I have a lr-300axl upper and the retractable/folding stock kit that YHM produces.
The folding stock puts your lower into a proprietary configuration to use only YHM's upper system
When folded it is quite compact but when retracted it is longer than my M4.
My M4 is more compact by 2"1/2"
I would like to convert my other uppers to GP but I will wait until a reasonably priced kit comes around.
On the other hand? I haven't had any problems with the gas system uppers I already have?