+1 Kaldor's recommendation.
When I started reloading for the M1A, I encountered chambering issues also (using purchased MG fired brass). I purchased a small base die (RCBS, I believe), but it did not resolve the issue. Come to find out that I was not bumping the shoulder back enough. Ended up having to use a bit more lube (lanolin/alcohol mix) and sized the brass twice at considerable effort in order to get the MG fired brass back into spec.
A little background on the MG fired brass also...the chamber of the MG that the brass was from was apparently really loose. Close to 50% of the brass as received had signs of incipient case head separation. Scraped the ones with signs of case head separation and ended up scraping the rest after 1 reload.
Ive had better luck with MG brass than you, but it can be a PITA to prep. Whether its worth it is up to the individual but it is good brass.
I have an entire process it goes thru to get it ready
Step 1: Inspect - screen it to get dirt out, throw out obvious junk
Step 2: Decap it all. I do this my Lee APP
Step 3: Cut crimps. Another inspect stage as well
Step 4: Tumble for an hour in SSTL media. I think this really helps make sizing easier, it deburrs it, and it will save your sizing dies from getting scratched
Step 6: Anneal it. Makes for easier sizing
Step 5: Size it on a strong single stage using lanolin lube. I use my old Hornady single just for this reason. I use a standard Hornady sizing die without the ball. If its blown out really bad, then I will turn the sizing die up a turn and double pass it with the second pass fully bottomed out on the shell holder.
Step 6: SSTL tumble to do final cleaning. I will run it 4 hours at this point so it is factory new clean.
From here I just leave it set until I need it. Then its a quick pass with a mandrel to set neck tension, a trim on the Giraud, and then load it up. Its alot of work, so it isnt for everyone. But over time I think its a great investment especially if you shoot an autoloading 308 which really tend to beat brass up as the LC brass seems to hold up really well. I dont doubt there are other brands of brass that will be just fine over the long haul, its just that I was able to buy large quantities of LC a few years back for a good price, so I just use that. I think at some point I will buy a Dillon trimmer and automate some of the work, probably taking a first pass on the single with the die turned up, then a final pass on the progressive to do the final sizing and trimming. However once its been processed the first time, Ive never had a need to size it twice, so I would go full progressive at that point and really burn thru it.
