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

[chuckhammer]

Had 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 Savage 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.

 

[JThorsby]

Can't you have a sizing die made with the same reamer?

 

[Steve Blair]

Is there some reason why a 7mm WSM would not do what you want?

 

[chuckhammer]

Is there some reason why a 7mm WSM would not do what you want?

The H20 capacity of the fire-formed case for this cartridge will be 92-93gr, or about 10gr more than either 7RM or 7WSM. According to my calculations with QuickLOAD, as well as others' real world experiences (7mm Practical, 7mm Gunwerks Long Range Magnum, 7mm Rogue, etc.), this should be sufficient to launch a 180 Berger Hybrid at 3200 fps from a 28" tube at under 66kpsi peak pressure. Should be within 125 fps of a 7RUM but with 10-15gr less powder. Maybe 40-50 fps shy of an STW with ~5gr less powder. Plus the generally superior availability of 300WM brass.

 

[Steve Blair]

The H20 capacity of the fire-formed case for this cartridge will be 92-93gr, or about 10gr more than either 7RM or 7WSM. According to my calculations with QuickLOAD, as well as others' real world experiences (7mm Practical, 7mm Gunwerks Long Range Magnum, 7mm Rogue, etc.), this should be sufficient to launch a 180 Berger Hybrid at 3200 fps from a 28" tube at under 66kpsi peak pressure. Should be within 125 fps of a 7RUM but with 10-15gr less powder. Maybe 40-50 fps shy of an STW with ~5gr less powder. Plus the generally superior availability of 300WM brass.

[br]
I shot a .300 WM in long range prone in the eighties. That experience was enough to convince me that I never want to deal with belts, again. If you just fire a few rounds hunting or at the sighting range, I'm sure your plan will work well. BTW, I have no problem finding quality WSM brass and I buy 500-1000 cases per purchase.

 

[chuckhammer]

The H20 capacity of the fire-formed case for this cartridge will be 92-93gr, or about 10gr more than either 7RM or 7WSM. According to my calculations with QuickLOAD, as well as others' real world experiences (7mm Practical, 7mm Gunwerks Long Range Magnum, 7mm Rogue, etc.), this should be sufficient to launch a 180 Berger Hybrid at 3200 fps from a 28" tube at under 66kpsi peak pressure. Should be within 125 fps of a 7RUM but with 10-15gr less powder. Maybe 40-50 fps shy of an STW with ~5gr less powder. Plus the generally superior availability of 300WM brass.

[br]
I shot a .300 WM in long range prone in the eighties. That experience was enough to convince me that I never want to deal with belts, again. If you just fire a few rounds hunting or at the sighting range, I'm sure your plan will work well. BTW, I have no problem finding quality WSM brass and I buy 500-1000 cases per purchase.

Agreed, it seems pretty easy to get quality WSM brass (especially 300) for not a huge cost. I was thinking of the RUM and STW.

The perfect case in my mind would be a WSM with the body stretched about half-way to RUM. Comes to a body about .350" longer than 7WSM. It'd have about the same capacity as the one I'm building but in a .120" shorter case and with the fat .550" body of the RUM and WSM.

I plan to headspace off the shoulder. Also, neck-size only, depending on how well the 37 deg shoulder prevents growth. Don't foresee how the belt will hurt. It actually serves a useful purpose in headspace control during fire-forming since I'm moving the body-to-shoulder junction forward of the parent case's location. With a beltless case I'd have to up-size the neck to 30-cal and down-size only part of it back to .284, thus providing a headspacing feature at the neck. Similar to what is down when fire-forming 30 Gibbs from .30-06 brass.

 

[chuckhammer]

Here’s the final version of my 7mm wildcat off the 300 Winchester Magnum case. Chamber reamer goes into production today at Pacific Gauge and Tool in Oregon.

I christened it the Valkyrie, a buxom Nordic blonde goddess who is the “chooser of the slain.” ;D It’s also a reference to the XB-70 Valkyrie, an experimental Mach 3 strategic bomber from the ‘60s.

QuickLOAD estimated ballistics (28” barrel; 66k psi max pressure; conservative):
Generic 100 gr Varmint Bullet @ 4175 fps
Generic 110 gr Varmint Bullet @ 4075 fps
Generic 120 gr Medium Game Bullet @ 3975 fps
Generic 140 gr Large Game Bullet @ 3775 fps
Generic 150 gr Large Game Bullet @ 3650 fps
Hornady 162 gr A-Max @ 3500 fps
Berger 168 gr VLD @ 3400 fps
Berger 180 gr VLD @ 3200 fps (what my chamber’s throat was designed for)

7mm Valkyrie (center) alongside comparable cartridges. Parent 300 Win Mag on the right. I drew this up in UG:

Chamber reamer print (not to scale):