I'm going to put together a 300Blk pistol for keeping in my truck.  I've decided against the 223/5.56 due to the velocity loss issues and reliability issues when barrel get real short where as 300Blk works better (reliable) with pistol length barrel/gas.  My question is how short can I go with 300Blk without effecting reliability and at same time have enough velocity for reliable expansion with various JHP type ammo.  I guess I'm just trying to decide on the optimal barrel length for the 300Blk. while still short enough to fit in a tennis racket bag or small gym bag.  I've seen some 10 1/2" uppers and pistols but they seem too long for my purpose and I was told some 7 1/2" uppers have reliability issues.  I guess that leaves 8"-9" uppers.  Any known reliability issues with uppers in such length?  Do they retain enough velocity from such shot barrel to expand JHP bullets?  Any suggestions?

~8ish inches.