DD, Some 7.62 chambers are long enough to essentially ruin brass on the first firing. And brass spring-back is a thing. Well.....unless you're sizing just enough to rechamber in that particular 7.62 MG.

People approach the MG brass conundrum in different ways, as you've read. Here's what I do.

I use a LE Wilson case gauge. Every piece of bulk-purchase (possibly MG fired) brass gets checked in the gauge before any processing. Can do it by feel, watching TV (if there's anything on). But the check is step one. Any pieces that are badly overstretched are culled. How much stretch? Usually pieces over 10 thou above max shelf (often they're more like 20 thou) will stand out from the rest. Suppose you could have a batch that are all no good, but haven't yet.

If you want to try and rehab the long ones, sort them by degree of stretch and try a few.

Anyway, you can use a std FL die to FL size the "good" pieces, which will most likely chamber in in any .308 Win.

My rationale. No matter what is done to make a badly overstretched case fit a somewhat spec .308 chamber, there's an incipient separation waiting to happen. Might be first reload, might be fifth. I don't need the aggravation.

Prevention vs cure. JMHO.