7 Mashburn Super Magnum Load Development

joelkdouglas

Handloader
Jun 5, 2011
1,310
3
Started this in the rifles thread, but I'm moving it here to update. It's become a reloading topic!

First range trip yesterday (25 Apr).

Fb4iVjL.jpg


The weather in ND is not optimum just yet...and the winds matched that white stuff.

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It looks like she will shoot once I get her all figured out. This pic is fire forming brass loads. There are 10 shots on here, 5 in the upper middle shot at 25 yards. Then moved the target to 100, shot 3, adjusted the scope 2 clicks left, shot 2 more. These are all IMR4350 with 139 Hornady bullets. I think it's 65 grains IMR4350? I'd have to look at my notes.

Cilx6Ae.jpg


Then I cleaned her to get the Dyna Bore Coat process all done, and moved on to 160 grain Sierra Matchking bullets with Retumbo. I hooked up the Magnetospeed for some pressure information...
73 grains 2977 and 2941 fps
74 grains 3004 and 2986 fps
75 grains 3084 and 3086 fps
76 grains 3119 and 3134 fps

I started seeing ejection marks about 74 grains. But oddly, two of the cases with 76 grains don't have ejection marks. There was also quite a bit of resistance chambering the rounds. I think I probably had the Matchkings seated into the lands, making the pressure spike. I didn't have a good way to measure the distance to the lands without fire formed brass; I'll be doing that today.

Then I put 3 rounds of 76 grains Retumbo with 160 SMK downrange...still work to do. I will try and make it back to the range tomorrow, and I anticipate loading 160 PTs first.

JfnGz2g.jpg
 
I had a lengthy discussion with Scotty and Matt yesterday, most of it about brass prep. For an intro to anyone new to wildcats (i.e., me), when you resize brass sometimes you end up with results that would be abnormal compared to factory brass.

For the 7 MSM and Nosler brass this means donuts on the inside of the case at the bottom of the neck. These donuts are a thicker part of the neck, and they grip the bullet more tightly, significantly raise pressures, etc. When I told Scotty and Matt about the pressure signs at less than 3200 fps with the Mashburn Scotty asked right away about the necks.

I do have a Forster mini lathe, and it has an inside neck reamer attachment. A picture is worth 1000 words...

zticl23.jpg


Gzydztd.jpg


Voila, no more donuts. I also neck turned the brass to 0.014 inches per side, as it was 0.016 with only 0.002 clearance for bullet release.

I just thought I might mention the above brass prep, someone might learn a bit about wildcats and brass prep.
 
I had planned on starting with 160 PTs today, but after the discussion about the donuts and pressures I wanted to try 160 SMKs again. I don't want to start sending PTs or ABs until I'm confident I'm not wasting them learning about the rifle.

After the brass prep I loaded up 4 rounds each of Retumbo at 76 grains, 77 grains, and 78 grains. If you recall yesterday 75 grains starting giving me extractor marks on the cases, but I was confident / hopeful removing the donuts would make the pressure signs go away. Scotty and I also acknowledged it's possible this rifle is just unique and makes extractor marks easily, maybe a sharp extractor edge or something. I asked Pac Nor to check out the bolt and they did, they said there was nothing abnormal.

Sure enough, pressures dropped. 76 grains gave 3097 fps, 77 gave 3140 fps, 78 gave 3180 fps. At 78 grains the bolt started being a bit more difficult to lift, but nothing like "forceful" to remove brass.

All loads with 160 tipped Sierra Matchkings seated 0.03 off the lands.
76 grains Retumbo:
6YbK2Lr.jpg


77 grains
YswPx76.jpg


78 grains
cZzUwFj.jpg


The more powder I added the tighter the groups got, a bit. 3 shot groups are hard to write home about.
And It looks to me like this rifle wants the bullets seated deeper in the case.
 
Handsome rifle, to be sure. With modern powders, especially the slower burning powders, as you approach maximum, it seems the groups do grow tighter. Your experience is verifying what has been observed with many other cartridges. Those are showing real promise. Great report, Joel.
 
Very cool Joel, I hope to add to your thread soon.

It looks like you're getting good speeds and I'd bet you'll find accuracy soon as you start seating them a little deeper.

That rifle looks great.
 
Loading up some more rounds tonight for a trip to the range tomorrow.

I'm going to start with 160 Partitions and adjust some seating depths with Berger's seating depth method. I know PTs aren't VLD bullets, but the approach works for all types of bullets.

