375 RUM Improved (Box Monkey Express)

[yorketransport]

It took much longer than I expected, but my 375 RUM Improved is done. The official name is the 375 Box Monkey Express (my company is named Box Monkey Distribution Inc.) I just started fire forming brass today and I hope to start working up loads soon. The forming loads used 375 RUM brass and 94.5gr of H4831 under a 300gr Sierra GameKing. I'm getting a very consistent 2625fps with this load with a SD of only 12fps! I had to neck the brass up to 40 caliber and then neck them down to create a false shoulder. The next batch of brass will be 404 Jefferey cases fire formed with the COW method.

The specs are:
- RBLP action T&T'd by SSS
- HS Precision stock (I don't know how long I'll leave it in this stock)
- Sightron SIII 8-32x56 LRMOA scope
- CSR 4 port Muscle Brake
- 30" Brux 1-10" .375 barrel (barrel work done by Sin Arms)

Here's the rifle


On the left is a standard 375 RUM, on the right a fire formed 375 BME with a 350gr Sierra MK seated to touch the lands.


The line up 6BR, 308 Win, 300 WSM, 338/375 Ruger, 375 BME


375 BME next to a 308


This is the average 100 yard 5 shot group while fire forming


The next loads to try will be the 350gr MK and the 235 TSX.

Andrew

[carpsniperg2]

Looking good

[deerslyr]

Looks sweet! Did you use just a standard savage long action for that build?

[Dan-o]

Wow.....

Is this your 1,000 yard rifle for Big foots?

Holy cow, how many ft. pounds at the muzzle?

What do you plan to do with it?

Cool build.   I might be afraid to pull the trigger.  How's the recoil?

And, thanks for sharing!

[bobcat]

Should be an awesome long range ground squirrel gun!   

[Biggerhammer]

Well done! Gotta love the super magnums! No replacement for displacement. Give some IMR 7828 a try in that bad boy. Great grouping for a fire form load.

[724wd]

well done!  that's a SHORT neck!!   

[Jamieb]

What magazine? or is it a single shot?
I like that one.

[yorketransport]

The recoil with this load was pretty mild, about like a 308. These were very light loads though. The muzzle brake is incredibly effective but I expect it to feel like a 30-06 with full power loads

The 2600 fps I'm getting is well below what I expect to get from finished loads. H4831 is too fast to be ideal in this case with heavy bullets. I just had some left over and figured it would work well for forming brass. H1000 or IMR 7828 are the powders I plan to focus on. The goal is 2900+ with a 300 gr bullet I wont know for sure until I'm done though.

The biggest gain over the standard RUM didn't come from the changes to the case but to the OAL. The loaded round with the 350 gr MK has an OAL of 4.2". That frees up a ton of case capacity. Th short neck will work fine since this is a single shot. No worrying about bullets moving in the magazine. I'm not worried about barrel life, but I should be at least in the 600+ range.

Andrew

[yorketransport]

I just ran the numbers and the changes to the case dimensions only gave me about 3.5 grain increase in case volume. The real gain came from extending the lead. Here's a comparison of net case capacity (grains of water) using the 300gr Nosler Accubond seated to different OALs

375 RUM with an OAL of 3.6": 94.6 grains
375 RUM with an OAL of 4.2": 111.3 grains.
375 BME with an OAL of 3.6": 98.2 grains
375 BME with an OAL of 4.2": 114.8 grains.

I could have achieved similar results by just having the throat extended on a standard 375 RUM. But what's the fun of that? If you're going to spend the time and money to build something, why not make it a little different. It's the same reason I went with a 1-10" twist. The standard 1-12" probably would have worked just fine.

As far as what I'm going to do with this, I really don't know. I only hunt 5-6 days a year, and that's typically with a handgun. And there really aren't many places to shoot really long range on the West side so this won't accomplish anything that I couldn't do with the 300 WSM, or the 308 for that matter. But it will make a heck of a porcupine gun!

If this project works out well I may put together a matching pistol. I hate to see the reamer just sit on the shelf and go to waste. So if any of you guys want to put together the ultimate long range elk gun, let me know. The reamer is going to get awfully lonely just sitting there......

Andrew

[Nape.257]

Great looking rifle.

The componets are smiler to the ones I used on my edge build.

Cant wait to see some 1000 yard groups.

[Biggerhammer]

It's way COOL Andrew! I like rifles like that. May have to take you up on that reamer offer and rechamber a .375 Ultra magnum on a 700 and run a Wyatts center feed extended box to fit those bad boys down.

What's the contour/ barrel dimensions on that beast?

[yorketransport]

It's way COOL Andrew! I like rifles like that. May have to take you up on that reamer offer and rechamber a .375 Ultra magnum on a 700 and run a Wyatts center feed extended box to fit those bad boys down.

What's the contour/ barrel dimensions on that beast?

It's a 30", straight taper to .9" at the muzzle. Including the brake it right about 32" over all. It's pretty muzzle heavy right now, so if I'm not careful it will do a nose dive off the front rest!

I've been holding out on getting a different stock for it. I wanted to get a feel for the gun before deciding on a stock style. I'm starting to think that I may need one with a little weight in the butt to balance the rifle out.

Just let me know and I'll send both the 338/375 Ruger and the 375 reamers your way. They're just sitting here calling you!

I just finished annealing all of the fire formed brass. I have to say, Tempilaq has sure made this easier! Since 375 RUM brass is pretty much impossible to find right now, I feel like I need to be extra careful with my cases!

Andrew

[wraithen]

How many reamers do you own man! Sounds you need to open up a local business. Call it: Gunsmith of Savages. That way savage owners will know you will work on their stuff! 

[Biggerhammer]

York,

 You wouldn't happen to have GO gauges also, would you?