KNS shows two pistons for the PSL, large bore and small bore.

Here is what they say on their website about this. I highlighted the interesting comment.
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Small Bore/Caliber pistons are for .223, 5.56mm, and 5.45mm. They work better in AK's of ALL CALIBERS with less excess gas.
Large Bore/Caliber pistons are for 7.6239, .308, and 7.62x54r.
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So as far as I know, the PSL only came with a 30cal bore, so large bore piston seems like the obvious choice, but then they made that comment that the small bore piston works better for all calibers.

Have any of you guys used the small bore piston with 7.62x54R? If so, how does it work with various ammo?

I'll be mostly shooting milsurp 147gr Czech silvertip and Romanian LPZs, but I do have some surplus 180gr Romanian and Hungarian that I use with my bolt action M39s.

I'll ask KNS about this too, but I wanted to hear comment from the shooters too.

i would go with the large bore. I also have a psl and have considering the piston change myself. Post if you hear anything from kns.