I went shooting and I noticed this when I was cleaning. looks like the bolt carrier smashed into the side? the pic also shows a piece of lint from a q tip snaged on it. this is in the upper where the bolt carrier group is housed.

http://img515.imageshack.us/img515/3379/ar017ej7.th.jpg


here is a bigger pic..

http://img515.imageshack.us/img515/8520/ar017iv6.jpg

What part am I even looking at?  

Upper receiver, cam relief slot.  Lock back the bolt on your rifle, point it to your left, and look down the magwell to see it oriented the way it is in the picture.

The cam pin bumps a little if the bolt carrier moves back faster than the bolt can fully unlock.  Let me guess, is it a 16" or other carbine-length gas system and a full length stock?

I'm told it's a minor, self-limiting issue, but I don't know if I trust my source on that.  Someone with more experience than me will have to speak to that.

Ok, the shadows had me confused.  

If it’s the upper receiver, cam relief slot we’re talking about then yes, some wear in that area is normal. Each rifle wears differently, some hardly, if not at all.

Here's three of my AR's, not in any order, one is a BM, RRA and Colt. All three have some degree of wear.

I've noticed that my carbines wear more than my 20 inch models. Must be due to the higher gas psi that a carbine sees sompained to a 20" model.

"Let me guess, is it a 16" or other carbine-length gas system and a full length stock?"



- yep.

Quoted: "Let me guess, is it a 16" or other carbine-length gas system and a full length stock?" - yep.

My carbines supported both types of buffer tubes.

Yep, my Dissy has a little of the same thing.  It has a couple little dings in the aluminum there.  They formed right away, and haven't changed since, but I'm still keeping an eye on them.

Carbine gas systems get higher pressure gas than the rifle length ones, and it really slaps the BC around.  (I don't think the full size buffer is that big a part of it.)  That's part of why M4s have faster cycle rates than M16s.

You can get something like this to lower the cycle rate, but I don't know if it's really worth it.  The overall wear rate I've seen hasn't been excessive.

Anyway, either it's normal wear, or everyone here is in trouble. :)

mine has it also, once it "fits" in it should not grow.

my 20" has marks like that on both sides of the cam slot. It happened the first time at the range and has been the same since. I guess with a rifle that has a moving bolt the parts are going to rub in some spots. The rifle is going to get worn in, it mike look worse, but it will work better.

There are two types of wear around the cam pin cutout.  The type shown by zach in the first post is due to the cam pin head hitting the side of the receiver after the bolt unlocks.  The type shown in QUIB's pics is due to wear between the carrier and receiver.

The second type of wear is completely normally and will eventually stop.  

The first type of wear depends on the gas tube length, ammo type, index pin location on the barrel extension, index pin slot in the receiver, etc.  Basically, depending on the above factors, the bolt may not fully turn the full extent (the cam pin won't move to the end of its slot in the carrier), before it unlocks from the barrel extension.  This allows the cam pin head to contact the inside of the receiver.  It has been my experience that this type of wear will eventually stop as well once the aluminum work hardens.  However, its possible that the aluminum can extrude out into the charging handle channel causing the charging handle to get a little sticky.  A little bit of filing will take care of that.