7 Mashburn Super Magnum Load Development

[truck driver]

I didn't think a standard M70 action was long enough to handle anything much over 3.40"or can you just change the mag box and put a short ejector in them?

 

[SJB358]

I didn't think a standard M70 action was long enough to handle anything much over 3.40"or can you just change the mag box and put a short ejector in them?

Yup, exactly. With the old push feeder I had I just put a shorter bolt stop in there along with taking the spacer out of the original mag box along with a new follower. Pretty easy deal. Should net you 3.6+ of space.

All of the P64, Classic and Post 70 LA M70's are the same length, it just depends on the cartridge on how much they are allowed to open up.

Winchester used the same action for the 22 Hornet through the 375 H&H for their guns up till 1964 and they really didn't change much till they made their first short actions later on.

 

[joelkdouglas]

2.735 CBTO shot better than 2.695 CBTO today.

The shot I pushed left was the 5th shot of the group. I was sitting at the bench, wondering why the 5th shot was claimed to be the tough one...and I pushed it. Or pulled it, or whatever. Even so I would take that errant shot in the field, it's good enough to kill a deer or elk out to several hundred yards. Looks like I need to shoot further and see how it does at longer distance.

On the chrono the average said 3175.

 

[SJB358]

WOW! Very nice!

Is that the 160 AccuBond Joel? Speed looks great as well.

 

[joelkdouglas]

Thanks Scotty! I should have said:

7 MSM
160 Partition
2.795 CBTO (0.02 off lands)
77 grains Retumbo
Nosler cases formed from 300 Win Mag
CCI250 primers

I honestly think it would shoot a bit better with more shooting. I'm out of practice


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[SJB358]

I think I would be A-OK with that load Joel.

Sounds like a perfect elk killer right there. I hear you on being out of practice. I need to get some ammo loaded for the MSM and get it ready to hunt myself.

 

[AFG270]

I think I would be A-OK with that load Joel.

Sounds like a perfect elk killer right there.

Joel, I was thinking the same thing, great elk load and nice shooting. That is a beautiful rifle, looks like you are all ready for elk season.

 

[joelkdouglas]

Thanks gents, I agree. I would put in two clicks right and go hunting with that.

Scotty, the pin gauges really helped identify the neck tension too. The velocity variance dropped to 13 fps max spread for the three shots over the chrono


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[SJB358]

It sounds like you have found a keeper Joel. I'd be happy as a clam with that load. Matter of fact that is about the same load I used in Wyoming the first year with the 160 AB's and it worked as well as I could want.

I am thinking now that we have the necks sorted out our lots of H1000 might actually act better and not give the pressure signs I thought we were getting from the powder lots.. Don't know for sure but I will be trying soon.

 

[joelkdouglas]

I'm going to give my H1000 a try for sure. This load came together pretty well but it's not perfect either!

Did you say around 75 grains?

I might put some of the Retumbo in a 25-06 case with a 115 Berger and see how that goes.


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[SJB358]

I'm going to give my H1000 a try for sure. This load came together pretty well but it's not perfect either!

Did you say around 75 grains?

I might put some of the Retumbo in a 25-06 case with a 115 Berger and see how that goes.


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I used 75.5 with the 160 AB's the first year. That was a 3200 FPS load in my old rifle. With the new lot of H1000 I was up to 75 grains with the 175 Partitions which ran about 3030 or so from the old gun as well.

I think Retumbo with the 115/120's is a pretty slick load Joel. I use RL25/RL26 with the 115 BT's and 100 Swift Scirroco's.

 

[joelkdouglas]

I wanted to go shoot at 100 yards again and I took some time today to get out to the range. Last week I had a pretty good result, but I was still getting ejection marks and I had a primer fall out of the brass, so I wanted to:

1) Back down another grain on the powder and see if the accuracy would hold up
2) If the accuracy held up, I wanted to chrono the load to see what the velocity would be
3) Test a couple of seating depths to see if there was any improvement

I achieved all 3 objectives by loading up 4 round groupings of 160 Partitions, 76 grains Retumbo, Nosler brass, CCI250 primers. I loaded up 4 x 2.740 inches CBTO, 4 x 2.735 inches CBTO, and 4 x 2.730 inches. I was willing to go back to the 77 grains Retumbo load, but I think it's too hot.

Now I really feel like I'm done with load development. Chrono reading on the 76 grains Retumbo was 3100 fps. Slower than what some others get with their Mashburns, but still fast enough for me.

2.735 inches CBTO:


2.740 inches CBTO:


I shot the 2.735 inch group first. Then I shot the 2.740 inch group. I didn't shoot the 2.730 inch group after because I was satisfied with the results. I will pull the Partitions and load them up again at 2.740 inches. No reason to waste good hunting bullets!

 

[DrMike]

Looking good, Joel. I'd say that will do quite well for hunting this fall (or about any time).

 

[Dwh7271]

Nice work!
Those should do for about anything you encounter.

 

[SJB358]

Nice. That looks great Joel. Still sorta surprised you are seeing pressure at 3100 with Retumbo. Makes me wonder if the Nosler brass is out of spec or something. No real matter since 3100 with a 160 is a powerful combo.

 

[joelkdouglas]

Thanks all!

Scotty I agree there's something going on different from your rifles. My throat is definitely shorter, and that could make a difference. The Nosler brass might be making a difference. The Partitions might make pressure signs at a lower velocity...I think it's probably all related. I have the RWS brass, 160 ABs, and 150 Scirocco IIs, along with the H1000 and IMR7977. I'm not done working at it, but now I have a load I can use this fall!

Also weird the velocity dropped that much--from 3215 or so with 78 grains, to 3175 with 77 grains, down to 3100 with 76 grains.

 

[SJB358]

Thanks all!

Scotty I agree there's something going on different from your rifles. My throat is definitely shorter, and that could make a difference. The Nosler brass might be making a difference. The Partitions might make pressure signs at a lower velocity...I think it's probably all related. I have the RWS brass, 160 ABs, and 150 Scirocco IIs, along with the H1000 and IMR7977. I'm not done working at it, but now I have a load I can use this fall!

Also weird the velocity dropped that much--from 3215 or so with 78 grains, to 3175 with 77 grains, down to 3100 with 76 grains.

I'm not surprised about the speed drop with Retumbo. Until that stuff hits the higher end of pressure it doesn't really speed up real fast.

I'd say you've got a great load. That's worth loading 40-50 of them and seeing what it does at distance.