The .444 Marlin was introduced in 1964, about 8 years after the .44 Magnum round. The .45-70 was of course, around, but not a lot of rifles were being chambered for it back then. In the 60's and until well into the 70's, the heaviest slug you could find for the .444 was the 265 Gr. Hornady, and you had to handload it. Now there are slugs up to 300 grains available making it much more useful, as well as powerful.
Brass is still available from places like Midway at a good price. After Marlin chambered the .45-70 in the same gun, sales of the .45-70 took off, and the .444 just sort of remained in limbo. Marlin has always sold enough of them to keep them in the catalog. What really made the .45-70 surpass it was the new high powered ammo from Garrett, and Buffalo Bore. They produce loads up to 500 grains for the .45-70, putting it into .458 Win. Mag. territory ballistically. Bottom line is the only "bad" thing for the .444 Marlin is the resurgence of the .45-70 stole a lot of it's thunder. It is still a hell of a lever action round. I picked up this Marlin .45-70 guide Gun a few years back. It was one of the first ones, and has the ported barrel. It's not too bad to shoot recoil wise, the ports help a lot. A lot of fun ringing the steel plates at the club range at 200 yards. The 405 grain jacketed Remington's really ring them loudly! Bill T.
