After making a couple 44x1.8"BGM rifles 1Biggun and I got to talking about an ar15 version last year. At the time there were cheap SS blanks with a 1:16.5" twist, I had an extra upper and a piston kit complete with handguards. So all I needed was a 450BM type bolt carrier, 450BM type follower and the rest of the parts were already on hand. Initially it was going to be a 44x1.7" but the cartridges looked very much like the 44x1.8" cartridges and would snap into the extractor when chambered. We tried a 1.6" and it was short enough that the bolt just pushed it forward and didn't lock it into the extractor where the firing pin could hit it. Not foolproof by a long shot but a little extra insurance should it happen. Run at the same pressure on average the 1.6" case is about 50fps slower than it would be at 1.7" which I can live with. Two barrels were made, one 16.5" and one at 14.5" with a welded on flash hider to bring it up over 16". A standard ar15 barrel extension was used and they were modified to big bore center feed style by milling the bottom lug down to make a big feed ramp. The same reamer as the 44x1.8" was used and a set of 1.6" headspace gauges were made.
Initially 30-06 brass was trimmed, expanded and reamed to make what amounts to a 1.6" long 44AMP. With the switch to a 1.6" case length came the happy coincidence of having the same case length and internal capacity of the 445SM. 445SM has a around 1000PSI more allowable chamber pressure than a 450BM so I back off of the 445SM data slighty for max when working up loads. Recently I made some more cases from 7.62x51 cases and found it easier and quicker than using 30-06 cases. Since the base dia is wider they are loaded with a standard 444Marlin die with lengthened seating stem and a 44mag taper crimp die.
I only have two 44 molds (240gr & 310gr) and so far have only shot jacketed in them. I hope to test out some of the cast ones next time out. I've also been looking at 350gr-400gr molds for suppressed plinking. Here's some pics of it being made. The pic with the paper plate is the first semi fire. Out in the back yard into sand at about 6'. The recovered 240gr xtp's are sitting on the paper plate.