I have a Bakal single shot 12 ga. and it doe's not say on the barrel what choke it is. I measured it and it is .680 , would this be a modified choke? Also it has a .120 wall thickness, could a screw-in turkey choke be installed ? Any info would be appreciated, thank's: Rick.

If you measured it correctly, that is a VERY tight full choke, assuming you have a normal or near normal 12 bore which is supposed to be .729 ID (which is the same as cylinder bore). Most people consider a full choke to be .040 (40 thousandths) constriction although some are a bit more. I've measured some brownings at about .045 but yours would be .049 which would make it extra tight. It might be so tight that it would disrupt patterns, only shooting and patterning will tell you that.