The maker of black powder pistols featuring a diamond with "SKA" is likely the Italian firearms manufacturer "Pietta." They are known for producing a range of black powder firearms, including revolvers and pistols. The "SKA" mark may refer to a specific model or series within their product line.
Depends on the caliber. A .45 caliber pistol uses about 40 grains of black powder. Contact the maker's website and download the owner's manual for YOUR gun.
Among other things, it COULD stand for Armi San Marcos- an Italian maker of reproduction black powder revolvers.
I think what you are seeing is in actuality PN which is a proof mark and stands for polvo negro or black powder. It is most likely a pietta or san Marco made gun.
Pistol or rifle? The ROUGH rule of thumb- start with the bore diameter. For a pistol, charge is 50% of the diameter- so about 16 grains of fffG. For a rifle, 100-200% of the bore- so 32-64 grains of fffG. You should check with the maker if they are still in business, and see if they have an instruction manual available. NEVER use anything but black powder, or a black powder substitute, such as Pyrodex.
Maybe 50 USD
I have found out that P. Streignard was a Firearms maker in Liege Belgium during the years 1820 through 1840. He specialized in finely engraved and incise carved Percussion pistols.
Without knowing the maker or model it would beimpossible to give you an accurate value. I can however steer you in the direction or the on line auctions such as gun broker, auction arms and guns america. You can search black powder revolvers and compare what you see to yous. That may help with the maker as well as the value. The PN you mentioned to is a Spanish proof mark (polvos negro in Spanish) or black powder.
Mendi- now out of business for several years, was a maker of reproduction (modern, in other words) muzzleloading black powder pistols. Values will vary with condition, but are not high- typically $75-$125.
Flli Peitta is an Italian gun maker specializing in reproduction black powder and single action guns. Imported in the US by several retailers and distributors.
A gunsmith
To even begin to give an accurate value I would need to know the condition, maker and model. If you don't know these you can compare yours to ones in the on line auctions, like gunbroker and auction arms. Look up black powder pistol or black powder revolver and compare photos to what you have, that should help identify and give you a value as well.
Request a manual from the maker.