Daryl thank you for your advice, I am reassured for shooting

iomskip - the cheekpiece makes it unlikely to have started life as a shotgun, but logically Husqvarna would have used basic actions for different guns/rifles

I am quite please with this extra "safety"

Lancaster - extreme thanks for the connection with Swedish moose hunters, I had not thought of that, but so logical instead of the India connection I had in my mind. The 577/500 should be an ideal stopper for even this large game

To all practical purposes the .500 and .577/500 in BP are identical in a practical situation

9.3x64mm - I do not know the proof load Birmingham used, they do not give them out AFAIK

The Martini and "Improved Martini are probably the stronger actions with the block contained in the action instead of the pins in the RB Daryl has made the good point that 4 pins must shear before failure
The most quoted RB failure involved a .500 Nitro in a 12.7 x 44 rifle!!

I have shot my 12.7 X 44 RB carbine with NfB with no problems in the past

However, as quoted on another forum, in a lightweight rifle 8 1/4lbs the 577/500 is a beast of a cartridge to shoot extensively. I have a WR "Improved Martini" and a "Howdah" DR in this calibre and after 20 shots (in the DR) you certainly knew you had been shooting!

TH44