7mm Dies w/37 deg Shoulder on 300WM Parent Case

[chuckhammer]

Have a nice conversation with Dave Kiff at PTG on Friday. I'm planning to have a reamer made for a 7mm-300 Win Mag "Improved" project gun on a 110 action. He convinced me that a 37 deg shoulder would be perfect. For comparison, 300 WM and 7mm RM are 25 deg. I also plan to go with .007" less body taper than the parent 300 WM (.496" vs. .489" @ body-to-shoulder). Dave said this falls within the minimum .009" per inch of length body taper needed for reliable extraction.

Few questions about dies:

Neck Sizing
I can do the initial 300--->7mm neck size reduction using a standard 7mm RM neck sizing die raised up .120" on the press to account for the longer 300 WM case. I did some trials this past weekend and it went well. Although this is a neck-only resizing die, I noticed it still has a shoulder. Since my chamber will have a 37 deg shoulder I don't believe I can use this die for neck sizing my fire-formed brass. Do the bushing-style neck dies have enough shoulder clearance to be able to re-size the entire neck of this case? If so, since the body taper is slightly reduced (case is .007" fatter at the base of the shoulder) will this prevent a typical 7mm RM or 300 WM bushing-style neck die from working with standard bushings?

Bullet Seating
The 7mm RM RCBS Gold Medal micrometer seating die works with a straight 7mm-300 WM since the shoulder is identical. However, with the 37 deg fire-formed shoulder, I believe this die's shoulder will prevent me from fully seating bullets. Is there a better option? I guess I can turn the die's shoulder back on a lathe but would rather find a cleaner solution.

Body Sizing
With the sharp 37 deg shoulder Dave tells me I may not need to ever bump the shoulder back with a body die. However, if the need does arise, I'm guessing I'll need to order a custom die? I'm not aware of any mainstream SAAMI or wildcat cartridges with a 37 deg shoulder.

Thanks for any help you can offer.

[Apache]

My first question is: What is the purpose of the wildcat you're wanting to build? Just for something different or a specific goal?

Next: Is there a particular reason for using the 300WM as the parent?

[chuckhammer]

Several purposes:
- Never built a bolt gun from parts before. Done work on 1911s and ARs. Wanted to try something new and it appears the barrel nut + floating bolt head design are the perfect classroom.
- Just came of a few years' effort (about 360 rounds) of precision loading for a M70 .30-06 that never liked heavy bullets. I'm wanting to go with a heavy barrel gun to eliminate sharply defined nodes and see what kind of accuracy my loading and shooting skills can obtain with heavy/high BC bullets.
- It will serve primarily as a hobby bench gun but I'd like room to move into prone on the outside chance I decide to get into F-Class.
- 300WM chosen as the belt provide easy headspacing for fire-forming of wildcats. And the brass is fairly easy to find.
- Thought hard about a straight 7mm-300WM but want to go with something unique, hence the changes to body taper and shoulder angle.

[Apache]

Savage is the best for wildcat experimenting and ease of user 'smithing........as well as excellent accuracy potential without a lot of fuss.

I would suggest a different parent case that does not use a belt for headspacing......unless you know how to headspace off the shoulder of belted cases. Belted cases are not the best thing for accuracy....plus there are other troubles when it comes to FL sizing them. Trying to size all the way down to the belt is problematic at best.

I would also suggest an evaluation of case life and barrel life with the intended purpose. Case life as well as barrel life will be rather short. Personally, I love barrel burners, but it is something to consider. About the time you get a good load figured out the barrel is almost gone. Works OK for hunting but hard for target use.

You're talking about a "chunk of change" to do this, especially when talking custom dies, so make sure you think it all the way through. Not trying to discourage you in any way but just making sure you think it all the way through.

I would suggest looking at the WSM, or SAUM case.......running them through a long action (what you would need for the 7mm/300WM) means you can take advantage of seating the bullets long also. If speed and power is what you want you may even look at the 375 Ruger case.....capacity would be slightly more than what you are proposing and there's no belt to deal with...dies would be easier to deal with also....or just a straight 7mm RUM with a reamer designed for accuracy. That one would be a single shot though unless you're lucky enough to own one of the Savage RUM actions.

