Guys, there's no need to re-invent the wheel here. Buy or borrow a .308 Win. FL sizing die. Lube the cases well. Neck the .444 Marlin brass down a bit at a time until it will JUST comfortably fit in the .309 JDJ chamber. Clean the case. Prime it and load it. Shoot it. FL size etc. from then on with the .309 JDJ dies. Lots of people have been doing this for 20+ years. You should lose no more than about 1 case per 100 if you lube properly.
Minnesota1 : 52 gr. of 4350 with what bullet and primer? Sounds a bit hot. 49-49.5-50 gr. of 4350 with a 165 & FEd. 210's is about max. WORK UP to this as Max.! Not much fireforming involved with the .309. The advice to start 10% low is always good, working up gradually, in 1/2 gr. increments, a couple rounds at a time.