The MEC 600jr is a great shot shell press and parts are available to convert them to load any of the common gauges.
But, the change overs are not convenient, easy or simple. You might do it once but never again.
I have four 600jr's, one set up for 12, 20, and 28 gauge as well as one for .410. When I was shooting competitive skeet, I loaded and shot all four gauges.
So, I recommend buying a MEC 600jr already set up for 20 gauge.
Powder i controlled by bushings that fit in the charge bar. MEC charge bars have s fixed cavity of the weight of shot desired (1-1/8 oz, 1 oz, 7/8 oz etc). There are universal adjustable charge bars available for the MEC 600jr. Then shot and powder charges can be dialed in with the charge bar.
I preferred the MEC charge bars for all gauges except .410 but with the universal charge bar, you do not have to buy a drawer full of bushings.
There are two sizes of bottles available for the MEC presses. The large bottle can be handy for shot as you do not have to fill it as often.
There is a baffle available for the powder bottle. It screws in line between the bottle and the press. It can help getting more consistent powder charges.
MEC makes a tray for stacking shells of insertion in boxes. As the shell comes off the press, put it in the tray. When 25 shells are stacked, the box slips over the stack of shells and you have a box of shells.
I have the primer feed system on two of my presses. It was ok but it took constant watching to make sure a primer was fed to the seating station. I found it as quick as placing the primers by hand. No real benefit on the 600jr in my opinion.
Make sure you always put the filling plugs in the bottom of the bottles after you refill them. When it comes time to empty the bottles, you flip the bottles and charge bar over. If the plugs are not in place, you will spill shot and powder everywhere. I guarantee that you will only do this no more than once.
Hope this helps.
