375 RUM with 350gr MK


Well-Known Member
Jul 5, 2008
Has anybody tried the 350gr MatchKing in the 375 RUM? I have this urge to put together a heavy long range 375, but I don't really want to deal with the CheyTac case or action sizes. The RUM seemed like a decent option. I'm not sure how fast this case can move a bullet that heavy, but I'm expecting somewhere around 2700 fps.

I ordered a box of the 350gr Sierras and some 375 RUM brass so that I could put together a few dummy rounds as inspiration. Those should be here in a few days. I'm not sure what the OAL will be, so I don't know what action I would need to use in order to have a repeater. I'm not really worried about that though.

You might call/email Sierra and ask what twist is best for that long, heavy bullet at the speed you'll want to shoot it. A 10 or 11 inch twist may be what it needs.
i have a 338 edge now and i was thinking the same thing that the 375RUM with the 350 would be good long range i would have the throught cut deep so i could seat the bullet out . my action and stk is set up for single feed. When your shooting past 500 yds if you miss usily the criter you are shooting at just stands there and lets you try agin so a fast reload was not an issue for me . buy the way my edge is for sale.
I have sold my 338 edge and built a 375 rum with a 32 in barrel and I put a JP recoil eliminater on
Loads so far are 350 smk with 87grs imr 7828 and 91grs RL25 both shot about 1/2moa with RL25 being a bit tighter now time to put them over the crony. Ihave no pressure signs and expect to get arond 2800 fps with the long barrel that will get the 350 smk with a BC of .800 the same trijectory as my 300 mag shooting 178 a Max at 3150fps out to 1000 yds
Since that post I built a 375 RUM/404 improved to shoot the 350gr MK. I ended up with the 350s at 3050fps from a 30" barrel. I'm pretty happy with how it turned out but I really don't shoot it much. I end up shooting the Striker in 284 Win more. Go figure!

Here's a picture of a standard 375 RUM next to my improved version and my 375 next to a 404 case.

Is that what they call the box monkey? And what is your load I might rent a reamer and ream it out some but maybe ill leave it alone if I can get 2800 it will do what I want. My 338 edge pushed the 300 over 3100 and I backed it down to save on brass so I'm only 50 gr heveyer so ill se when aether gets better ill get more time at the range it is hard to find load data on the combo so I'm going from what I can find to start
Is that what they call the box monkey? And what is your load I might rent a reamer and ream it out some but maybe ill leave it alone if I can get 2800 it will do what I want.

I have to admit that this is pretty funny. I called it the Box Monkey Express (BME) after my business Box Monkey Distribution Inc. just for fun at first but the name seems to have caught on. To my knowledge I have the only reamer and the only barrel chambered for this particular version of the 375/404 Improved. It is listed by Pacific Tool and Gage as the 375 RUM Improved 40* I believe. I have the reamer print number if you're interested. A fully fireformed case holds 122.7gr of water on my scale. Honestly the biggest improvement over the standard RUM came from being able to seat the bullets out so far. My usable case capacity with a bullet seated just off the lands is 113.7gr water. My OAL is 4.175" and this isn't quite touching the lands. Definitely not going to work as a repeater!

For loads I've played with a few different powders but H1000 is pretty much my standard. I get .5 MOA accuracy, which I feel is more than acceptably for a caliber like this, and an average SD of 7fps from my loads. I've hit higher velocities with other powders but without pressure testing equipment the velocities just seemed too high to be safe so I backed it down to around 3050 fps. Lets face it, a 350gr bullet at 3000+ fps is more than enough for the average shooter! I imagine that Retumbo would work well too, I just don't have any to try.

If you have any specific questions about the loads you can shoot me a message.

I have a friend who loves his 338 Edge but is now looking at building a 375 RUM to shoot 350s. Can you tell me why you used the 404 brass instead of 375 RUM brass?
I've used both RUM brass and 404 brass.

375 RUM brass needs to be necked up and then necked down to create a false shoulder when fireforming. It's a lot of stress on the brass, but I have 50 pieces which I've formed this way. Fireforming them is easy though. I just load up starting load for a 375 RUM and have at it. They shoot very well while doing this and still give 375 H&H performance with a 300 gr bullet.

404 brass just needs to be necked down and fireformed with a little bluedot and some cream of wheat. I happened to have 60 pieces of Norma 404 brass sitting around when I came up with this idea, so this was a perfect use for it.

If I were to do it again I would probably just run a straight 375 RUM and have it throated for the 350gr bullets. That would save the cost and trouble of custom dies and a custom reamer and still get you some very impressive numbers. If I wanted more performance than that I'd just move up to something along the lines of the Allen Magnums. RUM brass is hard to find and not the best quality, 404 brass is hard to find and expensive when you do find it. The 375 Allen Xpress will give the same performance and use better quality Lapua brass. I'm running mine off of a Savage and at the time there weren't any Savage actions available to handle a Lapua sized case head. Other wise I would have gone with a 375/338 Lapua improved.

So the case length is the same as the RUM? what is the freebore on your reamer?

Case length is the same, but RUM brass comes up a little short once it's formed. Freebore is long!:D It's setup so that the base of a 350gr MK sits right at the top of the shoulder.
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