For starters, I would forget about the powerbelts out to 150-200 yards and certainlythe 245 grain. They will not group that well at those ranges and really run out of gas quickly.
You will be very dissapointed if you shoot a deer on up with the 245 PB and even the 295 PB. They just fragment too rapidly and don't deliver the terminal impact.

The key to accuracy with ML's is consistancy and patience.

Everything about your shooting procedures need to be consistant (cleaning in-between shots how and with what, loading pressure and seating force, primers, powder charge, barrel temperature, projectiles, etc). The more consistant, the better your results.

I am getting the impression that perhaps your frustration is getting the best of you a bit. Don't fret, it happens. Just settle down and focus on your shooting procedures.

Also, are you certain you did not pull a shot and throw off your groups? It happens as well. I develop loads with a locking benchrest (HySkoreUltimate Sighting Rest) with a remote triggering device. That way I know exactly what is happening downrange without my causing problems or any sighting variability.

With pellet's, you are going to be limited in tuning your loads, but you should be able to obtain good accuracy with two 50gr T7 pellets. I would opt for loose powder myself as I have always found the best accuracy somewhere inbetween, not to mention loose powder is much cheaper. But to get started and to keep things more simple, perhaps stick with the two pellet approach.

I would play it safe to begin with and stick with Thompson/Center projectiles to begin with. If you end up calling T/C they will not be able to help you with any other projectile type anyhow.

The most reliable and accuracteprojectiles I've foundfor the 50 cal 1:28" Omega/Encore/Black Diamond is:
240gr/44/50 TC Mag Express/Hornady XTP
240gr/45/50 TC Mag Express/Hornady XTP
250gr/.452" TC Shockwave/Hornady SST

The problem you will find with pellet's is dialing in your best accuracy along with the highest velocity. Three 50gr T7 pellets may not group at all, plus you and your rifle will not like the recoil.

If you go the route of loose powder (I would), go with either 777 2FG or 3FG or Black Mag'3.

For your Omega, with the 3FG powders, start about 80gr and top out about 110gr. For 777 2FG, start out at about 90grains.

Last weekend from my Omega, I was putting the 250 Shockwave/ Hornady SST inside of 1.5" on averageat 100 yards with 95gr Black Mag'3 and the Remington Kleenbore primers.

Same thing with the 45 cal Hornady XTP/TC Mag Express and 85gr 777 3FG & the Remington Kleenbore Primer. About 1.5" on average.

I swab in-between each shot with a damp patch(not moist or soaking wet)50/50 mix of 99% alcohol and Windex followed by a couple of dry patches. Bone dry clean bore.

If you find yourself grouping around .5-2" on average at 100 yards, then that should be considered excellent accuracy. Especially if not shooting from a locking bench rest.

For the 200 yard shooting, I would seriously practice alot at that range. You will need to sight anywhere from 3-6" high at 100 yards depending on your projectile and powder charge.

Three shot groups.

Good luck!