Charleton Hall Auction reports the gun weighs 11 lbs, 8.6 ounces. This lets out its having been at one time a rifle I believe. All the Reilly rifles I have recorded have barrel length under 28." It also seems light for a big bore rifle (see the weight of 15963 made almost at the same time as this gun, which weighs 18 lbs 10 oz).
I went through my database looking at 8 and 10 bore Reilly's and unfortunately almost none of the advertisements recorded their weights. 8 bores seem to range (from limited data) from 9 lbs to 16 lbs. 10 bores from 7.5 lbs to 10 lbs. There are a couple of Reilly owners where who own Reilly 8 bores who might provide the weights of their guns.
-- 12bore owns 14983, the first extant Reilly gun with 2 rue Scribe, Paris
-- GaryD007 owns 30295
I can't contribute any more than this. There are collectors here who own big bore 19th UK shotguns who can opine more on the subject. Still it's an unusual gun.
Here is the Reilly gun in my database which most closely resembles the specs for 15625's - 18688, 8 bore SxS shotgun, 36" barrels, pistol grip stock (no weight mentioned)(and with that pistol grip, it might have had a 2nd matching rifle barrel to go with the shotgun barrel).https://www.rockislandauction.com/detail/76/3417/em-reilly-co-double-barrel-shotgun
SN 18688, 8 Bore SxS Shotgun, Reilly & Co., Edward M. 36 in solid rib.
But 15625 looks more robust (15625 was originally a pin-fire, now converted to center-fire) so I'm wondering if 15625 also had a separate matching rifle barrel)...there are a couple of extant Reillys with a pistol grip stock but with both rifle and shotgun barrels:
Hummm another one...15255, 10 bore SxS pin-fire shotgun....no weight mentioned but with pistol grip.http://silverhawkauthor.com/small-a...shotguns_610.html?printable_version=true