BigAl I honestly would have agreed with you on the milled reciver 3 years ago. I built that varmit 223 on a dare an a bet with spare parts. I would have been thrilled with A one inch group. to get sub 1/2" groups as easy as I did still seems to good to be true.
the truth is IMOA that the reciver flex is not as much of a issue as with a bolt action due to the scope being on the barrel and not on the reciver. all the sheet metal reciver does is hold the trigger grop and the carrer in place. I have read posts on other boards were guys claim there milled reciver guns are not as accurate as stamped. It is possable that a stamped reciver may dampen barrel harmonics or perhaps they cool better or maybe it dampens the trigger group vibration .
Harmonics and the bolt being sqare to the barrel, bolt flex, an firing pin weights, and lock time, barrel tension all come into play but after shooting my varmit build over 1500 rounds at varmits I really am certain that almost any calibure on a stamped reciver can be made to shoot sub 1" moa with a good barrel, chamberd correctly and really good ammo. I will likely find that my new varmit II version may be only slightly more accurate than version I or not at all. I feel at some point ,likely at the sub .5" group range there is going to be a wall that the AK system will not allow you to do any better. it will be a combination of bolt flex, lock time and the swing of a heavy hammer into a not perfectly centerd firing pin. or the .080 or larger gas port causing the barrel to warp at a certain temp or possably the barrel pin causing tension on the top half of the barrel only, or the 23mm limit on barrel OD, but something will prevent it from going super accurate (1/4" groups). I intend to play around with these CHEAP guns and see what the limit is.
As far as the 7.62x39 goes the round is limited by its case capacity for getting much in the way of velocity out of it but bolt guns can shoot very very accurate with it. I got the reamers I got cheap takeoff barrels so what the hell, build it throw in some good ammo and see what it does. It will be a nice light hunting capiable pretty stainless barrel shooter. Like I said I have shot several 1" groups with a 12.5" pistol in 7.62x39 so I know the potential is there.
If anyone want to donate a .30 cal match grade barrel blank or a milled reciver to the cause i will be happy to put it to the test. LOL