Witness marks in upper from BCG cam pin

[danpass]

Normal, it will get to a point and stop. The big cutout is because the cam pin needs a place to go when the BCG is in battery as it cams to the side when the bolt locks in place.

I know. But either the cutout radius is too small or the center of the cutout is too far forward.

So I really see no reason for it to be 'normal' in the design.

I may as well take the edge of a file and put in a relief to avoid having the parts 'help' each other out as a form of break-in.

[steyrman13]

Is this typical?

I just noticed so I checked my RRA NMA2 that I've fired 10x as many rounds thru and it is there as well though much less obvious.

About 200 rounds

Is that a crack in the upper? Right passed the marks you are concerned with?

[Safetyhit]

Is that a crack in the upper? Right passed the marks you are concerned with?

That's the bolt carrier and I was just wondering the same thing based upon the third picture down.

[steyrman13]

That's the bolt carrier and I was just wondering the same thing based upon the third picture down.

Ahh small phone, small pics. Look in first pic too though

[ASH556]

Just a thought here and perhaps someone with more knowledge like IG will respond:

Your 20" A2 RRA has a rifle length gas system, which will yield a smoother and slower-operating bolt. The BCM carbine bold is operating more quickly and violently. I'd wager that has something to do with the wear difference you're seeing.

[danpass]

Is that a crack in the upper? Right passed the marks you are concerned with?

grease line

[MarkG]

grease line

grease + carbon = lapping compound

[Safetyhit]

grease line

What are your thoughts on the third pic? I was thinking grease on that too but the line seems too distinct and irregular. Bad machining finish perhaps?

Secondary from your now settled issue but still possibly noteworthy.

[sinlessorrow]

I know. But either the cutout radius is too small or the center of the cutout is too far forward.

So I really see no reason for it to be 'normal' in the design.

I may as well take the edge of a file and put in a relief to avoid having the parts 'help' each other out as a form of break-in.

Parts rub and wear until they mate perfectly. Its normal.

[danpass]

What are your thoughts on the third pic? I was thinking grease on that too but the line seems too distinct and irregular. Bad machining finish perhaps?

Secondary from your now settled issue but still possibly noteworthy.

Just the camera angle. That 'line' is where the rail of the BCG rides and pushed it's grease down.