I will second what guncat said. I first took my JM out with winchester white box and was ready to sell it then. Part of my JM not working was my fault too. I'm horrible about not cleaning my gear good enough. Took it all apart, soaked everything, took the spring washers or whatever you call them off the piston and reassembled and mine runs like a champ. I'm using herters low brass and it mows through them like butter. The white box would stick in my chamber like crazy. Extractor even tore some of the lips off because they were stuck so bad. My gunsmith friend told me to use some of the finest steel wool and cross hatch the chamber. It still jammed.
Also had the lifter welded by a local shot so no more biting my thumb either!
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