I believe the top rails are 4.0 mm wide from the outside of the receiver. The receiver sheet metal is 1.0 mm wide, so that would put the edge of the rail about tn the center of a 6.0 mm drill bit diameter. The top rails I believe are rough cut to 5.0 mm wide before stamping the step and final machining. I have references the making of the receivers in Russia including a general overview of the process but the description doesn't include all of the specs. 6.0 mm is the minimum rail gap, so somewhere between 6.0 mm and 6.3 mm should be ideal and may not have to be exact.
also I always sand down the edges of the top rails on my receiver builds to remove nicks and burrs
I've always used a 1/4" piece of keystock cut to about 5 inches long. I clamp the rails into place with the keystock setting the spacing. One clamp near the front, and one near the rear. Then I put one spot weld up front, and one at the back. Then pull the clamps and keystock. THis has always left me with perfect alignment from the rails to the trunnion. YMMV.
Also on my latest build for grins I polished the upper rails as well as all wear surfaces on the bolt/hammer/trigger interface. (I usually do this but not to this degree, or to the top rail.) Boy did that makes the bolt smooth as glass when operating the bolt/carrier. Slicker'n snot!!!!
Just went to look at your pics, and it looks to me to be the top rail width. Yeah you have to shave those down quite a bit. Measure the width of your carrier in the slots that ride on the top rail, and then slowly file/sand the sides until you get to that same measument, then go a thou or two till the carrier slides freely, and doesn't bind. Then check your bolt again, I bet she slides along just like it was meant to. Right now it's not wide enough for the bolt to slide, and I'm surprised that you can even get the carrier to move at all.
Even if I could have made that receiver work, it doesn't hurt my feelings to be moving on to a new one.
2ndAmendican: I measured the slots of the bolt carrier, and that measurement came to .0977". The gap between the upper receiver rails on the new receiver measure 0.897". Move it? Ha. I'll do some filing as you suggest once everything is riveted together.
The thing I'm seeing with this receiver is that the grooves in the front trunnion do not appear to line up with the upper rails on the receiver--perhaps this will be solved when I file the upper rails, but it looks like the bolt will have to step up when it goes from the receiver into the trunnion. See photo:
The trunnion appears (to me, anyway) to be 100% properly located in the receiver, so I don't think that's an issue:
When I temporarily install (clamp) the lower rails in the receiver so that I can fit the bolt between the upper and lower rails, the left rail (with the ejector) sits considerably lower than the lower groove in the front trunnion. I can post pics of what I mean, if that would help.
What I will do at this point is file down the upper rails until I can get the bolt carrier to fit. Perhaps that will, as mentioned above, help the bolt to slide as it was meant to!
Red, I see what you're talking about, but the top rail appears to be located properly. in this picture the alignment looks perfect:
Yeah you definitely need to open up the width by filing the top rails though. Try to keep the amount you remove even between both sides. You don't want one too narrow and the other too wide. Of course you need to have the center support installed first before you do your final trimming/filing.
Good luck with it.
The distance between the top rails is suppose to be 25.0 mm. Measure the width of the outside of the receiver, subtract 25.0 mm and divide that number by 2 and that should give the rail width from each side of the receiver as measured from the outside of the receiver to the inside edges of the top rails.