Well Affordble is the Key word. I agree with everything Mooncoon said.
But if your trying to stay under say $1000 then its gona be some cartridge thats not so Easy to Load.
I like 44 Russian 45 Schofield 44 Sp capable guns.
Im listing a Remington 1875 soon Antique that shoots both 455 Domion Factory ammo and 45 Schofield loaded to 45 Colt factory veloictys.
But Afordable thats a Hard call.
What do you want it for? just a plinker or a knock down caliber !
The RF Adaptor guns are good for up close stuff.
44 Russian 455 Webleys or 45 Schofield are great power house guns in the antique world. Anyone says there not wont get out in front of one i garrentee that!
Frist gun a 455 Webley MKI or MKII would hook you for life, did me

well mine was a Enfeild NWMP MKII in 455 but my second gun was a Sweet MKI in 455.
Buy some cheep 32 RF and you could be turned off. Buy a good Quality RF gun tho and you should be Happy.
38 RF is still got a kick to it loaded in Dixie cases with the 22 Acron caps and a full load of BP.
You can buy a 38 RF gun for under $1000 easy to get them Dixie cases and 22 Blanks and 38 Cal lead wad cutters.
Hope this helps theres lots to choose from eh.