Chris, Roman is the usual font for gun/rifle work, either upright or italic (slanted to the right). All lettering is capitalised with the capitals being slightly larger than the rest of the letters. Keep it small, large lettering looks ungainly.
Depending on the maker Old English is also used, but this is very unusual (Rigby's use it all the time, Purdey's occasionally). Old English font is much larger than Roman.
Sometimes, very rarely, you may see block lettering but more often than not a machine is used for this. Small block lettering is very difficult to make look nice. My advice is stick with Roman and you will never go wrong.
Remember though, lettering is one of the toughest jobs an engraver has. Any misaligned or wonky letters will be seen immediately. Unlike a dodgy scroll which can hide amongst the rest of the design everyone sees lettering every day so know what is should look like. Are you working on the barrel? If so, make sure you are thoroughly adept at cutting lettering on convex surfaces. Also remember to back cut the serif lines before you cut the serif itself.
I agree. I think Roman is the best choice for barrels. Where does this lettering go on the floorplate? I also like Copperplate style. It's a classic block but I'm not sure if it's appropriate for every purpose.
My personal preference is Roman that has be squashed down just a bit. I like the look of the slightly shorter, wider letters.