I was assembling a couple of lowers using parts I bought from Fulton Armory, and when I got to installing the bolt releases, neither would fit into the slot in the reciever. Close inspection of the latches showed that the castings were pretty rough, and had numerous bumps that are enough to prohibit them from fitting in right. How do I smooth the surfaces or otherwise abrade the raised areas enough to get the parts to fit?
Those parts are a pain because they’re head treated. If they’re so tight they won’t even fit in snug… that’s a pain.
I’d try to find someone local who has a spare part you could just try to confirm that it’s the part and not the upper.
You can file on it, but it’s a slow process because of the hardness.
Right, I tried a file, but only seemed to take the finish in the high parts off.
Both lowers are Fulton Armory (and, consecutive serial numbers), so the machining is probably identical.
If memory serves me, the slot in the upper receiver should be 0.155 +0.002.
I measured a couple of Colt bolt catches. Both came in at 0.152 with dial calipers.
According to a Colt drawing I have (which may or may not be correct), the slot should be 0.167 +/- 0.010. The hole diameter for that detent and spring is 0.156 +/- 0.003.
Joe Mamma
Edited to correct typo in spec!
You mean +/- 0.010.
We sure hope.
Yes, I just edited my original post for clarification. Thanks for picking that up!
Joe Mamma
No problem. I saw it and thought, “Hmm, that’s a lot of variance,” and had to go check my schematic. 
Blueprint (for reference):
http://biggerhammer.net/ar15/cad/ar15_lowerblueprints_unknownorigin.pdf
Well I got a couple of noname bolt releases from my LGS, and they fit in with no problem. Apparently, Fulton Armory parts don’t fit into Fulton Armory recievers.
Glad it Worked out. FA has dissapointed me lately.
If you don’t mind me asking, were the bolt catches from Fulton marked in any way?
No, there were no markings of any kind on these bolt catches. The parking was a very light grey, for what it’s worth.
I recently found one of this type to have an undersize RP bore.
Would have been a cluster had I not attempted to dry-fit it for a function check, prior to driving the RP.
It had a “coarser” than typical, tactile (not tactical) surface on the paddle as pictured…
For reference, typical “finer” tactile paddle surface…
You’re referring to FA, correct?
No, this one in particular was shipped by PSA.
If you dry-fit the assembly as standard practice, using the correct diameter drill bit shank for a temp pivot, you will find this crap.
I tape the drill bit in place and let the roll pin chase it out as well.
My FA looks like the top one, and the noname one looks like the bottom one. I did not test fit the pin in first, but I will now. They could not be forced into the recieve slot anyway, so is a moot point though.
I had the exact same problem the other day. The bolt catch from a PSA kit had function problems until I reamed out the rolling pin bore by hand with a drill bit. Very frustrating.
ETA: Belay that last. I did need to apply power to that bit in order to open the bore. Flexeril fogs the intellect.