I bought a 338 Federal and I'm trying to break it in. I bought a box of Federal Fusion ammo to shoot and reload the brass. I can't find any more cheap ammo so I bought a cheap box of speer hot core .338 200 grain bullets and I'm gonna load up the empty pieces of brass to continue breaking in and to determine if the scope I've got on the rifle is not holding zero cause I'm getting horrible groups. My question is that I've only got Remington Large Rifle Magnum primers and RL 15 powder. I'm thinking that I might use the starting load of RL15 and use the Magnum primers. Is this possible or should I continue to wait until I'm able to find regular large rifle primers? I'd like to determine if the scope is bad and not holding zero and send it back sooner than later.

if those are the 9-1/2m primer, go ahead i use those in my 7x57's with excellent results.
since you are gonna work up a bit you should be fine.
i know this round is based on the 338 and the original wildat was the 338-08 so you may be able to use 308 brass,i use the longer pieces of 308 brass for my 358 win with excellent results.

Ive also had terrible groups with Fusion ammo in my Ruger Hawkeye 338 Federal. Maybe one day they will turn loose of unprimed brass in that caliber.