Issues with my "new" 1950 Marlin 336A

[Bill in Oregon]

Took my first shots with the rifle yesterday, using cast handloads with John Taffin's 20 grains of 2400 under the Lee 358-200RF bullet. I started at 25 yards, as I had just installed a Williams Fool Proof receiver sight and Fire sight up front. Remarkably, withthe rifle on an inexpensive plastic rest and taking a 6 o'clock hold I put the first three shots -- loaded singly -- within a half-inch of the point of aim, and two bullets in one hole. Great, I told myself. I have a shooter.
Moved the target back to 50 yards, loaded five more and the second round hung up on the lifter, locking the lever up tight. Peeking down in I could see that the next round in the magazine tube had slipped out just far enough to catch the lifter and tie the whole thing up. I pushed it back into the magazine tube with a small screwdriver and proceeded to chamber the round on the lifter, fire, and repeat, with the same lock up, the same solution, etc. Maybe it was my getting a little rattled over the feeding issue, but when I walked up and looked at the target, I didn't have a group, I had a pattern.
Discouraged, I fired another five-round group with the same jams and same results on target. With just a few rounds left, I plinked offhand at our steel bison silhouette at 220 yards, went two for two, then one for three on a 12-inch gong at 220, walking the final shot down after striking just high. Hitting anything with iron sights offhand beyond 200 yards is pretty gratifying to this old Boomer, so that helped take some of the sting out of the accuracy and feed issues. I am still a bit baffled at how I made the long shots when the closer shots on paper were so disappointing.
Suspecting my loads and especially my cast bullets, I did my best to slug the Ballard-cut bore and got .360 in the grooves as near as I could guess with seven-groove rifling. The only sizer I have in this caliber is a Lee .358 push-through to seat the gas checks, so that is the likely main culprit on the accuracy front. I probably need a .361 cast bullet to get better results and the Lee mold drops them about .359 from my scrap alloy. Will try some jacketed loads here shortly and see if they do better, but I may need a levergun surgeon to fix that feeding issue.

[marlinman93]

What does your barrel groove diameter slug at? Hopefully you slugged it to ensure the bullets you got fit groove diameter?
The overall length is pretty critical to reliable cycling on any lever action rifle, so hopefully you're staying within the spec for OAL on loaded cases. If OAL is good, then there may be an issue with the carrier, or rocker on the carrier. Might be the end of the lever that contacts the carrier/rocker, but that's not a usual wear issue.

[Rockrat]

You might seat the gas checks with your .358" die and then powder coat the bullets. Should bring up to .360-.361" and might improve things.

[Blaine]

If I've had trouble with my hand loads, I'll shoot some factory stuff and see what's what and go from there.

[GunnyMack]

Scrap alloy, too soft and stripping lead in the bore/ leading ? I say this only because it shot well with a clean bore at first, granted it was up close. Go back to the range and clean between groups? Undersized bullets of course a possibility. The powder coating is a good option. Most likely not a crown issue.
Shoot some jacketed for accuracy testing.
I'd also look at springs, might be weak/ taken a 'set' over the years causing the hitch in its giddyup.

[earlmck]

I can help with the jam thing. That is the classic "Marlin jam #1" check this post for directions from me and from NKJ on fixes.
viewtopic.php?f=1&t=32740&hilit=+marlin+jam

As for the shootin' issues -- that load will want a pretty hard bullet for the 1800fps+ velocity levels it runs. Good alloy, or powder coat would be my prescription.

[Bill in Oregon]

Earl, a belated thanks for the guidance and the link. I will try a lower-velocity load with Unique -- someone on Cast Boolits said about 12 grains -- and will message Mr. NKJ to see if he would be willing to do that heat bend for me.