Reply with quote #1
One of my buddies recently put together an AR-15, he's had the lower for a couple years and just recently purchased the upper. They were both new and in unfired condition, DPMS I believe. He's got the 1-9 twist 16" barrel.
He's only been getting like 2 1/2 " groups @ 100 yards with the military surplus ammo like 55 and 60 gr. mostly FMJ I believe. So he bought some Federals with 55 gr. Nosler Bal. Tips. Still only like 1 3/4 - 2" + groups. So I told him to bring it up and I'd try to work up some reloads for him with the heavier 60 + gr. stuff.
I got out my Stoney point cartridge OAL gage and using 2.260 the max length that will fit his mags. We found out he has a jump (freebore) of like .135 using 68 gr. Hornady HPBT's. So I tried some 55 gr. Sierra's I had and it was .150.
I'm a bolt gun guy, but I have to believe that is to much throat isn't it ? Is this normal for semi-auto's ? I usually set my bullets like .025 to .035 off the lands for my bolt guns. But the only way we can get anywhere near that close would be to feed it one round at a time.
Could this be his accuracy problem with factory rounds ? Or is there some other trick to AR's ? The barrel has a flash hider on it, so I'd assume being brand new the crown should be okay.
We took off his scope and fired iron sights @ like 40 yards and the groups were still like 1 1/2 to 2 " with very eratic spreads. One bullseye, one an inch high and left at 10 O'clock, one right and low etc.
Reply with quote #2
That's a fairly standard AR chamber, probably SAAMI spec. Not only is the freebore long, it's probably quite a bit over bullet diameter. For magazine-length loads, bullets that tolerate a large jump such as the 68 gr Hornady, the 75 gr Hornady BTHP, or the 69 gr Sierra are the ticket. You'll probably be able to get better groups with bullets such as the 80 gr Sierra or the 75 gr Hornady A-Max seated close to or into the lands.
DPMS sells magazines that allow 2.43" OAL; they're the size of a standard 20-round magazine, but they only hold 10. The extra length is obtained by using a single stack with the case head in the rib. A modified bolt catch is required, and it doesn't work with standard magazines. The magazines and bolt catch cost considerably less than a new barrel, so it might be an option if he's determined to get small groups from the magazine.
Reply with quote #3
I am very new to the AR game. wanting mine for a short range carry varminter 100 200 yards I have had good luck with the 52 Berger varmint pills seated to mag length. Mine is a RRA and shoots consistent sub 1/2" at 100 yards off the bench.
Reply with quote #4
Thanks fellas, I'll tell him about the single stack mags, and the bullets. We're just feeling our way through.
Reply with quote #5
With a 1-9 twist barrel, you're approaching the limits on bullet weight/length with the 68/69 grs. Some 1-9 may stabilize the 75 gr., others will not,( not all barrels will be the true twist rate they are claimed to be: some faster/slower). The 80 gr. are out of the question for a 9 twist, check the bullet box labels, ( they call for 1-8 or 1-7). Suggestion: seat the bullet out, ( the Sierra 69 hpbt# 1380 is one of my favorites), and single-load.
Reply with quote #6
DD, I agree with Ronemus. You will find better accuracy in a rifle such as RRA as mentioned by brfun. The RRA A-4 varminter have a "Wylde" chamber and a Wilson air gauged barrel which is designed for better accuracy. I too have been able to load at mag. length and obtain sub. 1/2 MOA at 100 yds. with careful brass prep of IMG brass. Bill
Reply with quote #7
Try some 60 gr Hornady TAC ammo. My Bushmaster 1-9 loves this ammo, but would only shoot 1.5" with military 55's.
Reply with quote #8
I like the new barrel idea. Wislon is ok. Krieger, hart, schneider, rock creek, obermeyer, or satern is where all the accuracty comes from. they have tons of different twist selections, contours, rifling profiles, and lengths to fit anyones needs.