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]

Originally Posted by SPR123

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]

Originally Posted by brentc

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]

Originally Posted by SPR123

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]

Originally Posted by SPR123

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]

Originally Posted by calib

.020" jump with what bullet?

180's in his signature.