I have not experienced any FTF caused by resting the mag on the bench. And I don't know exactly how or why my POI shifts. This may be a peculiarity of only my 15-22. Others have no problems using the magazine as a bipod.

My POI definitely shifts, depending on just how much pressure I place on the mag. Presumably, this would be a non-issue if I sighted in and kept exactly the same mag pressure for every shot. But this introduces another variable into my shooting routine--like breath control, sight picture, trigger control, mag pressure, etc.

Test your weapon by firing several groups, half with magazine rested, half with the handguard supported. Same ammo, same distance of course. And post your results here--curious minds want to know!