Based on what I've read in this forum about RL33, which is quite a lot, it will be on the slow side for a 250 gr bullet out of the 338 RUM. It does very well with a 300 gr bullet in a 338 LM Imp and it might wok well with a 300 gr bullet out of the 338 RUM. However the RUM is about 10 gr or more, less capacity than the Lapua Imp. 33 does best with heavy for cal bullets.
You will never know how much it has cost my generation to preserve your freedom. I hope you make good use of it.
~ John Quincy Adams
Although I run 284RUM, as opposed to 338RUM, I'm thinking that maybe I ought to chime in. After hearing numerous people [including a 6.5/284AI BR guy that I was shooting along side, at a public range] talk it up and recommend that I try it, I finally did. I really wasn't expecting the results that I got. In order to achieve the same velocities and pressures that 95gr of RL-25 produced (that's the absolute max charge, using 139gr SSTs), it took an incredible 110gr of the 33 (It was given that # designation because the 338 is what it was specifically desgined for).
The only way to get 110gr into a  RUM cartridge is to fill it to the top of the neck, tap it numerous times so that the powder settles down to maybe 1/2 way down the neck, then top it off once again with additional powder. Obviously, seating of the bullet will then yield a highly compressed charge. Now you may (probably DO) think that sounds totally nuts (it is actually MY fist foray into such territory), but pressures were closely monitored, and DID NOT exceed those generated by 95gr of 25.
Now 95gr of RL-25 IS a few grains above the optimal accuracy charge. But my hopes that the equivalent charge (110gr) of RL-33 might be able to produce better group sizes simply did NOT happen.
In summery, RL-33, which also seems to produce much more smoke than 25, did not show ANY advantages for the 15 additional grains of powder "hoggishness" that it necessitates. I'm not all that surprised that it is too slow, even for 250gr 338 cal bullets. Maybe it would work OK in something with a 35-40" barrel?
P.S. Those results were produced in a 26" barrel - I will be stepping up to 32" in about 2 months.
IME your bullet is too light for RL-33. It will work with a 7 RUM with 180s and when Berger gets it produced, the 195.
I have used it extensively and it only seems to work best with the heaviest bullets for caliber. So far have tried 180s in 7mm/375 Ruger, 230s in 300 RUM, 110s in 257 weatherby, 105s in 6mm-284. Tried it with a 162 a-max in a 7 rem mag and it didn't beat Retumbo, actually ran out of powder space just like you did with your 7 Rum.
I'd say it WILL work well with a 300 gr bullet in a 338 RUM but doubt there an advantage with a 250 gr bullet.
The Reloader powders are given their number designation for their burn rate. 22 is slower than 19, 25 is slower than 22, 33 is slower than 25 and 50 is slower than 33.
True that higher #'s indicate slower burners. But similar to 33 being developed for the 338, RL-50 was given that exact # because it was developed specifically for the 50BMG.
What I don't know, is what the "33" designation on bottles of rolling rock beer is supposed to indicate.
P.S. - Although 33 was developed specifically for the 338, Alliant does state right on the label that it is "ideal for large magnum calibers". But unless I'm mistaken, if you read up on the history of RL-33 on Alliant's website, they do say that it was designed for the 338, which is why they chose "33" as the # to give it.