Mike - your question about shooting in the UK - I am most jealous of you guys in the US and elsewhere who can shoot these rifles easily

I have .577 and .577/450 on licence and irregularly shoot them, although only on a range
There is an anomaly that I can chop/change them from obsolete calibre to legal firearm albeit with time consuming paperwork - peculiar and illogical bit much better than not being able to shoot them at all

I also am a (police) approved "servant" of a Registered Firearms Dealer friend and as such, if the rifle is temporarily put on his records I can legally test and shoot it, although I do not abuse it

I have used Nitro for black using 4831 - 40% of black in a .577 Snider but did not use a magnum primer (essential IMHO) and had unburned powder everywhere and potential for detonation ! so:
All my shooting of them now with BP only, (what most of the were designed for) plenty of information around for that and reasonable accuracy
The .577/450 I use the lighter carbine bullet 405 grain instead of the 480 grain rifle bullet, ahead of 85 grains of BP it is easier on my old shoulder!

Thanks for all your positive comments, it is a pleasure to communicate with like minds

Tony