Many moons ago I shot skeet with two friends that are talented and retired machinists. They were always making creative shooting stuff for themselves and each other which they always shared with friends.
One of them made several sets of SHOT bushings (7/8 oz. thru 1 1/8 oz.) to fit MEC bars. They worked exactly like powder bushings. We used them in 502 bars for Mec 12 or 20 gauge progressives but it would be the same for single stage loaders and any gauge.
Basically, he enlarged the shot hole in the bar. He then machined shot bushings whose O.D.'s matched the larger shot hole. The I.D.'s of the shot bushings were cut to whatever size necessary to drop 7/8, 1, 1 1/16, or 1 1/8 oz. of #9 shot.
It's such a simple concept I've often wondered why Mec doesn't offer powder AND shot bushings.
The same creative friend bought a used 366 loader that didn't include Hornady powder bushings. So... he machined an adapter bushing. The O.D. of the adapter fit the existing (and much larger) I.D. of the powder bushing hole in a 366 bar. The I.D. of the adapter was cut to match the O.D. of Mec powder bushings. The adapter allows him to use his extensive collection of Mec powder bushings in the Hornady 366 loader. Pretty clever, no?
I remember those well sissy!! A new bar to drop a different powder charge, or bore it out! Before I found epoxy, I even used lady's nail polish to build up the ID for less powder too.
I've set up 5 complete heads for my loader and change them out with a quick change screw which holds them on the machine. I can switch loads pretty quickly without going through all the guessing which bushing number to use and weighing powder drops to make sure. I still weigh the charges when using a new lot number powder, just to make sure.