Len's new 7 LRM - first day at the range

[Len Backus]

No, I haven't shot any more at all. Can't wait for spring!

I usually think of Retumbo as needing a couple more grains of powder to get the same velocity as H1000...so that doesn't surprise me.

 

[brentc]

Len, I tried the Retumbo and the H1000 over the weekend in my 7mmLRM. From what I read, the Retumbo was going to produce more speed over the H1000 with the same powder charge. I wanted to deliver the 180 VLD's above 3000 fps. I loaded 5 rounds with 72 gns of Retumbo and 5 rounds with 72 gns of H1000. The Retumbo had an average muzzle velocity of 2985 with a 2994 High and a 2976 low. The H1000 had an average of 3028 with a 3040 high and a 3017 low. So i guess there is no real advantage of going with the Retumbo unless you look at the ES of 18 on the Retumbo and 23 with the H1000. I'm still testing both but the H1000 looks like it might be my powder for the 7mmLRM. Have you tried anything different from your original post?

Nope, the same charge weight is not how it works. Retumbo is a slower burning powder than H1000. You should see at least one extra grain of powder more with Retumbo vs. H1000. If you were to load retumbo to the same pressure as your H1000 load, you would see the increased velocity you've heard of. I'm betting you would see at least a 25 fps increase with a max load of retumbo vs. a max load of H1000. However, you should evaluate your load on more than just a velocity increase.

 

[SPR123]

Nope, the same charge weight is not how it works. Retumbo is a slower burning powder than H1000. You should see at least one extra grain of powder more with Retumbo vs. H1000. If you were to load retumbo to the same pressure as your H1000 load, you would see the increased velocity you've heard of. I'm betting you would see at least a 25 fps increase with a max load of retumbo vs. a max load of H1000. However, you should evaluate your load on more than just a velocity increase.

I understand that just velocity increase is not the only evaluation. How would you go about finding the "Happy" load for a new cartridge such as the 7mmLRM?

 

[brentc]

I understand that just velocity increase is not the only evaluation. How would you go about finding the "Happy" load for a new cartridge such as the 7mmLRM?

Here's a simple way that works for many. With Retumbo, I'd start with the equivalent charge weight to your H1000 load and work up in .5 grain increments until you start seeing pressure signs. (ie ejector marks, hard bolt lift) Once you hit pressure, back down to the next lower increment and evaluate accuracy. There should be an accuracy node close. Once you find the node begin to fine tune by changing seating depth, primers, etc. For expectation management, your Retumbo load should end up weighing 1 to 2 grains more than your H1000 load.

 

[moombaskier]

I understand that just velocity increase is not the only evaluation. How would you go about finding the "Happy" load for a new cartridge such as the 7mmLRM?

I went with something close to what Gunwerks came up with;-). They use 72 grs of H1000 lit with a Fed 215 primer....at least thats what they were using when they did the work for me. They were also .025" off the lands. My sweetest spot is .020" jump, 72.8 grs of H1000 lit with a 215M.

 

[calib]

.020" jump with what bullet?

 

[brentc]

.020" jump with what bullet?

180's in his signature.

 

[moombaskier]

180's in his signature.

Yep, it's the 180VLD. I wanted a cartridge that would shoot the 180VLD at 3100fps or more without having burn too much powder, fit in a std long action magazine and not be a barrel burner. The decision also was made based on the fact that Gunwerks was able to do the work and have the barreled action back to me in less than 2 weeks! The rifle shot sub moa during load development, but the load listed will consistantly put 4 shots inside of a 1" bullseye at it's 200 yrd zero.

 

[victorc]

Thanks for a great thread, Len. Thanks to all who contributed. I just ordered a 7LRM reamer from Genwerks today to build one for myself.

