I slugged mine over the weekend and found my bore is .313
They supposedly vary from .307 to .314 or .316! The tighter ones are supposedly the "tack drivers" some guys have and can shoot handloads well....
Which is probably why my ten yard groups are 4-6".
I was advised to try Privi Partisan ammo in it it I can't find actual Yugo surplus ammo, since that is the same Serbian factory that made all the Yugo surplus too, so it is INTENDED for the M57...
That being said, if that doesn't work, I guess I go on my "quest" like I did until I finally found an M38 carbine that shot worth a crap... sell this one, buy another, slug/shoot it, sell it, buy another, slug/shoot it...until I get one with a bore closer to .308...
I slugged mine with a .375 Hornady round ball from my son's .36 Navy...used a section of a broken aluminum cleaning rod and it swaged it down after a few good raps with a hammer from the chamber end, and then pushed it through the bore nicely, with a good impint of the lands and grooves. then I miked it land to land....or is that groove to groove since it's backwards?