More trouble than they're worth, IMO. Barrels need special tools, generally speaking, to get locked down properly & headspacing is a bitch for an idiot like myself. This is why I buy a gun & not a kit.

My .o2

the  build is simple IF you have a barrel press.  the barrles need to be pressed into the trunion(sp).. if  its already installed (like aparts kit) you gotta drill it out,, take the old torch cut reciver off,,  press it into the new reciver.  Make sure it has proper bolt gap( headspace) and thatsit,,
need a US reciver,, clipped and pinned grip frame,  US parts semi auto fire control group. and then you gotta weld the rear site on,, make sure the triple frame is on right,,, ect ect ect.....

if you have the tools,, it can be done. but personally all the g3/cetme recivers they make these days are no where near what the orignals were.. buy a clone ,, rebuild for there,  or drop the cash on a custom build from a cetme/hk builder,, its all on how much $$$ ya wanna drop.