MEC 600 Jr in .410 shot drop issue

I have a brand new 600 Jr for .410. Wanting to load #7 1/2 shot, and can't get a consistent drop. My shot has been stored dry since I moved quite a few years ago, and haven't needed to load any for any of my other gauges since before I moved (12 years). Shot seems to have oxidized I guess, has a light gray appearance and quite a bit of dust. Using an Adusta-Drop-Charge-Bar from Ballistic Products. Tapping on the shot drop tube seems to get a stuck charge to release but the drop varies from 320 grains to 370 grains. Then it will drop 170 grains. Any ideas would be great.

In 12 ga they use a large shot kit which really means the step up to a 10 ga drop tube to accommodate #2 and up to BB for steel shot .

I don't know how you would get a large shot kit for a 410 . This may be a situation where the adjustable bar won't work because of alignment to the tube and chamber shape. You may need to have round holes lined up for a straight shot or maybe just a different funnel shape .

As a patch , play with the bar push timing . What I mean is move it in such a way that it sort of dribbles the first 1/2 of the shot charge then slap it or whatever your move is to finish the stroke to the powder drop into the bar . Unfortunately it will take a lot of focus on that one detail . I only have MECs in 20 2-1/2 ,16 ,12 ga set up for 2-3/4 lead , 2-3/4 steel , 3" steel and a 2-3/4" steel Grabber , the 20ga needs only a quick adjustment to load 3" .

I've resigned myself to brass cases or roll crimping for the 410 because used 410 presses just don't come up for sale .

I'll give the timing thing a try. This 600 Jr is much different than the 650 I learned on as a kid. I have one for 12 gauge, and a Lee for 16 and 20.
I know one of the things done back in the day was to machine the charge bars to accept Texan bushings. I have a boatload of bushings, but the charge bar I have that is machined for them is for my 650.

I have adjustable bars for all but the grabber 3 of them are for steel or multi metal . I bought a lot of loaded ammo when it was cheaper to buy with the same components that it was for me to load them . I think if I get back to ducking around I have enough steel to hold me 3-4 seasons.

My issue was being able to find 410 ammo. I've always saved my hulls, so I purchased the 600. I may have more than enough of 12 and 20 gauge to last me 10 years, 16 not so much.

Messed with it some more, but couldn't get repeatable results. I'm guessing you are correct about the chamber shape. The 7 1/2s are too large for that small hole and the get hung up. Probably work okay with #9s.

UPDATE: Had some hardware issues with my chargebar. Vendor I bought the chargebar from sent a hardware kit and I replaced the parts I had an issue with. I have it throwing shot charges with 1 -3 shot. Charges hang up in the drop tube sometimes, but I have a wooden dowel that I use to tap the tube and it usually drops.