I know you are asking Evl, but...
The factory SBS has a 12" BBL. And they likely reamed the ports to compensate. It works for high pressure loads.
To shoot 3 dram from a 12” bbl, it's best to move the gas block toward the rear of the weapon to increase impingement time. Saiga's are easy to do like this. Veprs are more involved. Would be better off starting with a 19" bbl that runs exceptionally well.
Evl's take may differ. Whatever he says will carry far more weight than me.

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