Cut/paste below from: http://www.bergerbullets.com/getting-th ... our-rifle/
-------------
Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

Shoot 2 (separate) 3 shot groups in fair conditions to see how they group. The remarkable reality of this test is that one of these 4 COALs will outperform the other three by a considerable margin. Once you know which one of these 4 COAL shoots best then you can tweak the COAL +/- .002 or .005. Taking the time to set this test up will pay off when you find that your rifle is capable of shooting the VLD bullets very well (even at 100 yards).
 
joelkdouglas":14tfzz2e said:
Loading up some more rounds tonight for a trip to the range tomorrow.

I'm going to start with 160 Partitions and adjust some seating depths with Berger's seating depth method. I know PTs aren't VLD bullets, but the approach works for all types of bullets.

Cut/paste below from: http://www.bergerbullets.com/getting-th ... our-rifle/
-------------
Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

Shoot 2 (separate) 3 shot groups in fair conditions to see how they group. The remarkable reality of this test is that one of these 4 COALs will outperform the other three by a considerable margin. Once you know which one of these 4 COAL shoots best then you can tweak the COAL +/- .002 or .005. Taking the time to set this test up will pay off when you find that your rifle is capable of shooting the VLD bullets very well (even at 100 yards).

That is usually too much of an increase with Partitions Joel.

Wish you luck buddy.
 
SJB358":20q4dshx said:
joelkdouglas":20q4dshx said:
Loading up some more rounds tonight for a trip to the range tomorrow.

I'm going to start with 160 Partitions and adjust some seating depths with Berger's seating depth method. I know PTs aren't VLD bullets, but the approach works for all types of bullets.

Cut/paste below from: http://www.bergerbullets.com/getting-th ... our-rifle/
-------------
Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

Shoot 2 (separate) 3 shot groups in fair conditions to see how they group. The remarkable reality of this test is that one of these 4 COALs will outperform the other three by a considerable margin. Once you know which one of these 4 COAL shoots best then you can tweak the COAL +/- .002 or .005. Taking the time to set this test up will pay off when you find that your rifle is capable of shooting the VLD bullets very well (even at 100 yards).

That is usually too much of an increase with Partitions Joel.

Wish you luck buddy.

Thanks Scotty--it turned out I wasn't able to get that much variance with seating depth anyway. I was able to get 0.01 off and 0.05 off, but I wouldn't have been able to get 0.09 off. So I loaded up 4 at 0.01 off, 4 at 0.03 off, 4 at 0.05 off. I think when I move to ABs I will be able to get 0.075 off, but it's gonna be tight. Almost all the bullet bearing surface will be in the neck.

I haven't noticed that PTs are that sensitive to seating depth anyway.

I also inside reamed and then outside turned all the brass last night. Even rounds that I had previously removed donuts I was able to outside neck turn a donut area off at the bottom of the neck.
 
joelkdouglas":wznd1jit said:
SJB358":wznd1jit said:
joelkdouglas":wznd1jit said:
Loading up some more rounds tonight for a trip to the range tomorrow.

I'm going to start with 160 Partitions and adjust some seating depths with Berger's seating depth method. I know PTs aren't VLD bullets, but the approach works for all types of bullets.

Cut/paste below from: http://www.bergerbullets.com/getting-th ... our-rifle/
-------------
Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

Shoot 2 (separate) 3 shot groups in fair conditions to see how they group. The remarkable reality of this test is that one of these 4 COALs will outperform the other three by a considerable margin. Once you know which one of these 4 COAL shoots best then you can tweak the COAL +/- .002 or .005. Taking the time to set this test up will pay off when you find that your rifle is capable of shooting the VLD bullets very well (even at 100 yards).

That is usually too much of an increase with Partitions Joel.

Wish you luck buddy.

Thanks Scotty--it turned out I wasn't able to get that much variance with seating depth anyway. I was able to get 0.01 off and 0.05 off, but I wouldn't have been able to get 0.09 off. So I loaded up 4 at 0.01 off, 4 at 0.03 off, 4 at 0.05 off. I think when I move to ABs I will be able to get 0.075 off, but it's gonna be tight. Almost all the bullet bearing surface will be in the neck.

I haven't noticed that PTs are that sensitive to seating depth anyway.

I also inside reamed and then outside turned all the brass last night. Even rounds that I had previously removed donuts I was able to outside neck turn a donut area off at the bottom of the neck.

I was wondering about that Joel. I am not sure why your rifle seems short in the chamber area, but so far it doesn't really look like it matters too much.

