As a long time Ultimate Firearms owner, the RUM's daddy, I'm going to give you some advice. DO NOT shoot BH209 through your RUM. Remington will not warrant the rifle and breech plug if you gas cut it. This I will tell you also, it isn't a matter of "if", its "when" it will start gas cutting the breech plug. Your shop dealer should know better, and just because your friends haven't had a problem yet, it certainly doesn't mean that they won't. Sooner or later, my bet is they will.
If you gas cut a breech plug, plan on sending the rifle back to Remington for the replacement/repair. Also, plan on anywhere from 6 to 16 weeks to get the rifle returned. Now that would be a great thing to have happen a week before hunting season.
Gas cutting can be caused from a couple different issues, but the nitrocellulose properties of BH are a factor. Next is improper head space, along with PP brass primer carriers. Some of that brass can vary in head web thickness as much as .020". Its impossible to get a complete seal of the brass to the nipple with PP brass like that. Some of the brass is known to have the flash hole off center by up to .010".
The absolute best advice I can give you, is to send your rifle off to one of three builders and have your breech plug replaced with the new ASG breech plug. Doing so, eliminates the brass quality issues, the plug won't gas cut and you can shoot BH209 without any fear of breech plug issues. The new Arrowhead Sporting Goods (ASG) breech plug is extremely strong and is more than capable of RUM maximum charges.
It'll save you a ton of money over time and one heck of a lot of work and headaches. Guys are starting to get their RUM rifles done and just love the new breech plug. ASG has now created the breech plug for the Ultimate Firearms Inc. rifles. I sent mine off last week, its complete and should be on its way home tomorrow.
The three builders are: Arrowhead Sporting Goods, LR Customs and Bestill Creations.
PS...…….. To KNOW your scope settings, shoot the different ranges and determine the additional MOA you'll need. Let them know what those are and they'll make the custom turret. I've used the custom turret, but would rather dial now.