I shoot a TC Omega. 100 gr.777pellets and 250gr. SST. This combo shoots great with a 2" group at 100 yards.
This combo has taken many whitetails in the NorthEast.
This November I will be hunting Elk in Arizona.
Do you think I should increase the powder to 150 gr. and re-sight in for the Elk?

I would use a heavier bullet. Ron

I use 100 grains of 777 for elk and bear in CO. I started with 80 grains and worked up to 120. At about 110 grains my group starts getting worse. See if your weapon will shoot that much with accuracy if so go with the max load.

+1 More bullet and perhaps more powder. However, my Omega does very well with a 300 grain bullet and 100 grains of either T7 or BH 209.

Dittos on heavier bullet. Plenty of powder, but use a bigger bullet. 300 gr minimum if you use a solid copper barnes bullet, minimum of 350 if using a lead/lead alloy.

My current elk load is 90 gr of BH 209 and a 350 gr Hornady FPB. Know lots of guys who like 350 gr nosler partitions in a sabot.

The 290gr Barnes will work ok for you. Much better than the SST.

I doubt 150gr will be your most accurate load.

I have never hunted elk with a muzzle loader. But if I had to I would look for a heavy bullet, 300 grain or better. Then I would push that as hard as accuracy would allow. In the bullet design I would look for penetration, and then expansion as uniform as possible. So I would look at Barnes, nosler, shockwaves, or even a big heavy conical. I don't shoot an omega but I do have a black diamond. I do know someone with an identical rifle to mine took a elk with 110 grains of triple seven and a 375 grain all lead buffalo bullet ssb.

If I were hunting tomorrow, it would be shooting a 500 grain all lead conical bullet and 100 grains of triple seven out of my White rifle. Good luck with your hunt.

While 100 grains is the most efficient load for feet per second the most effective load with 777 is 130 or 135 grains my experience is that more than 135 is only detrimental to both accuracy and velocity.

I would seriously consider going with loose powder instead of those pellets. You would be able to go up in 5 grain increments with your powder charge and find out how high you can go without losing too much accuracy.
Good Luck

A couple of things I might suggest to you... Remember 100 grains of T7 pellets are not like a 100 grains of loose T7 or BH. Even a 150 grains of T7 pellets is not really a 150 grains of T7 more like 120 grains of the more energetic powders like Swiss BP, loose T7 and BH.

T7 pellets are manufactured to provide approximately the same energy as regular Black Powder or any of the Pyro powders. So theoretically your 100 grain pellet load is approxmately like 85 grains of T7.

Your Omega might do just fine with 150 grains of pellets, you will not really know until you shoot it. I normally shoot 110 grains of loose T7-2f from my Omega and have used 120 grains of loose T7 with a 300 grain Nosler Partition for elk. In most cases this is probably more than you might need but in the instance of an angry bull @ 100+ yards the extra may be needed, especially with just one shot.

In all honesty the 250 might work just fine in a lot of instances, especially shooting cows but again for instances where more might be better - I would feel better with a heavier bullet and at the same time a stronger bullet like the Barnes mentioned or a Lehigh/Bloodline. If Nosler were still making or you could find some .458-300 grain partitions they would be excellent also also. One thing I might suggest, if you were to decide to stay with a normal copper/lead bullet you might also look at a .458 bullet (45-70), they are built to perform from a rifle.

Kinda just thinking out loud... or in black and white