AAC 300 Blackout upper with HK MR556 lower problems.

[A1SPAM]

Having feeding issues with the above combination. May get 3 to 4 rounds out before having a feed malfunction. I did change the buffer and buffer spring as recommended by AAC website. I tried factory ammo (super/subsonic) inaddition to reloaded super/subsonic ammo adjusting C.O.A.L. Still no improvement. I am beginning to think its just the HK MR556 lower. I do not have another lower to try with the AAC upper. Is there anyone else with the same problems. Thanks for your time.

[Hkbeltfed]

What mags?

[A1SPAM]

Mag Pul M3, FN, and HK mags. Despite Mag changes still same outcome.

[maw1777]

You have to isolate the upper and lower to determine the problem.

Check your feed ramps, Barrel extension. Inspect everything with a flashlight, it could be a manufacturing defect in the upper or Lower.

[maw1777]

Is your 300 blk factory ammo in spec?

[E-man930]

What type of feeding malfunction?
What type of spring and buffer did you swap in?
What type of BCG is in the gun?

[aguila327]

Without more detail this will be a tough diagnosis.

Find a buddy with an AR and swap lowers. Its not very common for the lower to be an issue especially since its chocking three or four rounds in.

Sent from my SCH-I535 using Tapatalk 2

[A1SPAM]

Thanks guys for your interest and suggestions. I did check the upper and lower with a flashlight. Everything appears normal. The BCG is the AAC "full auto" BCG that comes with the 16 inch AAC upper. I swapped out the HK spring and buffer with a Standard GI Buffer Spring and Carbine buffer from AAC. The factory ammo I had was only 20 rds of Remington 220 gr subsonic and 20 rds of barnes 110 gr supersonic. I regret not taking measurements of the factory loads, at present I am all out of factory ammo and only have the capability of testing with subsonic reloads (I am using Accurate 1680 powder, 10.9 gr, O.A.L 2.089 with a light crimp, sierra 220 gr bullet, CCI small rifle primer, velocity 1090 FPS). The malfunction appears to be the BCG recoiling back just to the groove before the rim of the cartridge, I see scratch marks in the groove and the scratch mark extends onto the body of the case). It appears the BCG is not recoiling back enough to catch/clear the rim of the case. As the BCG moves forward, it catches the groove on the case and brings the cartridge forward into locking lug area just outside of chamber and locks up with BCG jammed onto the mid-body of the cartridge. I am going to tryout out a different lower from a buddy of mine this weekend as well. Any further suggestions would be greatly appreciated.

[maw1777]

Contact manufacturer for warranty repair

[darr3239]

Yep. AAC made the upper so talk with the experts.

[A1SPAM]

Thanks guys. Will do.

[Boba Fett v2]

Tagging to see what the outcome is. I've got an 9" .300 AAC upper on the way which will be mated to a Noveske SBR'd lower.

[maw1777]

Let us know what AAC says, hopefully customer service is good.