Good luck on whatever you decide........a word of warning though......wildcatting in VERY addictive. ;)

[chuckhammer]

Plan is to headspace off the shoulder and neck size only. Agreed, the belt is lame but at least here it's useful to headspace while fire-forming.

Considered the .375 Ruger case but have read how much work is needed to form this tye of 7mm brass.

I'm thinking I can get 1500-2000 rds out of the tube before she's done. Life should be a bit better than an STW, anyway.

Thanks for the input, Apache. I hope to become an addict!

[TC260]

Plan is to headspace off the shoulder and neck size only. Agreed, the belt is lame but at least here it's useful to headspace while fire-forming.

I would plan to headspace off the shoulder to begin with if that's what you want to headspace off of afterwards. The problem with headspacing off the belt for fireforming is that the case won't be formed properly to headspace off the shoulder when it's finished and minimally tapered cases don't grow in length very well on subsequent firings (same reason Dave said you probably wouldn't ever need to bump the shoulder back). So it's going to be a challenge to get them headspaced the way you want.

I shoot some weird wildcats too so I hate to say anything negative about someone else's project but since you're going custom top to bottom, try to avoid belted case headaches.

[chuckhammer]

I would plan to headspace off the shoulder to begin with if that's what you want to headspace off of afterwards. The problem with headspacing off the belt for fireforming is that the case won't be formed properly to headspace off the shoulder when it's finished and minimally tapered cases don't grow in length very well on subsequent firings (same reason Dave said you probably wouldn't ever need to bump the shoulder back). So it's going to be a challenge to get them headspaced the way you want.

I shoot some weird wildcats too so I hate to say anything negative about someone else's project but since you're going custom top to bottom, try to avoid belted case headaches.

No worries, thanks for your input.

I suppose the chamber could be just short enough to permit headspacing off the shoulder immediately after fire-forming? My first draft of the print shows perhaps too much chamber length. Looks like a .070" gap at the .420 diam shoulder datum. I sent it to Dave yesterday. He mentioned getting back to me with suggestions so this will probably be one of his comments.

[Jamie]

Don't over think it. I have a 7mm Dingato ( 7mm-300 WM) and love it. You can launch 180 Berger's at impressive speeds.

[chuckhammer]

Don't over think it. I have a 7mm Dingato ( 7mm-300 WM) and love it. You can launch 180 Berger's at impressive speeds.

Any changes to the parent case for the Dingato or just neck down and shoot?

[TC260]

I suppose the chamber could be just short enough to permit headspacing off the shoulder immediately after fire-forming?

Hmmmm, I hadn't thought of that :becky:...I suspect it could work but I'm not looking at the drawings. Either way, the false shoulder technique discussed in the other thread would work just fine. It would basically just be treating a belted case like a beltless case.

[Jamie]

Just neck down and shoot. The 7mmI Pratical is close to what you are doing, 30 or 35 degree shoulder

[BobT]

The 7MM Mashburn Super is also very close to what you are looking at, tapers to .494 at the body/shoulder junction but only has a 30 degree shoulder. This is another neck down in the FL sizer and shoot wildcat boasting speeds of over 3000 fps with the 175 grain Partition from a 24" barrel. Dave Kiff already has the print for this one it's #15106. I have been fooling around with it for a little over a year and it's good stuff.

Bob

[chuckhammer]

The 7MM Mashburn Super is also very close to what you are looking at, tapers to .494 at the body/shoulder junction but only has a 30 degree shoulder. This is another neck down in the FL sizer and shoot wildcat boasting speeds of over 3000 fps with the 175 grain Partition from a 24" barrel. Dave Kiff already has the print for this one it's #15106. I have been fooling around with it for a little over a year and it's good stuff.

Bob

Thanks for the print number. I think the Mashburn has a shorter case overall and also a longer neck, giving it a bit less capacity. I measured a straight 7-300 (neck-down only) at 89 gr H20 for an avg weight case, new/unfired. My improved version after fireforming should approach 92 gr. I believe the Mashburn is closer to 85 gr?

[BobT]

Thanks for the print number. I think the Mashburn has a shorter case overall and also a longer neck, giving it a bit less capacity. I measured a straight 7-300 (neck-down only) at 89 gr H20 for an avg weight case, new/unfired. My improved version after fireforming should approach 92 gr. I believe the Mashburn is closer to 85 gr?

