I have a Colt 7.62x39 flatop upper that I purchased from a friend about ten years ago. The deal was too good to pass up at the time. After sitting in the corner of a safe for all those years, I decided to make it into a complete rifle, for shooting our problem feral hogs.
I'm guessing the bore diameter is .310, but in all the components that were included in the deal, all the bullets and loaded ammo were .308 bullets! The guy I bought all this stuff from, has passed...so no info is available there.
I just called Colt, and the first guy(FNG!) said they didn't make the upper in that cal., but someone next to him straightened him out! But, nobody would tell me, for liability reasons, what the bore diameter was for those rifles! They just said to "use the ammunition it was designed for!" What a DIKHED! That's exactly what I want to do! I just prefer to load my own!
What I'm trying to avoid, is the hassle of slugging the barrel!
So, if anyone knows for certain, or could provide a link to documentation of what bore diameter Colt used for their rifles, it certainly would be a big help!
Thanks in advance!
1D

Look up "slugging the bore"

Hi... I'm new to this forum, but not to reloading and shooting, I've been at it for over 35 years now.

I have a Colt Sporter Carbine (dumbed down AR-15) in 7.62x39 I bought new many, many years ago plus a Colt replacement flattop upper with a rifle length barrel for it.

If I remember correctly, there was an article in Shooting Times magazine when the rifle came out that mentioned that the proper bullet size for the Colt was a nominal .311, and that the Ruger Mini-30 (7.62x39) had a .308 bore.... making reloading interesting.

I don't reload 7.62x39 so I don't have any experience there... and I've only shot Russian Milsurp out of both Colt uppers the few times I've used it, and I'm assuming they were loaded with the larger diameter bullet. It worked fine with acceptable accuracy.

I'll try to find the article to double check that info, I'm a pack rat when it comes to gun magazines.

Thanks, and welcome to the Campfire!
I really would like some definitive answers.....
1D

I have the ER Shaw barrel 16" and 1x10 twist. It is a .310 but I reload .308 in 200gr and 220gr at 1100 ft sub-sonic and it shoots good. I think you will be good using the .310 bullets most caompanies sell at 123gr or 125gr.

My Colt 7.62 X 39 barrel slugged .309". So far it shoots Winchester 123gr SP's the best. (Factory ammo.)