Tony/Marraki is spot on and has been at the .577 game for a long while.

My own experience with NE loads is with my 1902 Webley .577 3” with 28” barrel that weighs 11lbs 6oz. It is about my pleasure tolerance

with the equivalent of the old 90 Cordite 650 grain bullet at a chrono’d 1950’/sec.
I started with the “1.19 x Cordite load” of RL-15 and backer rod then worked up to equal the old velocity claims, no reason to change.

Easily loaded down a bit as well…..
- Mike