goes up to real world speeds.

If you shoot a light, short, fast bullet out of a fast twist barrel, it's rpm will be much higher than a longer heavier slower bullet which was designed for that barrel.

Now we are spinning the short bullet much faster than we are spinning the long bullet when the converse is supposed to be ideal. Short bullets need to spin slower to maintain stability. We are defeating our purpose.

The ideal rpm speed for the .223 bullet is the speed at which it just barely stabilizes, but does indeed stabilize. This allows it to easily tumble on impact and do the most tissue damage.

When Eugene Stoner invented the AR-15, this was his plan, and this is why he correctly believed that such a small caliber would be fatal even in FMJ in upper body hits. It is all of that when fired from the correct barrel. He believed in the 1:14 twist after tests, and the 1:12 twist AR-15/M-16 came to be only after it was found that 1:14 wasn't enough in sub-freezing temperatures.

Light, fast, short bullets fired in 1:7 twist barrels are inefficient and do indeed occasionally disintegrate.

Gone again.

 

[whphel]

I thought those were 1" squares on that target not 1/2" so not too bad concidering the ammo being used.