Hi Nitrox
Yes it is .577 Snider
It is totally illegal to shoot in the UK

What about Australia? (is that where you are?)
I have a Snider rifle on certificate so have ammunition but if I put in the pistol
5 Years in jail!!
I did shoot one at Cal Pappas' Alaska shoot a couple of years ago, recoil "stout' but was admonished for using 2 hands!! The guy then shot 4 shots one handed!

They are really collector's pieces (plus fun shooting if legal) their accuracy in doubt past 20 yards??
How many were actually in Howdahs is very arguable, the only record I have seen as a back up gun was a 12 bore with one barrel RB and one LG (00 buck to our US friends)
I have a DB "Howdah rifle" (posted here) with 19" barrels in .577/500 which I have shot with a dealer's certificate

TH