Sunday, May 15, 2016

Rate of twist, bullet weight, and a confused look on my face....

I've been shooting mostly 55 grain FMJ ammo in my AR's because they shoot it well, and I have me a fair bunch.  On occasion, I've shoot the 62 grain green tip in them, with mediocre accuracy.  Certainly noticeably larger groups than the 55 grain bullets.

But the thought keeps ringing in my brain:  My AR's have 1 in 7 twist rates.  They are SUPPOSED to shoot heavier bullets better...... %$#@ it.

Before I begin ordering heavy bullets by the bin full, I thought I would just give something a shot.  I bought a single lone box of Hornady 75 Grain Super Match ammo.  This stuff costs like gold, and by accounts is one of the best AR match loads on the market.

Want to see what happened?

This rifle has a Spikes heavy barrel upper with a mid-length gas system, a 1/7 twist, and a 3x fixed Nikon optic on it.  I shot it from the bench, on a decent rest but still hand held.  My heart beat was weaving the cross hair a good inch across the bull as I did my breath, breath, squeeze.  The groups were fired three rounds at a time, with cooling time in between.

The target on the left, 55 grain PMC Bronze bulk ammo (The one flier was called).

The target on the right, the 75 grain Hornady $uper Match.

I shall note.... if I had shot the 62 grain M855, the group would have fallen between these two in size.

I'm open to suggestions here.


Grog said...

The stag link is somewhat redundant for info, reading your comments on the groupings, but it's still good info.

So, my thought is your gas system needs to be adjusted.

I'd also suggest trying different ammo, not all name brands have the same accuracy, because of powder burn rate, etc. And the .223 has a different shoulder taper than the 5.56, if the ammo isn't fully seating into the chamber that might have some effect on the groupings.

Long Island Mike said...

Dont expect much from 556 chamber shooting 223 ammo. Go with hybrid Wylde chamber or pure 223. Maybe try 6.5 Grendel ! Check out article in American Rifleman...

Long Island Mike said...

Also is there a free float handguard on that bad boy ?

Anonymous said...

I'm not seeing a major problem here with the right target.
I just went out back and fired 5 rounds at 100 rounds with a 1:7 5.56 SS NATO barrel suppressed and got a 3 MOA Vertical by 2 MOA horizontal centered 2 MOA above target bull. Hornady Match 75gr. .224" BTHP 23.0gr. H335 OAL 2.250" Ave. 2657 FPS. I did not rezero the Nikon P-223 4-12X40mm. @ 10X for this ammo. Bench with bipod only for support and my shoulder.
I don't see a way to post a picture of my target here.
I imported your targets into OnTarget and found 20 holes with a group size 4.020" 3.839 MOA. 3.396" W X 3.679" H. Considering you only had a 3X scope this seems quite acceptable.

Your left target is 9 holes with a group size of 2.901" 2.770 MOA. 2.615" w X 1.013" H.

Assuming 100 yards and a 1" grid for the target.

Anonymous said...

Stupid editing. 100 YARDS in my previous post.

Anonymous said...

I suggest a better, higher power scope.
It's me anonymous again.
I hope you will post this comment.

100 yards in my previous post.
I went out back again with my AR, the suppressed 5.56 and shot my 100 yard zero ammo.
10 rounds of Hornady 55gr. FMJBT w/cannelure, 25.0gr. CFE223 OAL 2.230" Ave. 2776 FPS.
1.447" group, 1.382 MOA, Far Shot from dead center .755".
I have created an imgur account and will upload my target and rifle picture.

Anonymous said...

Speaking as a guy who also happily shoots 55's out of his AR because I have a plentiful stock, I would nod sagely at those who tell you heavier bullets should work moar better in your barrel, tell them you'll try it, and then keep right on shooting the 55's because your rifle seems to work with them jes' fine.
You could get a bunch of the 75gr $upermatch projectiles and start working up loads as the factory ammo you were using doesn't seem matched quite right to your rifle.
But adjusting the gas system? What does that mean, drilling out the gas port? Shudder.

David said...

Hi Teach. I would say that just because your 1:7 barrel will stabilize heavier (longer) bullets doesn't mean they'll be more accurate at short range than lighter bullets. My own 1:7 carbine actually shoots 55 grain Steel Match better than any other factory load I've tried. Over many 5 shot groups fired, group size average is better than 1.5" at 100yds. Yes, I've tried the expensive heavy bullet match loads, and 55 Steel Match is still better. Even tried 75gr Steel Match, and 55 was better. Heck, even my handloads show a preference for lighter bullets when it comes to 100 yard accuracy. 55gr GameKings shoot better than 75gr MatchKings. Whoda thought?