Marlin 336 jams
How fast are you operating the lever? If may be that you are going too slow.
Is the round not coming back onto the lifter? I have seen a 336 that had the magazine tube cut a bit short, and the rim dropped down between the end of the tube and the receiver. I chamfered the front of the receiver where the tube enters, and it then worked fine.
Is the magazine spring weak? If the jam happens when you load only one round and try to chamber it, then this is a possibility. If you have a couple of rounds in the magazine, then try to chamber one and it jams, then it is probably all right.
The 336 has a fairly simple loading mechanism. When the lever is closed, the loading lifter is at the bottom of the receiver. This lifter is almost "T" shaped, and when it is fully down, it allows a cartridge to be pushed back by the magazine spring onto the lifter. When you work the action, the lifter is raised upward to guide the cartridge into the chamber while at the same time the bottom of the "T" blocks another cartridge from coming backwards from the magazine tube.
As long as the magazine tube has the correct spring pressure, it will push cartridges back onto the lifter, EXCEPT IF THE AMMUNITION IS OVER LENGTH. In that case, it may jam.
Measure a cartridge. My old Lyman handbook gives 2.70 loaded length for the 45-70. However the "Handloaders Manual of Cartridge Conversions" gives a loaded length of 2.55 inches.
What bullet weight are you using? 300 or 405 grain bullets are shorter than the 500 grain ones, and should feed better.
I would measure the cartridge lengths. If they check out, take it to the range. Load 3 or 4 cartridges into the magazine, then operate the lever smartly.....do not baby or drag it slowly. Without firing the cartridge, operate the lever again, and cycle all cartridges through the rifle.
It it operates all right, then you have solved your problem. If it still jams, then take it to a gunsmith, as there may be a problem in the feed rails, or in the lifter.
