Please forgive my ignorance,but I've got a Bushmaster postban M-4 upper that won't fit
correctly on 2 Rock River lowers that I have.
The front pin/hinge goes together fine,but the
rear of the upper won't close down into the
lowers(post-ban if it matters),there seems to
be interference at the rear lug of the upper
causing a 1/16-3/32 gap,and not allowing rear
takedown pin to engage.
Thank tou for any help that you can give me!!!

If your using an offset pin, turn the pin with a screwdriver so it moves the upper foreward. You will find that the pin can move the upper forewards and backwards.

Both of these manufacturers use the push pin front hinge, so it's not going to be the kind of problem you can sometimes get with a preban Colt.

Wouldn't try to say for sure with out eyeballing the gun, but I suspect that the bottom of the Bushie lug is hitting the shelf under the takedown hole on the RR lower reciever.  Rock River uses a higher shelf than the other manufacturers, as a way to pevent the insertion of drop in auto sears.  Get something that will leave a mark by just touching; a piece of chalk, an old lipstick, etc, and coat the bottom of that lug on the upper reciever. Then try to close the two pieces a couple of times and see if the marker stuff is coming off on the upper surface of the lower reciever channel.  If so, your solution is to file off just a little of that lug. Take it just a couple of strokes at a time, and quit as soon as the upper will go down far enoughto allow the assembly pin to seat.

If the lug is not hitting that channel, you're going to have to take that marker and coat different places on the underside of the upper, to see where it is hitting.

i had the same problem. i lightly tapped downward on the upper with a hammer, putting a towel over it first. then i did the same thing and tapped the rear takedown pin in. then i took it out and shot it. after that, the rear pin goes in and out nicely with finger pressure. not sure if this is recommended...

Rear lug is a bit too wide on the Bushy upper,for the RRA lower usually.

Use an alcohol marker or Dykem blue to indicate where the interference is,and remove aluminum with a fine file in increments,untill a slight interferrrence fit is acheived.
Slow is good and don't get greedy or in a hurry.
Nice and tight is GOOD!

Best of luck and never let the metal win!

Cheers!
S-28

Thats where the manufacturer of one of the products probably did not have the M16 tech. package from Colt and probably reverse engineered measurements instead of purchasing the blueprints to do it properly. (It seems common with a few parts mfrs. out there from my limited view-but I could be wrong). I understand that bushies are made to specs. but I am not sure of RRs' source for recievers or machining diagrams(I like RR products though very well made-but I have 2 RR uppers that I have had to fit (minor, but fitting none the less) to my Colt and Bushy lowers.

Other wise it could just be tolerance stacking? not 100% though on either.  

     Thanks everybody for the helpful info!
                     NormC