I ham using a 12 ga MEC 600 Jr that I've used for years. It has developed the tendency to jam the shot load. I mean after powder is loaded and wad is set, I cannot smoothly more the shot bar to the right to load the shot. It seems to be hanging up on a piece of shot almost every time. This is using a 7/8 ounce bar. What have i done? help!

Guess I thought all MEC's did this, at least mine do. I simply bang the bar with the palm of my hand to shear the offending pellet. The bigger the shot size, the more often it happens. If I'm loading 8's it almost never happens. In my experience you're not doing anything wrong.

I inspected the bar and it was perfect. The seat for the shot bottle however had a problem. There was a chip out of the rubber gasket in the bottom of the screw cap! I replaced the rubber gasket (just happened to have a spare!) and all is well!

If your charge bar is an old one (mine was) and doesn't have the white cushion plastic "dog bone" insert for the shot bushing, buy your self a new charge bar. Mine works soo much better.

I find raising the handle a bit before dumping shot is the answer on my MEC 600.
Just experience with my loader.

Reclaimed shot that has not been cleaned well will create flow troubles.
I spray dry silicone on the bars and primer tray and allow to completely dry, this seems help.

I have two 600s, three 650s, a Grabber, and a 9000H, and none of them stick unless something gets between the bar and frame. Once it was the brass washer under the powder bottle had moved because it was worn, and a few times shot or fine powder. But it's not normal.

I add powdered graphite to the lead shot to help it flow through my MEC loaders and PW loaders ...... Jent