Something that has always fascinated me about shooting the Modern Sporting Rifle (aka “AR15”) is the complex relationship between a barrel’s rate of twist and the effect it has on bullets of varying weights.
I went through my ammo cans and selected four different weights of ammo for today’s test. Actually, I found that I had six weights but decided to skip the tracer and the super-lightweight varmint tip stuff. The four rounds you see here are (left to right): 50 grain ballistic tip .223; 55 grain full metal jacket 5.56; 62 grain light armor piercing 5.56 (aka “LAP” or “Green Tip”); and 77 grain open tip match 5.56. Manufacturers are American Eagle, IWI, Federal, and Black Hills respectively. The prices on AR ammo generally tend to go up with bullet weight: from around 40 cents a round for the 50-55gr stuff up to about 95 cents a round for the 77gr stuff.
My rifles are very similar: both have 16″ barrels, cheap red-dot optics, and an average amount of junk bolted to them. The one on top has a Palmetto State Armory Freedom upper with a 1:7 barrel twist. That means that the bullet makes one complete revolution in 7 inches of travel as it goes down the barrel. A fast twist like this supposedly favors heavier bullets.
The rifle on the bottom has a DPMS Panther upper with a 1:9 twist rate. Slower twist rates can be more versatile and shoot a wider range of bullet weights more consistently. I will confess that this was my first AR, it has had quite a few more rounds through it, and I am more comfortable shooting it.
The weather today was clear, in the mid-40s, with a 5-10 mph breeze coming from behind the shooting position. I shot seated using a Caldwell bag resting on the railing of my deck. Targets were orange adhesive dots on cardboard stapled to a 3’x3′ sheet of plywood that was screwed to a tree about 75 yards away and on a level with the shooting position. I did not use the Harris bipod on the PSA or the iron sights on either rifle. Three shots were taken in each bullet weight per target and then the cardboard was swapped out then the same 12 shots were taken with the other rifle. Both rifles were fouled and warmed up for the test with six or seven shots of 55gr. There were zero malfunctions or flyers today and I used a Barska spotting scope to check each three-shot string as I went.
The PSA upper AR (1:7 twist) went first:
My warmup shots went in the center with the first three high and left. I compensated for the grossly out-of-zero red-dot and got on the sticker with my next three warmups. Then I shot my way around the smaller targets. I had loaded magazines with 12 rounds each in batches of three increasing the weight progressively. Ignoring for a second how poorly I shot this gun today, you can see that the groups didn’t tighten up at all until shooting the heaviest weight bullet. In fact, the 62gr group was the largest at 3″ – double the 1.5″ group of the 77gr. The other two weights shot similarly mediocre 2.25″-2.50″ groups. As luck would have it, my best group was that second warmup string of three in the center of the cardboard at 1.25″.
My DPMS shot noticeably better regardless of bullet weight. That “Big Dipper” in the center was my 7-shot warmup string. The optic on this rifle was obviously holding zero and it has a smaller dot, which hides less of the target as range increases. Interestingly, the groups increased with bullet weight from 1.5″ and 1.5″ to 1.75″ finally opening up to an appalling 2″ with the 77gr Black Hills ammo.
Shooting lighter 50 grain ammo, 1:9 wins…barely.
Shooting 55 grain – which incidentally is the overwhelmingly most common weight shot by civilians and military out of AR-platform rifles around the world – the 1:9 is noticeably better. Also, the group looks nearly identical to the 50gr group shot by the 1:9.
I have bought and shot a lot of 55 grain FMJ ammo but I’d feel a lot better in a defensive situation if my magazines were loaded with 62gr Green Tip. One simple reason: the bullets tend not to self-destruct when they hit something hard or hit something at an extreme angle. Sure, a 62gr bullet only weighs about 13% more than a 55gr one and it’s traveling a bit slower so it will drop more as range increases but it will deliver about 6% more energy on impact. The really valuable thing about Green Tip ammo is that is has a steel penetrator core that will go through things that will make 55gr disintegrate – things like sheet metal, trees, windshields, and cinder block. No, Green Tip is not .308 but it’s the closest you’re going to get with your AR15. That said, I am delighted with the 62gr group my DPMS shot today.
Why would anybody spend nearly a dollar a round on 77gr AR ammo? Well, if you had a 20″ heavy barrel match rifle or a bolt-action with a 24″ barrel, this stuff might be the next best thing to loading your own. In a 16″ carbine-length barrel it’s clearly wasted. Neither of my rifles was laser-accurate with 77gr but you will notice that this group was the tightest shot by the 1:7 twist barrel.
All of this proves…what? That I can shoot pretty much whatever I want through my DPMS and it’ll work? That there might be something seriously wrong with my other AR? That spending more on ammo isn’t always a guarantee of accuracy?
I just realized I could’ve made this test much more fair by simply swapping the two uppers on one lower. I didn’t because I’m lazy. Also, my DPMS lower has a heavier trigger that has a bit of gritty creep, I would’ve bet against it being more accurate in this test. But the fact that I have the option to put either upper on either lower and shoot until I magically turn into Sgt York is another reason why these black guns are so popular.