I used some alum. jaw guars to hold the reciver. the alum. left some strecks but a trip to the caustic tank will remove the stains. ther was no locktight on the threads just macinal (sp) lock from the barrel ring.
pic 1 is of a FAL the barrel has a ring the butts up to the reciver and locks it in to place after 110+ lbsft is used. on a kit build the reciver stub is removed from the barrel with slot is cut to open up the reciver threds and a cold chisel is used to open up the slot, breaking whats left of the threds then the reciver can be removed from the barrel with out damage. pic2 is of the ring on the barrel. Im planning on boring out the stub to 22mm and cutting the barrel down to 22+mm for a press fit then set head spacing and tack weld stub to barrel then cross pin the three (stub, barrel, reciver). at 22mm this will give me a .15x" wall on the stub and still keep the barrel DIA. in the safe range.
you should get a fal they are realy easy to convert to .243 or somthing based on the .308 case. muttman