If you go below 105mm it becomes prohibited and you can not legally make a prohibited. But I believe you can shorten a pistol barrel from it's original length to 105mm. Gunnar at Armco has shortened a few long barrel Magnums to 110mm (about 4 1/4 in.)( it looks like he allows a 5mm safety margin in length). If it is already prohibited I guess you could make it any length you want, because you can put a longer barrel on some of the short barreled prohibs and make them restricted. Which ever way you go I believe they have to be VERIFIED as to changed barrel length.