HI, what actual velocities are you guys getting from the 338 RUM mag and 338-378? With what length of barrel?
I've been thinking about building a long range gun and it seems that the 338 bore will give better velocities than the 30 with these huge overbore cases.
I already have a 30-378 reamer, but the one I did with a 24" bbl didn't shoot very close to the claimed velocity.

I've got a win M17 action and I'm thinking about a MRC 32" bbl or one from Pac-Nor. They go up to 40".

I need to keep the complete rifle weight under 16# to hunt with in Idaho.

HI, I have a 338 RUM it has a 26" barrel. I use only Hodgdon powders . H1000 & Retumbo are the only ones I chrono'd . I worked up to Hodgdon's max With H1000 & 210 Partions I get 3170 with the crono 15-20 ft from the muzzle. With Retumbo and 250 Partions I was still 1 gr. from Hodgdon's max and it was chronoing 3065 again 15-20 ft from the muzzle . There was no sign of pressure but I backed it off until it was chronoing 3000. Both have very low spreads Neither load is piticularly accurate running between a 1" - 1 1/2" @ 100 yards. I just ordered some 225 Accubonds to play with I'll see how those shoot. Good luck KH

Lar45: Let me add this insight for your decision making consideration.
My close friend and Elk killing "machine" Ben, had a 338/378 Weatherby Magnum custom Rifle built for Elkin several years ago.
His Rifle is built on a Ruger #1 action and sports a 30" stainless barrel - well it WAS 30" inches before he tried to tame the recoil of the relatively light Rifle with a muzzle brake!
The Rifle shoots pretty well and he has brought to bag quite a number of Elk with it as well as Mule Deer and an Antelope!
I was spotting over Bens shoulder from the prone position (and three feet behind him!) as Ben harvested a wonderful 7x7 Bull Elk here in Montana. One shot kill it was, but, the incredible muzzle blast - and shock wave of that single shot deafened me for the rest of that day!
Ben stokes up his Rifle pretty well for "max" performance and he gets three firings on each case before he replaces them!
He shoots his Rifle each year at our homemade 1,000 yard range for pre-season sight in AND he is always trying new bullets in this rig. This gets expensive at $2.00 per piece of brass and all the powder those cases hold AND the price of the top quality "premium" bullets he tests.
Ben got wind of the 338 Remington Ultra Magnum and looked into it ballistics wise and cost wise. Last summer at one of our range sessions he mentioned to me that if he had it to do over again he would go with the 338 RUM. I think he was miffed that he was on his third firing and was disposing of the brass again!
I just mounted a new scope last week for another local fellow that had a beautiful factory stock Weatherby Rifle in caliber 338/378 Weatherby Magnum. He dropped it off while I was away on a Deer Hunt. I did not find out what happened to his original scope for this Rifle. It was the middle of our Elk/Deer season here in Montana when he dropped it off so your guess is as good as mine what happened to the original scope? I dropped it off with his neighbor when I was done with it so again I have not heard what happened as yet to the original scope!
I am sure my friend Bens Ruger #1 custom 338/378 Weatherby weighs around 8 3/4 pounds! I am glad to see you are going heavy with your Rifle! That will tame some of the hellacious recoil that these cartridges create!
I have left a message for my main man to call me regarding his velocities but we are in the last 4 days of our Hunting season and he may be gone.
If brass prices are going to enter into your decision making then be sure and check out your sources for the latest prices of the 338/378 Weatherby as compared to the 338 RUM.
Good luck with whichever you choose!
Hold into the wind
VarmintGuy

I've got a Mark V with a custom Shilen Select Match barrel in 338/378 that I had built several years ago. It was originally a 30/378 from the factory but I got sick of the muzzle brake and wanted to step up to 338 so I had them put the biggest barrel on it that would fit in the stock in the longest length they could turn out which happened to be 29" finished. I can get between 3400 and 3500 fps with 200 & 210 gr. bullets, 3300+ with the 225's. 3000 fps with 300 gr. Matchkings etc. I was actually able to get some Barnes 160's up to 4000 fps. a while back. Anyway, hopefully that'll give you an idea of the velocities vs. barrel length. The gun is heavy, and still kicks like a mule but I don't have to deal with that nasty blast/noise from the brake. Let me tell you that I have no regrets whatsoever. The gun is a shooter, and will reach out there and touch'em. Oh, almost forgot, the gun weighs around 12.5-13 lbs.

I am using 99.0 H1000 with 225 TSX and getting 3050 out of a 26". I also have used Retumbo (with 250 Swift A-Frame)and gotten 3040 with very consistent spreads. Needless to say I am anxiously awaiting the 250 TSX)

Oh yes, this is a 338RUM.....

IV