Not too long ago, I made an online purchase of an AR15 upper receiver, without BCG or charging handle. Reputable operation - I have bought their products before and was quite pleased with the quality of what they provided.

Tried to mate the new upper tonight with a Rocker River lower that usually serves as the lower for my 18" WOA SPR carbine. No go.

Problem is the rear lug on the upper is too wide to fit into it's channel in the lower. Looked up the dimensions on the AR15 lower blueprint, measured some lugs with a micrometer, and determined that the spec calls for a lug width of 0.5" +- 0.004". The lug width on the new upper is 0.54".

I don't want to return the upper to the OEM. I want to fit the lug to the lower receiver. So suggest tools/methods I can use to delicately reduce the width of the lug.

Thanks,
PJP

handmill




File

I just use a small needle hand file to match them up, this is not an uncommon thing when you are trying to match uppers and lowers from different companies.

A buddy of mine, just puts in the pivot pin and them slams them together a couple of times, he does not have the patience I do!

My RRA lower also fits snugly.  As mentioned, removed a little material from the upper to make it fit.

A flat file or Emory on a block of wood....

If you're correct in your .540" lug width you'll have a hell of a time with a file.  You should really cut it in a mill to ensure you only take off .02" per side.

Edit: spelling

If it is anodized, then it should show marks exactly where it is hanging up.

As far as using a small hand file, you are trying to take just enough off to have it snugly fit the lower, try to take .02 off each side is not as important as shaping the lug to fit the take down pin recess.  Normally when I have a problem with them, it is the back corners on the lug that is hanging up and not the sides, because the radius in the back of the recess, so I normally work on the those rear corners lightly until it fits.

+1

Uppers are hard coat anodized.
The manufacturer should have no problem taking it back.
Having to ask how to DIY is a pretty good indication you should send it back.

Unless you are purchasing matched upper and lowers you will often times run into small issues on fitting, virtually all of the non-matched sets I have purchase have needed a small about of work on them to get a good fit.  5 minutes with a small flat needle file and you will be in business, a set of needle files at the hobby shop run less than $5.00, it would cost more than that to send it back and wait.