I bought a brand new Colt EPR 11.5" URG and had an issue where a properly torqued Aimpoint developed some back and forward movement on the receiver (ultimately affecting zero). Can you guys take a look at the pic rail on this Colt upper receiver?

Look at the back of each of the receiver pic rail lugs. It looks like there's some material built up that wasn't machined off before anodizing. I don't think this is right unless it's a Colt thing??? It only took about 20-30 rounds of firing to cause my Aimpoint to start moving. I can tighten the mount and then after a few rounds, it does the same thing.

I have reached out to the dealer that sold the upper and they said they have another EPR that shows the same thing. They aren't saying it's "good", just that another EPR 11.5" upper exhibits the same thing. I have also reached out to Colt, and although there is still a dialog going; the conversation doesn't look too promising.

The top picture shows where the optic wears down the raised metal. The side picture shows the build up on the back of the lugs. The camera only pics up a few spots right in front of it, but all lugs show the same amount of material.

Any ideas? My Aimpoint has been through a couple uppers and has never done this as far as losing tension or wearing the upper like this. The old uppers also don't have the raised material.

What mount are you using?

I don’t think that minor blemish can cause the issue.

Not right/spec.

American Defense mount. Never had a bit of trouble with it before. I inspected it closely after the first time it loosened up on this upper and it showed no signs of damage that would case this.

I’m familiar with adm mounts since that’s all I use.

They grab the side of the picatinny rail so I don’t think that blem can cause a mount to loosen up.. once it’s tight it should stay tight..

I guess you already tried to adjust the tension on the Adm mount.. don’t you?

Yes. Your rail is fucked and out of spec. Seems to be machining issues. I.E. a dull bit pushing material out of the way rather than removing material, creating a burr on the edge of the cutting terminal.

How this works in regards to your scope mount, is the mount applies a certain amount of pressure to the upper that when full surface area is in contact, appropriate purchase is achieved and the optic does not move during firing.

Apply that same pressure to a significantly reduced surface area incapable of maintaining form or friction under that amount of pressure the mount applies vs. inline forces of the cycle of the weapon, and it will not generate an appropriate amount of surface tension/friction, allowing the optic to move when enough force is applied (object at rest= scope, object in motion=rifle: scope will attempt to remain at rest until hard stops engage and scope is forced to move). This happens on both the rearward stroke(unlock, extract, eject, cock) phases of the cycle of function and during the forward stroke (load, chamber, lock), so it uses the full range of motion the specifications and tolerances allow.

As this happens, the smaller surface area experiences wear and deformation, thereby negating applied torque value during installation.

Bad news, the upper is out of spec.

Good news is that as the high points wear down and full surface area contact is achieved, this will stop happening. If you don't mind cosmetic issues, you'll eventually have a working upper capable of holding zero.

This.  Could break out some glass and sandpaper; hone down those burrs to make it flat; and then "refinish".  Or, hit up Colt for replacement upper.

Excellent post. Decide if you want to send it in or fix the problem yourself.

Sorry for the delay guys. Forgot I even made this post.

Ended up getting a brand new replacement URG (no thanks to Colt). In fact Colt completely ghosted me on this issue. Not going to buy any of their new stuff. Milspecretail took care of me.