I'm thinking it could be a modern 50-70 when loaded with lead or .510 Barnes or Woodleigh, etc. hunting bullets. Granted it is .125" shorter, and case capacity is undoubtedly less due to the heavier brass....but the brass is heavier, and rated for higher than 22500 psi.

Anybody got Quickload? If you would, use a generic 50 BMG bullet, and a .510 midweight(450-535) hunting bullet if the data is there.

300 WSM brass, shortened to 1.625.
Base diameter ahead of the extractor groove .550
Neck diameter loaded .541
Neck brass thickness .0155-.016
Case weight empty 210g
Case full to water to the top 284.7g

Here's what it looks like, next to a 500 A2, also loaded with SA 690g BMG ball.

Assuming a 24" barrel, a 0.35" seating depth, and a 64,000 psi rating, I get 535 gn at 1900 and 450 at 2000. Assuming a .45" seating depth for the 690's, I get 1700 fps. These numbers include a slight reduction in pressure to allow for unknowns, a reduction QL automatically applies; if you work with a strain gauge and chronograph, you might eek out another 50 fps. The class of powders are estimated to be 2400/4227, 2400, and Re7, respectively.

will you be headspacing off the casemouth???

Thanks for the info! That's a lot more velocity than I expected it to predict.

I did some measuring on the 690's. There is actually .720" seated in the case with .400" of that being a boattail that tapers down to .400 O.D.

Yes, I'm planning to headspace off the case mouth.

Seating such a long boattail that deep really cuts into case capacity. QL predicts about 1450 with a powder similar to 2400.