My first BP rifle was a LYMAN 50 cal PLAINS RIFLE.
Like you, I had to go through a learning curve. I had to find out why my groups were always around 6 inches at 100 yards, rather than tighter. Many of the people in the Monashee Mountain Men were convinced the rifle was pretty but a dog. Nope, I borrowed some round balls off a fellow because my supply had fallen out along the rendezvous trail. I had everything else I needed, FFFG powder, caps, and carefully cut patching material. The patching material I had on hand was alter linen. I had purchased it from a fund raiser at the local Catholic Church. I had heard that it was the best patching material a shooter could ask for. It was lovely stuff, all around .015 thick and extremely consistent.
The round balls the fellow gave me, were wrinkled, with a huge sprue stub. Beggars can't be choosers and I was thankful to him for being kind enough to help me out. He probably had 5 pounds of them in his possibles bag. They were cast of pure soft lead of course.
I loaded up my first round and LO and BEHOLD, the rifle shot to point of aim at 50 yards. I was using a 75grain charge of well tamped FFFG under the round ball. It was stout but I hunted with 90 grains of the same powder, under the same ball.
At first I thought it was just a lucky shot. Nope, that rifle shot better than it ever had and the only difference was the poorly cast round balls.
One thing I learned that day, DO NOT TAP the side of the volumetric powder measure. It just doesn't make any sense to do so. This was pointed out to me by the fellow that gave me the round balls. It leads to inconsistent loads. Not only that, in the 50 cal muzzle loader, it just wasn't necessary. A grain one way or the other of the sweet load made little if any difference under 200 yards. He was right. When the bore diameters are that large the difference is not even noticeable.
It wasn't until you mentioned you were shooting an Omega that I understood why you were loading so much powder.
Your rifle is way stronger than my old Lyman, which still loves FFFG x 90 grains on my brass, volume powder measure.
Once it was pointed out to me the reason those ugly round balls shot so well, in about a month, the rifle redeemed itself and I actually became a competitor at the matches.
The round balls I was given, measured exactly .500 diameter. The balls I was casting with the Lyman moulds that were included in the package I bought new as a starter kit, were only .490 diameter. In most 50s, that ball would work fine. Mine of course had a ten thou larger diameter bore. It actually measures .507. The rule of thumb is, the patch and ball should measure about .015 over bore diameter to get a good seal and very slight obduration of the ball, when it is rammed down on top of the powder. It worked like a charm on my rifle and subsequent rifles thereafter.
Your Omega on the other hand is a whole different ball game and you are likely shooting a polymer encased jacketed bullet of 44-45 caliber.
Those bullet configurations shoot very well and make for a nice hunting round.
Back to your powder measure. If you don't have one of the adjustable volumetric powder measures, get one. The only other option is to make one up that holds exactly the volume of powder you use for your particular load.
I like the volumetric measure with the little brass funnel on the end. Two reasons, it eliminates spills and when it is a cold, rainy day, I can add another 5 grains of powder by volume easily.
One thing about a volumetric measure, it is darn consistent, unless you insist on buggering around with a good thing, by tapping it to get more powder into it. They were not made with that in mind.
BP cartridge loaders may be a bit more fussy when it comes to charge weights. I will be finding out shortly. Somehow, IMHO, a grain one way or the other isn't likely to make much if any difference shot to shot on the target.