Clymer makes the reamer I have, or PTG can make you one. Really might only need a .300win mag reamer. All you are doing is lengthening the chamber body by .15 or whatever the dimensional difference is.
If you have a barrel vise and action wrench to remove your barrel, you can probably improve your barrel. I use a large tap headstock and wd40 for lubricant. Also own a RCBS Precision Mic and use it to set headspace. I figure that I want my chamber to be just about .0005" over the length of brand new unfired brass. Run the brass through a .300 win FL minimum sizing die (if you want a standard) before you mic it.
I am not a velocity chaser. Have never seen anything decent come out of Ruger when it comes to precision centerfire rifles. The .338/300 will get you about the same performance as the .338rum, 330 Dakota, or .340wby. I have improved 4 barrels for the cartridge; 3 rem 700 BDL sporter barrels and a Pac-Nor in Palma contour for my TRG42. Also have a .338 TRG-s that will likely be "improved", but hasn't yet.
I extrapolated my own data using the 250gr Sierra and Hornady bullets and XMR4350. Started off with 65gr and worked up to 78gr. Fired more of the 79gr than any other in the BDL and it was plenty accurate.
Last year I decided to standardize my gear. I am selling off all my 7mm and .338 brass and pulling the 200gr smk from my .300win loads. The 200gr will be used in .30-06 and the .300win cases I plan to keep will all be necked up. I have also setup a .338-06 in 22" barrel for walkabout rifle.
Plan to try some 300gr match bullets in the .338/300 TRG42.
I have a couple of spare .338/300 BDL barrels. If anyone wants to try the ctg just email me and we can get you squared away. email@example.com
As far as what works; in a BDL barreled action with my light loads, the .338/300 recoils very mildly. Would have expected much more from a non-braked barrel with 250gr bullet. I never saw any significant pressure signs.
What better magnum case is there than the .300 Winchester? A lot of match ammunition is mfd in the chambering, cases are cheap and plentiful, you get much more room to seat bullets in your magazine box, and you burn lots less powder.
Only thing I am thinking is getting a reamer with improved shoulder and no belt. Would be real easy to lathe off the belts (if you have a lathe) and I know Ackley shoulders are simply more beneficial to precision.
Alliant touts its Reloader 17 powder in the .338win. That stuff may really be The Ticket. Maybe it would be Cat's Meow in the .338/300 too? I wonder if any of the longrange benchers are using it in their .300/338s?
I know the .338/300 offers longer barrel life, cheaper shooting, and less hassle obtaining components. I also own a 274gr NEI flat point mold and intend to cast and shoot some of these in the months ahead. A 2200fps load with bullet cast in wheelweight alloy would likely rival the .50bmg for penetration. My .454 Casull with .345 gr ww cast bullets will penetrate 27" of Cottonwood slab and not deform.
If you aren't slaving to beat the wind any of the High BC .338 bullets can perform and give you great barrel life. I have owned two .338rum, .300rum, and a 30-378. While they were fun to own and shoot, (I never did detach a retina!), with barrel life in the 400-600rd range and getting under 70 loads per pound of powder just took the fun out of ownership for me. I am a hunter, not a stand shooter or a longrange game dispatcher. I figure the Mk4 scopes I own with 100moa and 75moa of adjustment enable me to shoot milder rounds and still get hits.
The beauty of the .338/300 is; if you own a .338win you don't even have to rebarrel, just ream it, neck up your .300win brass and go shoot.