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?

[yorketransport]

York,

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

I certainly do! And I have a die reamer for the 375 BME. Why pay $300 for custom dies when you can pay $120 for a reamer to make your own.

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! 

Right now I just have the 338/375 and the 375 BME. If I had my own lathe, I'd probably have a lot more. I just don't have time or space to have too many more projects going. Right now I have 4 project guns which need some attention. There's the 6mm BR pistol, 338/375 rifle, 358 Win, and now the 375 BME. Plus I have a 308 Norma Mag barrel which should be done in a couple weeks. I should really spend less time building, and more time shooting!

Andrew

[Biggerhammer]

You have to have them to shoot them!

[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

[Biggerhammer]

That's what I did with 450 Ultra Magnum, I ordered .404 Bell cases from Buffalo Arms. Used cornmeal to partially fire form them. Then seated a 500 gr Hornady into the lands, so the case head was pushed tight against the bolt face and ran a mid range load of Win 748 to fully form them.

[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 as I expected, 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. 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

[Biggerhammer]

Cool set up York! Maybe some RL-22 and a 5 thou jump or so. Welcome to the super magnum recoil club! My right eye is sometimes fuzzy for a little while after a bench session with my 450 Ultra Mag. Like you said, STRAIGHT BACK! recoil.

[yorketransport]

RL-22 and IMR 7828ssc are the next to try.  Then it will be a pile of RL-17 and a 235 TTSX. That should be screaming!

Andrew

[Biggerhammer]

You should be able to flat CRUISE that 235!!!!! It would make a great Rockchuck bullet out of that chambering.

[Frank7mm]

Do you still have the reamers? What load did you settle with? What Velocity? I was thinking of a 375 RUM but this looks like more fun.

[lghtnquik]

7500lb-ft of energy hahahahahahahahahahahahaha I guess why not!

[yorketransport]

Do you still have the reamers? What load did you settle with? What Velocity? I was thinking of a 375 RUM but this looks like more fun.

Sadly, I haven't had much time to work with this rifle yet. Just as I finished the rifle I had an unexpected business expansion which will consume all of my time for the next few months. I went from a one man corporation to buying two more vehicles and hiring two new employees over the course of 2 weeks. This now puts me working 6 days and more than 80 hours every week.  I'm glad that the business is growing, I just wish it wasn't at the beginning of hunting season!

Right now I have a load using 114gr of H1000 under the 350gr MK moving at about 3050fps with single digit standard deviations. The groups are not as good as I want though. It's hanging in the .75 MOA range and I really expect better. I need to fine tune the seating depth and charge when I get time. I've tried a few other powders (IMR 7828ssc, H4831, and Reloader 17), but none gave me the results I wanted. Reloader 17 was throwing 300gr Sierra boat tails at nearly 3400fps, but it gave pressure spikes, so I dropped that powder. The extreme spread was over 30fps but the groups were amazing. It's a shame that the powder was so unpredictable.

I still have the reamers and gauges, but I would probably just go with a standard 375 RUM unless you are just looking for something different. Most of the gains I'm seeing are from extending the throat to increase case capacity and the 30" barrel.  If you stretched the throat on a standard RUM you would be pretty close to the same numbers without the need to fireform. I will say the that I'm not happy with Remington 375 RUM brass. It's only good for 2 firing before the primer pockets open up. The Norma 404 Jefferey brass holds up much better and actually give a slightly longer neck, which is nice.

I probably won't get much done with this until after the Christmas rush has passed. Hopefully I'll have a little more time to work on it then.

Andrew

[coachcw]

Bad Ass ! enough said .

[Biggerhammer]

Try some of the BELL .404 Jeffery brass from Buffalo Arms. It has worked well in my .450 Ultra and is stout stuff. I'm going to neck some of it down , turn the necks and try it in my .338 Edge when I have some time after the hunting season.

[b23]

You're primer pockets are blowing out because your load is over 80,000 PSI and if your still jamming those 350 SMK's the pressure spikes up to almost 88,000 PSI !!!  That's using your 4.2 OAL and 114gr of H1000

SAAMI spec is barely over 63,000PSI for the 375 RUM

You might want to think about backing it off a bit before you hurt yourself because you're flirting with disaster.

[yorketransport]

You're primer pockets are blowing out because your load is over 80,000 PSI and if your still jamming those 350 SMK's the pressure spikes up to almost 88,000 PSI !!!  That's using your 4.2 OAL and 114gr of H1000

SAAMI spec is barely over 63,000PSI for the 375 RUM

You might want to think about backing it off a bit before you hurt yourself because you're flirting with disaster.

Your numbers would be accurate if I were using a standard 375 Rum case, but did you enter the actual dimensions for the case I'm using? It's also worth considering the throat dimensions I'm using are not the standard RUM specs either. I assume that you're using QuickLoad to calculate those numbers.

I haven't had any pressure problems with H1000, only with Reloader 17. I can't fit enough H1000 in the case to really get into trouble. That's one of the reasons I started with that powder. And I'm not the only one who feels that Remington brass is on the soft side. I do appreciate your concern for me though. I wouldn't be too worried about it though. This isn't my first day.  If somebody else wants to try these loads they're going to need a gun chamber for this caliber first.

Andrew

[b23]

I actually increased you case capacity by 4 grains.  Per your post your imp. version is only giving 3.5 grains over a standard 375 RUM.  You're throating it so you can seat the bullet out to 4.200 OAL , Ok, great but that just gives you more useable case capacity because you aren't taking it up with a deeply seated bullet, also taken into consideration.  So unless you have some hybrid designed throat, nothing else is different and you are wellllllllllllllllllll over max pressure which is why you are getting crazy fast FPS and only 2 go rounds on your brass.  I'm guessing the primers go in pretty easy after 1 firing and you probably can't keep them from falling out after 2. 

Remember, pressure shows in a variety of ways.  You probably aren't seeing any heavy bolt lift because you have a case that is nearly straight with little taper and improved type cases hide pressure signs like heavy bolt lift.  You're producing FPS numbers a 378Wby. can't match and it has a MUCH larger case capacity.  Your chronograph is telling you to back off but maybe you aren't listening

Rem. brass certainly isn't Lapua quality I'll give you that but it last longer than 2 firings.  Unless you're way over pressure. 

Feel free to shoot it however you choose, it's your life.....