BTW: your primers should work fine, if they are still good. While I prefer Winchester Large Pistol, Rifle will be fine. Mag Rifle may push the powder up the barrel without ignition (NOT A GOOD THING!)

You current "unknown" powder charge should be safe in anything chambered for 44 mag. Case life won't exist ,but you could shoot them in in 444 Marlin also ( originally that case was developed from 303 British too.)

Back in the 1880 there was a cartridge called "44 Evens Extra Long", it was a 44-40 shot shell (something very similar to yours) then left full length and loaded with a 200 grain LRN bullet. Very early wildcatting! In worked in the Evens lever action rifle, but not in the 1873 Winchester. So it is lost to history.

Annie Oakley used shot shells to break the glass balls in the Wild West show when indoors or in the tent (outside she used something like a reduced velocity rifle load) I'm still thinking about your shells as Rat Medicine.

Ivan