I have built some Ruger MkIIs into the 358 STA which is longer then the 338 RUM.
That said, I would not feel comfortable doing it with the 338 RUM for a couple reasons. First, there are really no good replacement magazine boxes for the Ruger for this larger diameter round and feeding may be an issue.
Also, the larger diameter case of the RUM would make me a bit nervous with the small bolt lug on the extractor side of the bolt. I am sure it would handle it but in this day and age of lawsuits, it would not be something I would chance.
You may want to look at something like a 338 STW or 338 Jarrett. I have the Jarrett reamer and would get you very near to what a 338 RUM would get you and feed much better.
The conversion however is not cheap and you may be looking at less money to just get a Rem 700 receiver in the long run.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
That's the way it was until one day I was having lunch with Bill Atkinson (A&M Rifle Co., Atkinson Barrel Co. and Bill Ruger's right-hand man for nearly 20 years). The question was simple: Could the standard Ruger Model 77 MKII action be modified to accommodate the .375 H&H without a lot of busy work? Bill pondered the idea for a moment and said, in his usual thoughtful manner, "I see no reason that it can't."
We discussed the idea a bit further, and I asked if he might be willing to do the job, considering that he was still working for Ruger full-time. If he did the work on his own time, there wouldn't be any conflict, and he agreed to give it a whirl. (Before the rifle was finished, Bill Ruger Sr. died and Bill Atkinson retired from Ruger.)
Since I had a Ruger Model 77 MKII .338 Winchester Magnum gathering dust at home, it was donated to the project, and Ed Shilen agreed to supply a .375-inch barrel with a one-in-12-inch twist. All we needed was a long magazine box and follower to fit the magnum case, both of which Bill had stashed in his shop.
Bill removed the Ruger barrel and used it as a pattern for the Shilen tube and mounted the sights. The new barrel was threaded and the chamber was cut with a reamer borrowed from Dave Manson Precision Reamers. The magazine well was cut, fore and aft, to fit the longer box, and the ejector and bolt stop were faced off to let the bolt back to the rear of the magazine, about 3/8 inch. Presto - a 7 pound, 4 ounce .375 H&H that looked exactly like a standard MKII save for the lightly beaded blue/black finish on the barrel was the result.
One other thing: I'm also reminded this conversion would work out well for the .300 Weatherby Magnum, which is based on the full-length magnum base. Because the big, fat, long Remington Ultra Mags won't stack up in the magazine box with proper geometry to feed reliably, a conversion to the Ultra Mags probably doesn't make much sense. Yes, a lot of bunk has been passed around in the last few years about the obsolete belt on the standard magnum cases, that they don't feed reliably, etc., but if you believe all that stuff, I have a bridge over at Lake Havasu for sale where the temperature rarely, if ever, strays above 90 degrees.
[Quote] from Scovill's article
"Because the big, fat, long Remington Ultra Mags won't stack up in the magazine box with proper geometry to feed reliably, a conversion to the Ultra Mags probably doesn't make much sense." [Quote]
range it,check the wind, dial in correction, aim and only one shot