I would suggest you completely forget about powerbelt bullets. Completely.
Here's exactly what you want, FURY BULLETS. https://www.furycustombullets.com/webapp/p/223/black-powder-muzzleloader
The
Universal Fit bullets are what the majority of RUM and UF shooters are now using. They are accurate and deadly on game.
You will not make a better change to your RUM than switching to the Arrowhead breech plug.
https://www.arrowheadrifles.com/product/uml_replacement_plug/
The exact combination shooters of the RUM and UF rifles are using with excellent accuracy is from using BH209, so you need that Arrowhead system. Once you have it, the 320gr Universal Fit Fury bullet and 106grs by weight of BH209. They're shooting bug holes with that combination.
If you are on FB, there are two dedicated sites for Ultimate rifle owners/shooters. You'll find that Remington Ultimate or in Ultimate Rifles and Conversions are monitored for only the RUM or UF and Conversions. There is no arguing or BS. If you have a question someone will answer it correctly or, they will tag someone who will. There are more than enough other sites to discuss other brands/types of muzzleloaders.
Forgot to include so I edited........ I highly recommend you consider an LR Customs T-Rex muzzle brake. I also suggest that if you intend to scope the rifle, you purchase a HIGH QUALITY scope. Recoil can be violent with heavy bullets and a full charge.
I've shot thousands of rounds through UF rifles. Good luck.