I can check this evening and let you know for sure, I have brass ready to fire form and also formed cases on hand.
Bob

[BobT]

Okay chuckhammer,

With new WW Super brass, trimmed to 2.600" and sized in the Mashburn FL die H20 capacity is 84 grains, with the same case fireformed in the Mashburn chamber is 90 grains H20.

Hope this helps

Bob

[chuckhammer]

Okay chuckhammer,

With new WW Super brass, trimmed to 2.600" and sized in the Mashburn FL die H20 capacity is 84 grains, with the same case fireformed in the Mashburn chamber is 90 grains H20.

Hope this helps

Bob

OK, thanks for that. It appears to be neck-and-neck (pun intended) with my idea. My cases will be just a touch longer (trimmed to 2.620 vs 2.6000) and I'm looking at .496 (vs .494) at the body/shoulder. Length from belt to body/shoulder is 2.043 from the belt and 2.263 from the bolt face/case head. In order to fit the Mashburn's longer neck within the .020 shorter length, I believe these two dimensions are shorter by .100 or so, is that right?

[BobT]

Looks like the Mashburn is 2.120 from bolt face to body/shoulder junction and the overall case length is actually 2.625, I trimmed mine shorter because that's where all the cases cleaned up. The Mashburn was originally formed from .300 H&H brass and I have s sneaking suspicion that Winchester took a long look at Art Mashburns case design when bringing their magnums on line in the late '50s and early '60s, especially the .264 and .300 Win.

My chronograph quit about the 3rd. shot with the Mashburn so I don't really have any actual velocity data but from all indications I can mirror the STW with 2" less barrel. I am going to send PACT one more nasty letter and tell them what I think of the lifetime warranty they brag about and then I'll start looking for another chronograph. They have had mine since last July (the 14th.)

Bob

[chuckhammer]

Bob,
Now that I have the full set of case dimensions from you I'm going to build the Mashburn case in 3D CAD to see how it compares with my wildcat in the same file. I can generate a volume ratio between them so I'll have an idea of relative case capacity even though I won't model the case wall thickness.

7mm Mashburn Super Magnum (that's a really cool name, BTW)
.300 H&H / .300 Win Mag Case Head & Belt
2.120 Length from Bolt Face to Body/Shoulder
.494 Diam @ Body/Shoulder
30 deg Shoulder
2.625 Case Length

The neck length will fall out from there with a little bit of trig on my part.

I'd love to see the your MVs when you get the chrono sorted out. I'm running a $90 Shooting Chrony F1. I know, it's a cheapie, but it's worked OK for me.

My goal is 3200 with a 180 Berger Hybrid from a 28" tube. If needed, I intend to run it HOT ---> 66-68k psi. Brass and barrels are a consumable commodity. :p

[chuckhammer]

Here's a shot of the cartridges I drew up in Unigraphics NX. The Valkyrie is the one I built with Dave Kiff's help. Final design still pending.

http://img7.imageshack.us/img7/6610/o9le.png

[BobT]

Interesting lineup! I think your goal should be pretty easy to reach in the 28" tube. I'm curious about why you chose to go this route over the 7MM RUM? I have heard the LRM pretty much mirrors the 7MM Dakota, it will be interesting to watch its progress. My goal when I started dabbling in the 7MM stuff was the ultimate long range pronghorn rifle. Have you had good luck with the Bergers on game? I have not been impressed with it in the field and plan to stick with the Nosler stuff, I'm anxious for the Accubond LR bullets to get here. I flirted with the RUM and standard 7MM Rem. I will revisit the RUM in the future but the Mashburn has too much to offer over the Remington version. I also briefly considered the 7MM Allen magnum but the powder shortage and demise of the Wildcat bullet have sent it to the back of the line. I was camped next to a fellow last fall in Wyoming that used the Allen magnum to make a cold bore shot on a doe antelope at 1141 yards using the 200 grain Wildcat bullet, the shot was pretty well perfectly centered in the lungs. I wish I could shoot like that but he waits for perfect conditions to take a shot, in this case he sat for 5 days and took the shot late on the fifth day.

Make sure to keep us posted

Bob