Hey LGB,
While I also have three Pacific/Hornady 366 progressive presses, I still have and use the two MEC Sizemasters and the one MEC Junior press. All of the three single stage presses have Universal Charge Bars and the Auto Primer Feed mechanisms.
As far as comparing the Sizemaster and the Junior goes, that is much easier for me than trying to compare the Junior to the Mini Grand which I have never even seen good pictures. Both the Sizemaster and the Junior have been around for many decades, and both have had some changes made to them. However, to the best of my knowledge, MEC still has replacement parts for the early machines as well as the new machines.
Both machines are simple single stage machines that work a shell from the first station to the last. Both are made of stamped steel parts, and while the Sizemaster is made a good bit heavier than the Junior, the fact is the Junior is built quite sturdy enough to last through several generations of users. The Sizemaster uses a collet to size the shell, and the Junior uses a ring sizer die to do the same thing. You can hear many folks state that the collet sizer is much better because it full length sizes the shell. The fact is that both methods of sizing full length size the shell, and neither can be proven to be better than the other. Also, Pacific/Hornady and a few other higher end progressive machines use ring sizer dies on their machines.
Other than the Sizemaster being built a bit heavier and having a collet sizer rather than a ring die sizer, the only other difference is the Sizemaster comes with the Auto Prime feature included in the price. The Auto Prime feature can be added to the Junior for about $40 extra. Do I like the Auto Prime feature on my machines? Yes, without any doubt. The Auto Prime feature alone increases throughput on the Junior without any effort by the operator to work faster.
Is a Universal Charge Bar really a valuable addition to either of these presses? It depends on what you are doing. If you simply make a one ounce load and use whatever grains of whatever powder and never change, then you really are fine just using a single charge bar and powder bushing for that load. However, if you are likely to be making different sized loads for different purposes, then a Universal Charge Bar can be quite handy.
If you already have a MEC Junior, don't worry about it lasting as long as its bigger brother the Sizemaster. The Junior will last for several generations of users, and it will produce the same quality ammo as the Sizemaster. In fact, I can reload a shotshell more times on the single stage loaders than I can on the progressives. When a shell starts to get a little ragged, it does not go through a progressive loader so well, and I end up throwing them out. There are a lot of those marginal shells which could be reloaded one or more times more if I would reload them on the Sizemaster or Junior machines. Also, if you already have the Junior and you find you have the $40, you can add the Auto Prime feature, and you will be happy with the results.
Keep in mind, however, whether you buy a Sizemaster that comes with the Auto Prime or you add an Auto Prime to a Junior, you will have to install the Auto Prime and make the adjustments to get it working properly. To me, this is very easy to do, but some folks seem to expect you wave a magic wand and it works just like that. It doesn't! You have to read and understand the instructions for installation and for adjusting it to drop a primer properly. Believe me, if you take the time to install it properly and get it adjusted so it works properly, you will find it to be a valuable addition to your machine.
Best wishes,
Dave Wile
