AR15 Bullet Impact Movement

[Irate]

I have a Bushmaster AR-15 with a 16" barrel and I'm having some accuracy problems. On the first shot at 100yards, the bullet impacts about one inch to the right of where I'm aiming. On the second shot, the bullet is dead on. On the third shot, the bullet impacts about one inch left. Each shot moves the bullet further left until it stops about 3" left of the bullseye, making a nice group. Each of these shots were fired one after another. Obviously the zero is moving as the barrel gets hotter. The rifle in its current configuration has the military set-up, two piece plastic handguards attached to the barrel. I have a rail attached to the bottom with a bipod on the rail.

My question is, will a simple free float job fix this problem or is this something worse? I think there is unequal pressure on the barrel which is moving the zero, am I correct in this theory? Has anyone come across something similar to this before? I don't want to replace the barrel, but if I have to I will. Thanks in advance.

BTW, I used factory ammo for this test to make sure it wasn't my handloads.

Irate

[EricW]

Sounds like you have a loose barrel. You could try loctiting the barrel in and then cranking the nut down good.

[kurtm]

Quick check; pull the handguard off and bench rest off the magazine ontop of a sand bag. If it doesn't move, you need a free float tube, If it does, check the barrel tightness , after all the handguards are off! KURTM

[Irate]

I just wanted to post a follow up to my little accuracy problem. It was in fact the handguards moving the group around. I free floated the barrel and really tightened the nut and now that thing shoots under an MOA with reloaded 55gr ammo. I used a DPMS carbine free float tube and stock barrel. If anyone else is having the same problem, I definitely recommend free floating the barrel. Thanks for your input.

Irate

[wsimpso1]

The free float tube helps with consistency regardless of how the gun is supported (short of resting on the barrel). The group wander was almost undoubtedly a loose or burred barrel nut. Doing a decent job of installing a float tube fixes both issues. Good Job!

Billski