Over the past 10 years I've become captivated with the 7mm/.284 bullet and the factory casings pushing them. Living and hunting out west has made me a follower of the long range hunting order. My first rifle purchased was a Browning BAR Safari II w/BOSS in 7mm Rem Mag. Great gun, super accurate, killed several animals, great novelty rifle but not much good for long range hunting. Since then I've built other rifles around the 7mm Rem Mag, 7mm WSM and the 7mm Ultra Mag. The 7mm Ultra Mag being the most potent and accurate at extreme distances. On it I used a factory Remington 700 barreled action, Wyatt's extended box magazine, Timney Trigger and a VAIS muzzle brake all bedded into an H-S Precision sporter style stock. At an elevation of 5,000 feet and a temperature of 50 degrees it pushes a 180 grain Berger VLD at 3,250 fps. 3/4" to 1" three shot groups at 200 yards from a bench. Load is 92.0 grains Retumbo, Fed GM215M with a COAL of 3.820". From first hand experience I can tell you this bullet and cartridge combination is way to much for mule deer and pronghorn antelope.... like shooting prairie dogs with 95 grain VLD bullets from a .243 Win. But it's awesome on big bull elk and Alasken moose.

Now it's time to tame it down a bit and try this new 7LRM. Again, thanks for all your comments. Very helpful.

 

[Len Backus]

Good luck with your project. Please come back and report.

 

[AZShooter]

Len,

After reading your thead this past winter I partially blame you for causing me to make a 7mm/375 Ruger. I had a box of 375 Ruger brass I purchased a few years ago. I wanted to neck it down to 7mm but never got around to it. I followed this thread and it was the catalyst.

I decided to go about this differently than Gunwerks . I have a ton of dies and decided to see if I could neck down the 375 case with things on hand. I didn't try to move the shoulder for a longer neck.

Fortunately I have a Whidden 338 RCM bushing/FL sizer die along with 338 RCM Hornady dies. The RCM and 375 share the same shoulder angle as well as diameter. I began my quest to make the 7mm/375. After using six steps with a variety of dies I got to 7mm.

Ordered a .317" neck 7mm/375 Ruger reamer from PT&G. I have an old lathe and do my own barrel chambering.

I took a Redding 7mm Rem mag competition seater die and recut the body guide with my 7mm/375 reamer. It works great!

Turned the necks to .013" and used the Whidden 338 RCM bushing die with the appropriate bushing for sizing the neck. The only die I purchased was a RCBS 375 Ruger FL and seater set. I use the FL die for partial FL sizing.


First experiment was to rechamber a 7mm Rem mag. Barrel is a short 22" but it gave me a chance to try some things. Obviously it wouldn't give me the velocities but I could learn what powders would suit the case. Tried H1000 and then moved to RL-33 with a moly coated 180 hunting VLD. It shot well with the 180 going 2980 with 84 gr of RL-33.

A friend sold me a SS 30" #5 fluted Lilja 1 in 9 that was a 7mm STW with almost 400 rounds through it. After setback and rechambering it is 29 1/2". I began load development with the (no moly) Berger 180 target hybrid and 82 gr of RL-33 working up to 85 grs. 85 was slightly over the top. Settled on 83 grs for velocity of 3175 fps. Tested rifle out to 500M and it shot very well. Come ups matched JBM predictions using Berger's G7 BC of .345.

I realize the 7mm LRM has slightly less powder capacity but thought I'd share the results with RL-33 and 180 gr bullets. I would suggest if anyone wants to try this powder with the 7mm LRM to start around 80 gr and work up.

Your last comments on another coming back to report prompted me to share the above. Hope it is helpful to other 7mm/375 owners as well as similar variants like the 7mm-300 win mag or the 7mm Mashburn.

 

[Len Backus]

Thanks so much!

What an interesting project.

 

[AZShooter]

Yes Len it is fun.

I just installed another 7mm/375 chambered barrel on a third rifle last week! This one is on a Rem 700 using a rem 700 takeoff in 7mm Rem mag. The barrel is totally floated so all I have to do when it wears out is run the reamer into the next rem takeoff and install. Going to shoot the heck out of this package saving the long barreled one for big game only.

The first rifle's barrel was on the end of its life so I'll pull it and rebarrel to something else.

This rifle is in a Boyd's tacticool stock. Going to see if it suits me. The price sure was reasonable and the inletting was excellently done. Only $125! link:
Boyds: TACTICOOL REMINGTON® 700 BDL LONG ACTION FACTORY BARREL CHANNEL LAMINATE STOCK W/BLACK TEXTURED FINISH TactiCool

There is another benefit to the 7mm/375: There is no shortage of 375 Ruger brass!

 

[C.O. Shooter]

Can someone explain how the 7LRM compares/differences/performs against the 7STW?

Thanks