Hey topshot
I REBARRELED a 338 win mag into a 338RUM which is essentially the same as reaming your existing barrel out. I just got tired of pushing the 338 win mag hard to get any kind of velocity, lucky to get 2800 fps with a 225 gr bullet.
There are a couple of things you need to consider and you may have already done so. One is that the 338RUM cases are longer and your magazine may be a limiting factor on OAL
Mine had a long detachable magazine (Beretta Mato) at 3.685" and most Sako's I have had or loaded for had long mags also at 3.730" so it may not be a problem. But, in order to fit in my magazine, I have to seat pretty deep at 3.600", which is actually the manual recommended OAL.
The other thing is that I wanted to be able to seat close to the lands so I had the gunsmith ream the chamber so it did not have a long throat and I could actually seat close to the lands. In my chamber the distance to the lands is:
225 TTSX - 3.628"
225 AB - 3.620"
225 IB (Interbond) - 3.580"
This had a dramatic effect on chamber pressure and I can not load to book max and keep the primer pockets on my cases. When I first started out I loaded a string going up to the book max for 225 gr bullets up to the max of 98 grs of RL25. I got good accuracy
but my primer pockets were toast on those loads. I have heard the 338RUM Remington brass is soft so that may be part of the reason.
Now I am down to 94 gr RL25 in order to keep my primer pockets and still getting good accuracy but velocity is down to 3050 to 3080 fps. That is just fine with me since all I wanted was to hit 3000 fps anyway.
So if you have the barrel reamed out to 338RUM and you come to the decision about how long a throat you want, you may want to tell the gunsmith to ream to factory specs and put up with the jump. That way you can use manual loads and shoot factory ammo.