Reloading dies and chambers vary in size, as an example I have a standard Lee .223 die that will reduce the case diameter more than my RCBS small base die.

A Lee bulge buster is a push through carbide die that will size the base of the case to SAAMI minimums. Meaning your cases do not have to have a "bulge" to use this die. And this die will simply size the case your normal die never touches.

And when you use range pickup brass you always run the chance of picking up cases fired in a larger diameter chamber.

All my range pickup brass is run through a bulge buster for pistol brass and a small base die for rifle brass.

And remember after sizing the case wants to "spring back" to its fired diameter.

I buy bulk once fired Lake City 5.56 and 7.62 brass and all of it is sized first with a small base die. I also pause for 4 seconds at the top of the ram stroke to reduce brass spring back.

NOTE, for once fired 7.62 machine gun brass you can use a .45 acp carbide die as a "bulge buster". And then use a small base die to finish the sizing operation. I was told about this in the CMP forum and the sizing effort is greatly reduced. The .45 acp die is setup as a standard die with a shell holder and is not a push through operation like the bulge buster.