So I got sick of waiting on my Leader 50 A1 and bought an AR-50 to develop loads. That thing is HEAVY, but oddly comfortable to shoot. The brake is a beast, and puts up a cloud of dust like an artillery piece, but transforms the recoil into a push that is subjectively more pleasant than my 6.5 CM (which also has a Med Palma barrel and brake). Anyhow, I started with 750 grain AMAX loads with US 869 and was confused to find that despite the VERY long throat in this gun and loading to Hodgdon's COAL, I got pressure signs near minimum loads and a nearly stuck bolt 10 grains above the starting load (which sounds like a lot until you consider that the published max charge is 265 grains of POWDER). Undeterred, I also started working up loads for the 643-647 grain API (Silver Tip) bullets. There, I also got pressure signs near minimum loads.
The unexpected issues now are that 1) pulling .50 BMG bullets is hard (and I annealed very case neck before loading) 2) military API bullets run about 644 +/- 2.5 grains and vary about 0.1" in total length, which means that the lowest common denominator for safe loads leaves a lot of performance on the table for the shorter/lighter bullets 3) I do not have a good theory on why I'm getting pressure signs so low in my ladder despite a long throat (I measured something like 0.3" of freebore when using Hodgdon's 5.450" COAL in the AR-50). There may be too much neck tension, but I'm already doing the things that should mitigate that issue (I sized all of the military bullets through a .510" sizer die before loading and annealed /ized all of the brass).
In better news, the 6.5 CM that I built to practice with shoots 0.5-0.7 MOA groups using loads that I spent basically zero time working up. 6.5 CM from an aftermarket barrel is basically a cheat code for precision.