When you need a caliber that starts with "5"

[Drydock]

https://www.full30.com/watch/MDE5NzEw/577-caliber-bland-pryse-stopping-revolver

[Reverend P. Babcock Chase]

Quick, somebody call uberti.

Reverend Chase

[Blair]

Webley does produce a .50 (12mm) cal. percussion revolver for Heavy Cavalry going back into the late 1840's.
I have only seen two of these large cal. revolvers here in the States and they were both in rather poor condition.
The Webley was an open top design at that time, with a rather small dia. (for the caliber) threaded cylinder pin which screwed into the barrel ass'y, held to the bottom of the frame by a screw. Not a very strong mounting system for such a large caliber. This may account for those I have seen being in poor working order.
My best,
Blair

[Snidervolley]

[Snidervolley]

Not a pryse but a tranter solid frame boxer .577
Diminutive 455 for camparison