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11/20/2019 5:07:11 PM
Posted: 10/7/2013 4:52:03 PM EST
Noticed this gouge inside of my Core15 AR today. Rifle is their piston model (TAC M4). Everything appears to be aligned correctly. Rifle functions flawlessly. Charging the rifle shows no signs of sticking and rifle has a low round count. It is deeper than the pictures show. Any ideas what might be causing this or if it will become an issue later. Bolt and charging handle have no groves or marks at all.




Link Posted: 10/7/2013 5:44:23 PM EST
[Last Edit: 10/7/2013 5:46:10 PM EST by j3_]
Fairly common with 16" carbine barrels. Some worse than others. Never really seen it on the few 20" guns I have owned. Not postive on what causes the cam pin to do it worse sometimes than others.
Link Posted: 10/7/2013 5:53:47 PM EST
All my carbines have some varying degree of the gouge. Although mine seem to be a smaller than the one shown
Link Posted: 10/8/2013 9:44:44 AM EST
OP, you will see that same amount of wear on many piston ARs, mostly due to the lack of gas rings which would provide tension when stripping rounds during feeding. A POF roller cam pin will prevent some of the wear.
Link Posted: 10/8/2013 10:13:39 AM EST
I had the same issue with mi .308 DI.

My gouge is about 1.5 mm deep and less than a centimeter long. I have some pics posted up here somewhere.

I bought an adjustable gas block. I'm not sure if it fixed it or not, but I've ran about 400 rounds through it since then and it hasn't gotten any worse. I figured I may have been an over gassing issue where the bolt was getting ripped out of the chamber before the bolt cam was fully forward, which caused it to carve a groove into the receiver. That was my reasoning.
Link Posted: 10/9/2013 2:39:17 PM EST
Thanks for the replies guys. I have several more rifles of varying set ups and none show this type of gouge. This is the only piston set up however. It does have an adjustable gas block. Ill play with it some and see. I know it won't shoot the cheaper ammo in certain settings. The roller cam pin looks interesting. Might have to give it a try. Thanks
Link Posted: 10/25/2013 4:08:17 AM EST
As far as I can tell, this is typical for some of the piston systems.

In the standard AR, the bolt is initially unlocked by the pressure internal to the bolt carrier, actually pushing the bolt into the barrel extension. That pressure isn't there in the piston guns, and the cam pin drags on the upper, making those marks.

Link Posted: 10/28/2013 2:34:26 AM EST
I have a 16" mid-length that has this gouge....it will stop wearing once everything breaks in.
Link Posted: 10/28/2013 8:25:51 AM EST
I was doing some reading about differences between internal and external piston AR15's. The internal piston is what people mis-identify as direct impingment, and external piston being Adam Arms, PWS, LWRC, etc.. Anyways what was explained is the external piston's op-rod causes the bcg to twist as it is being pushed backwards, while the internal piston setup doesn't create this twisting motion when the weapon cycles. So, this is what I am thinking is happening; the bcg is twisting as the cam pin rotates when the bolt unlocks, creating that gouge.
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