I don't think it would be a great advantage to start drilling the Shot cavity to throw a full load. An adjustable bar would be a better choice, since you will encounter a fairly wide variation in shot density. The amount of Antimony and other ingredients would help determine the density. That can vary between manufacturers and lots from the same manufacturer. I just make sure I'm not over and load them up. The extra few pellets might not make that big a difference. If it bothers you, then try the adjustable bar.
Quack Shot has my vote. I purchased the adjustable bar and it has saved me time and money, just a few minutes and I can go from a 7/8oz load to 1 1/8oz load. No more need in buying up the different bars and powder bushings.
I've got an even easier solution. I started out in reloading with a normal fixed bar for my MEC. Once I found that I needed a different load, I then switched to an adjustable bar that I found to be a pain, as it would take me about a half hour to get it to drop what I wanted. I then bought a new (used) loader, and when I received it, found that the fixed charge bar had been bored out on the shot side to accept Texan shot bushings. So, I went and found a couple of Texan bushings and made a couple of my own that I could not find. Now, if I want to switch loads, I just pull the bar out, drop in the shot and powder bushings I want, replace the bar, and back to loading. Hope this helps, Josh.
Long back I found my situation much like Bill's. The MEC Bars I find to be pretty accurate. I can't say I've been off 1/16 oz. but, shot does have the ability to change slightly. Not enough for me to worry about. I find myself checking my powder drop @ every box or two. I bought a powder baffle way back when I went from a single stage to progressive, I like it. Should buy another for my single stage.
I know I'm not alone on this but, I don't like the Universal Charge Bar. Bought it with my old JR., I find it to be a hastle. Between bars and bushing you can get close enough and ever so slight differences don't mean that much. I use to find myself going crazy about my power drops. Talked with Allient people a few times over the years and they told me my worries are insignificant.
My 1 1/8th MEC bar was off about 24 pieces of shot. I took my 1 1/4 oz. bar and added 2 1/2 (or so) layers of electrical tape to the inside until it is now dropping a true 1 1/8th oz. ( 1.125 oz.) on my digital scale.
Don't drill it. Just take a little file and touch up the inside of the shot bar a little at a time until you get what you want. If you are shooting hard shot, the bar will be a little heavy if you then switch to chilled shot. I had the same issue with a 1 oz bar--threw very light charges. Good luck.
I might add that, back in the days when I used bushings I would use a counter sink and chamfer the top ID of the bushing until I achiever the weight I wanted. HOWEVER, I found that when I got a new lot of powder the altered bushing did not drop the same charge as the previous lot!!!
So, that is when I went to the adjustable charge bar.
My 1 1/4 oz. bar with electrical tape throws within 2 pellets of 1 1/8th oz. with #8's, #7 1/2's , and homeade 7 1/2's. Hope this helps also. Now, I just PULL and don't worry about my "lost" 24 pieces of shot.Have had 2 -25 straights since doing so(1 month)........in the WINTER/OHIO/BELOW 20degrees.I use nothing but Magnum shot.....never will use chilled (unless the economy gets WAY worse).
EE and dkunser have the answer. My press uses bushings. I enlarge too small holes until they throw what I want. I shrink too large holes with 3M brand Electrical tape. It sticks so well it will never fall out by accident. In fact, you have to work at removing it.
If you are that concerned about the 1/16 of an ounce of shot. MEC makes an ounce and three sixteenths bar which should get you your original ounce and one eighth load. That is what I started using. TJD
I think the ATA should go to number of pellets allowed rather than weight of pellets. That should solve all the problems with weight. I realize you will need a different number of pellets for each size of shot, but who cares? HMB