What kind of gun has a breech block that pivots up and to the right and can be pulled to the rear to extract the 10 or 12 gauge shell which is loaded from the rear and has a small crown mark with MF?
There were many of these converted muskets built from post civil war surplus in the US and Anglo - Zulu war in British empire. They are sometimes referred to as cannon breech, and are pre-smokless powder conversions. There were many armorers converting these guns to everything from .303 to 12 gauge. Your gun I believe is a Manufrance conversion. Manufrance is a well known french company, fonded by Étienne Mimard et Pierre Blachon in 1885 at Saint-Étienne. The one I have is a 12 gauge and I fire lite (50 grain) black powder loads with no problem. As for value I see them range from less than $100 to around 250 dependant on condition and caliber.