I think you have a great set up for the Partitions. Pretty sure you will be fine with only getting the 160 AB's to .075".

I haven't turned anything yet. I am going to shoot the fireform loads first and then tackle it.
 
I learned at least three things today. Also found a couple of seating depths with 160 PTs the rifle seems to like fine.

First thing I learned: That trigger pull is light. It caught me by surprise on my first shot, and I think that shot was a pretty significant flyer. I need to make the trigger pull a touch heavier, around 3 lbs is my preference.

Second: 78 grains Retumbo with 160 PTs is too much. I need to back down to 77 grains. Yes, I got velocity (I chrono'd two shots, one said 3215, the other 3180), but I also got pressure signs, heavy bolt lift, etc.

Third: Looks like I need to have a shoot-off between two of the seating depths.

All loads are:
7 Mashburn
160 Partition
Nosler cases
CCI 250 primers
78 grains Retumbo

All groups are 4 shots. I varied seating depth.

Seating depth 2.735 Cartridge Base to Ogive (CBTO), 0.01 off lands. The flyer to the left was the shot that caught me by surprise. It may still be a flyer...but I can't say for certain. I need to re-shoot the group.
5DkB8fE.jpg


Seating depth 2.715 CBTO, 0.03 off lands. Doesn't seem to like this seating depth.
3FDGzGo.jpg


Seating depth 2.695 CBTO, 0.05 off lands. I would go hunting with this.
BmKBuq7.jpg


Thoughts? I really want to like that 2.735 group...but I can't say for certain about that flyer.
 
Load up 6 a piece for each seating depth Joel and reshoot. Two groups of three should get you the best information on what's really going on.

Glad it has turned the corner for you.

I need to measure a couple of cases OAL to the lands. I'd like to see if mines like yours.
 
I still find it amazing you're getting heavy bolt lift at 3200... just doesn't jive but it could be 10 different things.
 
I'm with Scotty on this workup. You assuredly have some promising groups.
 
Here are some pictures of some different bullets and where they touch the rifling in mine..

If anything, you can see how I gain a little powder capacity with the bullet seating. Pretty similar to a 260 Remington vs a 6.5 Creedmore. The Creedmoor really has about the same case capacity as the 260 Remington in regards to a 2.8" magazine box once you seat the longer bullets down in the powder chamber..

Well, the 7mm Mashburn isn't overall a ton more than a 7mm Rem Mag or a Wby, but the bullets are seated out fairly far and the longer neck allows you some flexibility. If I were shooting VLD type bullets more often I would have likely had a little less throat, but for the hunting bullets I plan to run, most require a .050-.150 jump into the rifling, so it works for me.

175 Partition



160 Partition



175 ELD X



168 Barnes LRX



175 Sierra Gameking



150 Swift Scirrocco



154 Hornady

 
Those look great Scotty! I thought also that if my rifle throat was as long as yours I couldn't fit rounds in the Rem 700 magazine anyway. My (much shorter) rounds fit in the mag fine now but there's not a lot of extra room


Sent from my iPhone using Tapatalk
 
Agreed. If I were shooting those VLD sorta bullets we'd have to make up another reamer. As it sits right now I'm sticking with what we have. I think it should make the hunting bullets out.
 
What I would like to know is how your loading ammo 3.771" long and getting it to feed threw a standard long action magazine designed for a 7mmRM?
 
truck driver":1kda1xjp said:
What I would like to know is how your loading ammo 3.771" long and getting it to feed threw a standard long action magazine designed for a 7mmRM?

I am not loading any of it over about 3.675" TD. I would imagine I could be as much as .050" long on some of my measurements since a few of them may have pulled out of the case neck a little as I was pulling them out of the chamber/bore when I was doing the measurements.

What I was showing is how my rifle is set up for hunting bullets that I will most likely run. The Partitions and even the Accubonds, which I didn't measure any of have an ogive a little further forward than most of the VLD/ELD sorta bullets out there. The reamer I use is set up to just touch the rifling at 3.603" which was set up that way because an AccuBond typically likes at least .050 jump so I can be pretty sure most everything else will fit in the magazine when loaded to operating lengths.

Bullets such as the 150 Scirrocco tend to shoot better as they are seated deeper, so by starting them out there, I am not jamming that bullet way down in the case and am saving powder room. Same for a few others on there.

I will know more once I get some real measurements as I begin load work, but I'll take a guess it'll work real well once it is up and running.
 
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