45-70-405 or for heavy boars in open field being driven out a 45-70-500. This for USA, ammo available and couple 'moderate priced' doubles available from Austrian and Spanish shotgun makers. still couple thousand plain finish . 375 flanged/rimmed from British were the 'medium' bores for Africa, Indian Colonial shooters. German made/make double or sometimes drilling with 8x74R cartridge that has 9mm and 9.3 bullet variations. Texas has feral hog shooting, sometime driven into open and Tennesee. has a feral hop population that has Russian Boar DNA from old hunting club stock prior to WW1- war stopped most of boar hunting and some of boars escaped to breed the some of local escaped domestic sows of English breeds like Berkshire, Great White/yorkshire and the smaller razor backs that had free ranged 100 years. 45-70 is popular there in the lever action Marlin 1895 rifle or Guide Carbine and some double rifles are used as alternative to the double barreled shotguns with one slug, one buck load. double rifles or sometimes one rifle barrel, one shotgun barrel in over under or some of S by S start with .348 Winchester then go to 45-70 for medium power rimmed case. European hunting with drivers as in Sweden, Germany , England sometimes? 300 to 500 pound Boars getting annoyed at drivers and hunters could be dangerous- 9 or 9.3x74R with 15-18 gram bullets. would be good, 375 Flanged/R british Holland would be other readily available ammunition. and the American 45-70-405 may be available just like 30-30 . Some hunters use the .50 Kodiak double rifles, muzzle loaders that can take 100 grain powder charges and legal in some area where large cartridge rifles are not - Shotgun or muzzle loaders for some game like Iowa for deer. The Kodiak muzzle loaders have been used in Texas on feral hogs, 2 shots just in case beats single shot muzzle loader, If really worried, get a .600 express with very heavy bullet or the .577 double as used on Elephants- couple tons of energy at 200 meters, kick like mule expression will leave impression on shoulder.