7mm Rem Mag: help with ES/SD reduction

[lennyo3034]

I have a Bighorn TL3 with a 30" 7mm Rem Mag barrel. It is very accurate, but I need help getting more consistent velocities.

I know there are better cartridges for 7mm, but until I shoot this barrel out, it's what I plan to run.

I am currently using 71.90 grains of retumbo in Nosler brass. CCI #250 primers. 180 ELD-m at 3040 fps.

ES has gotten as high as 50 fps. SD is over 10.

I've test different charge weights and seating depths already.

First thing I was to try is a different primer as that's the easiest. Any recommendations on which primers to try? What should I do next after that?

 

[dkhunt14]

I've always used Federal 215. I have always used a faster powder like 4831. Matt

 

[STOMP442]

How are the numbers when you back it off a touch? If I remember right, 71.0gr of Retumbo is max load according to the Hodgdon manual. The last couple 7 mags I have worked up have really liked 69.5 - 71 grains of Retumbo with velocites right around 2975 - 3000 fps. Ive been using CCI mag primers in Nosler Bras with Berger 180gr Hybrids.

 

[lennyo3034]

I did while working up to my load. Not any better. I'm loading mine quite long 3.5"+ and I'm not seeing any pressure signs. It is not a compressed load.

 

[Zero333]

Change primers to 215m.

I had the same issue with ES/SD in a 300winmag and cci-250. Changed to 215m and everything got better.

 

[lennyo3034]

That'll be the first thing I try then. Thank you.

 

[FURMAN]

How did you do your load development? Is 50fps as low as you can get?

 

[Slim13]

71 grains of H1000 gives me the best ES and I use CCI 250’s.

 

[lennyo3034]

How did you do your load development? Is 50fps as low as you can get?

I had the magnetospeed on while doing the OCW test. Rifle seemed slightly more accurate at 71.90. ES was about the same for all.

I will note that my magnetospeed sporter does not have a lot of clearance from my muzzle brake, almost touching. It's possible the issue is with the chrono. I've had the rifle out to 1200 and have not noticed vertical dispersion on steel.

 

[FURMAN]

If you did an OCW and did not find a good es I would repeat with a different powder or primer. It would be better to do a ladder at a longer distance(as far as you can out to 1000 yards).

 

[lennyo3034]

I'll try fed 215 first. If that doesn't work, I have some H1000.

 

[searcher]

I had a 7 Mag built and intended to use H1000 or Retumbo. Gun would not shoot well with those powders, despite bullet swaps. I was a bit dismayed - having spent a bundle on the rig and wondering whether it was capable of shooting well. After trying everything in my stocks, I tried RL22. One ragged hole, great SD's, etc. I use Federal Large Rifle Match primers.

 

[Bart B.]

The velocity tests I did with 30 caliber magnums resting on bags atop a bench also had big numbers. But shooting them slung up in prone produced less than MOA vertical spread at long range.

I don't think good numbers can be had shouldering rifles for velocity tests. Totally free recoil or barreled action bolted fixed in place produce the lowest numbers. I and a friend got a 60 fps spread on average numbers with the same rifle and ammo holding them different against our shoulders. Others have reported higher numbers. Free recoiling 308 Win rifles have got ES and SD numbers one third those shot shouldered atop a bench.

Such is life when recoiling rifles move back while bullets go through their barrels. It's a masked variable. If the rifle barrel whips the right frequency in the muzzle axis, it'll compensate for velocity spreads. Slower bullets leaving at higher angles to the LOS than faster ones. If accuracy is great with big spreads in velocity numbers, that's fine with me.