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The government picked 106mm to eliminate any 4 inch barrels as they were fairly common at the time.. Wouldn't want to risk any manufacturer over sizing to allow people to have un modded ww1/2 relics.
The whole thing is crazy IF you are so worried about concealing and such why is there no OAL and why does barrel length matter at all... I mean really someone could easily make a semi auto that's barrel is 106mm and OAL length is 110 but a revolver with a 4 inch prohib barrel will be much much longer. It's like whipping before you poop just don't make any sense
That number, and .25 and .32 caliber were selected because this would prohibit some 2/3rds of the firearms registered at the time. Initially, it was intended that immediate confiscation occur; when the possibility of court action and compensation was considered, the grandfathering scheme was introduced.