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

[AKShooter]

As someone that reloads the .375 ultra mag I had to check this out, I'm impressed didn't think you could really get any more capacity out of it. Do you think that short of a neck is going to hurt you? accuracy or barrel life wise? got any idea how much more case capacity you've gained?

My load for the straight 375rum is 98 grains of imr4831, 275 bitteroot runs 2950 from a 22" barrel, my stepdads rebarreled m70 classic, hammers brown bears. As a Master Guide/Outfitter he has yet to see something hit harder, walks all over the H&H loads he used to shoot.

[rattfink]

300 grains at over 2600 FPS is very impressive. Great reading and pics.

[yorketransport]

With the full power loads, I'm hoping to get 300gr bullets up around 3100 fps. I won't know until I try though.

The real goal was to get the 350gr Sierra MK to 2900fps. I don't think this will be too difficult.

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 throat. 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.

Now I just need to pick up some H100 or IMR 7828ssc to work with. H4831 is too fast for the heavy bullets in this case. Once I get all of my fire formed brass annealed, I get to loading up some more test ammo.

Andrew

[dirtking]

Im glad to see ya finally got your rifle! Looks good!

[358Hammer]

Andrew
The rifle does not appear to be a super heavy outfit. What is the recoil comparison like with the heavy bullets?
Where does the neck length fall into for length .200-.250?

I have been shooting a .250 length neck wildcat cartridge for a decade without issue as well. Holds the bullets just fine.

Neal

[yorketransport]

I'm glad it's finished too!

Neal, the neck is .268" long. Dave Kiff at PTG thought that this was about as short as I should use with a .375 bullet. I think it's about perfect for the 350gr MK. This neck length has it so the base of the bullet shank is even with the base of the neck when seated to just touch the lands. The base of the boat tail hangs a little below the shoulder, but not too much. This gives me maximum case capacity and maximum grip on the bullet. If this were going to be a magazine fed rifle there might be a problem with bullets slipping in recoil, but it's a single shot so I'm not worried about it.

I hope to get out and test some of the heavy bullets with full power loads next weekend. I worked all day today but hope to finish annealing the rest of my fireformed RUM brass, and start fireforming some of the 404 Jefferey cases in the garage tomorrow. I'll see how that works out .

The rifle as it's pictured weighs 12.5#. Recoil with the 300 gr fireforming loads was nothing! They felt very similar to the 10# 308 I was shooting. I expect that even full power loads will only recoil like a 30-06. I can't say enough about how effective those Muscle Brakes are! While I would never think of shooting it without hearing protection, I didn't need to double up with plugs and muffs. Just a set of plugs or the muffs alone was enough.

I'll post an update when I get done with the next round of testing.

Andrew

[yorketransport]

I got a couple more cases fireformed. This time they were 404 Jefferey cases. These are a little easier to form than the 375 RUM brass. No necking up, just run them into a 378 Weatherby die to neck them down enough to get the brass to chamber. If I try to neck them down in the 375 BME die, the necks will crush every time. The shoulder angle is just too sharp.


I only got about 25 of these done before my wife said I can't fireform them in the garage any more. It wasn't the noise, but the smell that she didn't like.

Hopefully I'll get out next weekend to do some more load work.

Andrew

[Jeff518]

my shoulder aches just reading this...But, Wow, I have to give you immense credit for following through on such a successful new cartridge!! (..fire forming in the garage!!??)

[yorketransport]

I would fire form them outside, but then I need to deal with the neighbors. The wife I can handle, but the rest of the neighbors could cause a little trouble!

I'm still planning to get out this weekend to test the 350 MK and some H1000. This is where it will start to get interesting!

Andrew

[yorketransport]

Well I got out for the first round of full power load development today. So far I'm happy with the results.

This trip was using H1000 and the 350gr MK. H1000 is not ideal. It's too slow to get max performance. I started with this powder because I figured I couldn't fit enough into the case to get myself into too much trouble. I ran the powder charges up to 115 grains, and even using an 8" drop tube the loads were over compressed. The good news is that I was able to hit 3100 fps with no pressure signs. I could cycle the bolt with one finger and the brass practically falls out of the chamber. 3100 fps with a 350gr bullet! That's not bad at all! Plus I'm still getting single digit extreme spreads.

The accuracy wasn't as good a#s I expect, but that had a lot to do with the over compressed powder charges. The bullet was hard into the lands and in my experience the MK doesn't work well when jammed. A faster powder should let me seat the bullets back off the lands a little bit and hopefully improve the groups. Right now I'm geting around .75 MOA. I know it will do better.

The next lesson learned is that I need a better recoil pad. :chuckle: I rarely complain about recoil, but with the full power loads this gun is "peppy" to say the least. Recoil is straight back, but still feels about like my 9# 375 Ruger, or at least mid weight 338 Win mag. The recoil pad on the HS Precision stock really wasn't intended for this sort of caliber. It came off a Savage LRPV in 22-250. The 375 BME is slightly bigger I could also add a little weight to the stock. Right now the rifle weighs just under 12# which may be a little on the light side for a target gun in this caliber. It's pretty muzzle heavy anyways. A little weight in the but would help balance it pretty well I think.

I'll post more results the next time I get out.

Andrew

[358Hammer]

Great information Andrew, thank you.

