I have an older marlin 444 with a micro groove barrel. It chambers and shoots well, action works flawlessly, except when I open the chamber slightly to check if there is a round chambered and try to reclose it, it will not reclose on the chambered round. My brother in law has a newer 450 marlin and it will rechamber a round after the action has been broke open. Is there a problem or is it a model feature of the older gun? I reload and trim the cases to the published length before each reload if that helps.

This being an older gun, I'd be inclined to believe the action is a little worn! I had one once that would do that. So, when I checked for a loaded chamber and it would bind up, I would open the action to a point just before the round would flip out of the chamber and that took care of it. You might also try some a little graphite lube in the action.

This is just a guess, but it may be the next round in the magazine is being release a bit early and jamming against the chambered round. This theory would be all wet if it still does it with one round, preferably a dummy, in the chamber and an empty magazine tube. Then you have another problem. If it doesn't jam that way then most likely a worn/dirty part and sometimes a total disassembly and cleaning will cure it. Good luck.