What kind of I. D. do you think you'd need to be safe from the proj pass through?
well, if you go with the thought process for opening up a standard M14 flash hider to National Match standards, you use a #7 taper reamer and gauge it (with a bore cleaning guide, per instructions - CLICK HERE
) as you open it up - bit,by,bit ..... now a #7 taper reamer starts at 0.3297" and goes up to 0.4220" as you can see here CLICK HERE
but you do not need to open up at the barrel end of the flash hider to the max size
my best guess would be about 0.525" minimum to a (theoretical) max of 0.590" for a 0.451" diameter bullet ~~ now that is just top of my head guess work not sitting down with graph paper drawing to a scale of 10 to 1 for exaggerated view and assuming bullet flight is stable from the lands