Recoil is a relavent thing at best. It is relavent to powder in which those gases can be sheared off. Plus bullet which one can not do anything about when the bullet exits the barrel. The 350 grain bullet in my Lott is quite comfortable. Fill the case with 3031 and one starts paying attention even with a very good brake. Put a full charge of powder behind a 500-600 grain bullet we then have something even with a very effective brake that I no longer find cool to shoot. Remember I shoot the Lott in a handgun so can't begin to understand why someone would put something like a 375 RUM improved to ones shoulder.

Neal

[yorketransport]

For a couple years, I was on a quest to get the maximum performance out of a revolver. I ended up with a 10" BFR in 45-70. I loved that gun, maybe a little too much. After nearly 1000 rounds of Ruger #1 level loads using the 440gr hard cast bullets, I developed compression fractures in my right hand and wrist. I sold the revolver (reluctantly) and switched over to the Strikers, and lite caliber Contenders after that. Now I'll just stick to my "little" 338x375 Ruger Striker as my big gun. The Lott is all yours for now Neal. I need at least another year to let my hand and wrist heal. It's still not right.

Now I just put all that abuse to my shoulder so I can end up with a detached retina. Some how that sounds like a better idea. :)

Andrew

[DukeShooter]

Dang boy! You like to scare people at the range don't ya! Very nice! I have been drooling on the 110BA in .338 lapua but you give that one a run! Makes me want to get my 110 rebarreled! I was thinking about a .416 but your little gem sparks some interest! I'm 155lbs soaking wet so the heavy set-up and muzzle brake would definitely be a plus otherwise I think if I shot it I would have to scoot my chair back to the bench after every shot! Have you done muzzle energy computations?

Fire forming in the garage? Gee you like shaking the rafters don't ya! My wife would keraaap her bloomers if I did that!

Got reamers that can be rented???

The Duke

[yorketransport]

Dang boy! You like to scare people at the range don't ya! Very nice! I have been drooling on the 110BA in .338 lapua but you give that one a run! Makes me want to get my 110 rebarreled! I was thinking about a .416 but your little gem sparks some interest! I'm 155lbs soaking wet so the heavy set-up and muzzle brake would definitely be a plus otherwise I think if I shot it I would have to scoot my chair back to the bench after every shot! Have you done muzzle energy computations?

Fire forming in the garage? Gee you like shaking the rafters don't ya! My wife would keraaap her bloomers if I did that!

Got reamers that can be rented???

The Duke

I have been accused of just going to the range just to make everyone else mad!

I did get a little more load work done. I backed the 350gr MK down to 3050fps for now. I still need to work on the seating depth to fine tune the groups though. I may need to switch powders though since H1000 is heavily compressed with the current load. I really can't seat the bullets in any deeper and maintain an accurate OAL. The muzzle energy of a 350gr bullet at 3050fps is up over 7100 fpe. That's enough to get the job done! Using Sierra's published BC of .805 for the 350gr MK, with a 100yard zero and a muzzle velocity of 3050 the drop at 1000 yards is 20.44 MOA. A 338 Lapua with a 300gr MK at 2800fps and a 100 yard zero drops 25.45 MOA at 1000.

The current load I want to get ready is a 300gr Accubond for hunting. I did some tests using the 300gr Sierra Pro Hunter and Reloader 17. This time I was using fireformed 404 Jeffery brass in stead of the Ultra Mag brass. I won't say how fast they were going, but it's fast enough that they can't be very safe. No pressure signs, but I just didn't feel like they could be safe. I only fired 3 and I plan to pull the rest until I can verify the info. Hopefully I'll get out next weekend to do another round of testing.

Recoil isn't that bad. I'm only 150# myself and I don't get tossed around too bad. The weight of the gun and a really effective muzzle brake help a lot. In fact the gun tracks surprisingly well in the bags. Recoil is straight back and feels similar to a 300 mag. As for fireforming in the garage, it's not that loud. It sounds like a 22lr going off. But I bury the muzzle in a cardboard box full of old clothes to muffle the sound and contain the mess. High velocity Cream of Wheat and toilet paper can be pretty messy!

Andrew

[M.O.A.]

my shoulder aches just reading this...But, Wow, I have to give you immense credit for following through on such a successful new cartridge!! (..fire forming in the garage!!??)

why does everyone think this is so strange its just blasting off powder no bullet ;-} great looking round looks like a real thumper

[yorketransport]

I went out to do some more testing today. I'm chasing some accuracy problems now. 2.5"groups at 200 yards is unacceptable. I wasn't having a great day with any of the rifles today so there was not point in burning up ammo. I know something's wrong though when the 375 Ruger Hawkeye Alaskan, with a 2.5-8x scope, out shoots everything else at 200 yards!

I need to do some trouble shooting and then go out and try again.

Andrew

[mckenziedrums]

I apologize for bumping the zombie thread but google was failing me... Did you also build some Winchester rifles out with a 500 BME? Found a rifle and the seller swears it is a 500 BME and while at first I thought they meant B&M I stumbled across this thread.

[yorketransport]

Not sure if I'm too late to help you or not, but there is another line of cartridges called the Black Mesa Express. They were based on the 404 Jeffery case and are very similar in performance to the RUM cases.

[mckenziedrums]

Nah, hasn't come up at auction just yet. Truth be told I'd prefer the 404 based caliber for the lack of the belt. I think I came across that one too but I'll take another look. There's always the chance this was some kind of one off.