I gotta ask: in which Marlin and with what ammo are all these "jams" supposed to be happening ?
IMO, the ammo is the likliest culprit in the equation, especially if the .44 Mag or .45/70 is the platform. There are factory loads now with heavy bullets that are over the OAL spec previously standard. Remember? They had to make the cylinders longer on .44 Mag revolvers to accomodate them when their use became more popular and wide-spread, and some large percentage of the .45/70 loads using bullets heavier than about 425 gr. can't be seated to crimp in the cannulure without exceeding OAL parameters for most any repeating design.
In LA carbines and rifles, OAL is a critical factor for proper functioning and there isn't much leeway between "go" and "no-go": the room just isn't there without some expert reworking, and there are finite limits to what that can do.
The only feeding problems that I've experienced in my LAs break down into two categories: improper ammo and improper action manipulation.
None of the .357 carbines that I have will feed WCs reliably from the mag (no surprise) and have very different tolerance levels when OAL meets or tries to exceed SAAMI max even slightly. My Timberwolf is the most forgiving, and the Rossi '92 the least, followed closely by my Marlin 94. Both of the LAs need to have the action cycled "briskly" to do their best.
I've never had any feeding problems with my Marlin '95G, but I haven't done much shooting with bullets over 405 grs. For all of my practical needs, the 300-to-350 gr. JHPs and their cast solid equivalents do it all. YNMV, but if you really want to explore the 500 gr. world in a .45/70 you probably would be much better served by a Ruger #1, IMO.
While I haven't as much experience personally with .44 Mags in carbines, the rule-of-thumb used by most afficienados of my acquaintance seems to be that if it's too long to fit comfortably in the cylinder of a M-29, don't try it.
My Marlin 39M still feeds any .22 RF 100%, and God alone knows just how many rounds have been through it in the last 27 years or so. As long as it's kept reasonably clean and nothing breaks (or finally wears out), I expect that it will continue to do so long after I'm gone.