This has been 15 or so years ago so Iam fuzzy. I have 3 Marlin 1894 levers. A 24" octagan ballard 1-38 cowboy, a 20 " round 1-38 stainless and a 20" microgroove round 1-20 blued. IFIRC I saw very little difference between them. I shot a a range of bullets and loads including the 300 grain the OP mentioned as well as lots of 240 grain stuff through all three guns including my 7.5 redhawk. Range was 50 yards for a bunch of shooting then a few at 100 yards through the one rifle I was going to hunt with. I do not remember powder or amounts but all was fairly stout but not hot rod. I too feel the 300 is not worth it. 240s hit hard and penetrate plenty. Also 2 of the three 300 loads did not cycle well on my stainless gun. They were RNFP hard cast and OAL was just a hair longer than most my other stuff. One of the problem loads was commercial. The first handload was developed using the commercial OAL. The third load I modified and shrunk the OAL and problem solved. The only reason I bothered was the stainless gun I had put the XS sights and rail and a scout scope and wanted to take it and my revolver all with the 300s on a hog hunt. I had just finished getting the sights/scope mods done and I do remember being pissed that the stainless gun gave me this greif. So there could be older 1-20 microgroove Marlins out there. BTW have shot plenty of non jacketed through microgrooves with no issues.
When I was getting ready for this hunt I thought the 300s were going to be the DRT end all but truth be told the 240s are plenty. These levers are handy up close guns and I just don't see much value in pushing the 44 too fast or too big out of this type of action.
