Remington's Ultimate Muzzleloader Ignition Source features a U.M.L. ignition system, which uses a unique brass case with a Remington 9-1/2 large magnum rifle primer to completely seal the flash hole. This allows the shooter to load up to 200-gr. of powder for increased range, energy and accuracy. For use with the Remington Model 700 Ultimate Muzzleloader only. I believe saddlesore is confusing it with Remington® Kleanbore® Muzzleloading 209 Primers. These are 209 sized primers that have a lower amount of priming compound then 209 shotgun primers. They were developed for blackpowder substitute pellets (777, pyrodex, etc.). The theory is that a shotgun 209 primer when fired behind a pellet can push the bullet up the bore a bit before the pellets are ignited. Not only will this increase fouling (crud ring) but it also causes inconsistent ignition due to a variable sized space behind the bullet prior to ignition.This can affect accuracy. Western Powder specifically recommends against using the muzzleloader primers with Blackhorn 209.
