Guards have "Elite" PTRS that are much better than Penal PTRS when it comes to AI if I remember correctly. But I'm pretty sure it's nowhere near Mosin damage. It's an interesting idea but it really gives you no reason not to spam guards when they are already pretty good out the gate at AI since the LMG is usually an instant upgrade.
I was curious about this so I did some cheat mod 2v2 duels i.e. 2 ptrs vs 2 mosins.
Out of 10 duels PTRS squad won 7 of them. I did 5 at close range, and 5 at long range. PTRS won 3 out of the close range, and 4 out of the long range.
As far as I'm concerned the PTRS is at least equivalent, but likely stronger than the mosin. The PTRS doesn't fire on the move so there's a downside there, but neither do DPs.