take mag and bolt out, put aside... You don't need em until the stock/action are married.
rest stock on butt, place barreled action into space, hand-tighten action screws to prevent movement, but NOT tight at all, just barely snug.
Give the butt a tiny bounce on the floor muzzle up, and make sure the barreled action is centered and plumb and the recoil lug is truly back against the slot for it, then hand tighten the action screws just a little bit more. Keep muzzle pointed up.
The goal is to have the weight of the barreled action sitting as far back and snug as possible against the chassis before tightening. (Sometimes triggers' little screw heads can catch the chassis sides, look for raw aluminum marks when the action is out).
Bounce lightly on floor again on the butt, muzzle up.
Do both screws, front then back, to 25 in-lbs.
Then bounce the rifle on the butt again, just for good measure, lol. Keep it pointed up. Torque front to 65 in-lb. Then rear to 45 in-lb.
Put bolt in and cycle dry fire for function.
Then try the mag.
I've done the foregoing probably over 100 times and it works, lol. If it doesn't, call Vudoo.
Also, what trigger are you using, and what barrel channel did you spec in the manners?