I'm confused: The SSK Whisper are intended to be SUBsonic, and the 510 Whisper is short as it needs but a little bit of powder. Now, the Peacekeeper is much bigger. If you want that, why not simply go with the 500 Jeffery?

Our 500 Phantom is a direct contender against the 510 Whisper, the 750 Amax will get no more than 1040 fps from this stubby, but we use the shoulder to head space, rather than the belt.

The newest 510 Whisper variant we saw used 338 Lap Mag brass that was cut short and had a "step" machined in to form a 90 degree shoulder to head space. Novel approach, rather radical, but should work.

OK, I'm with the program now ;o) So let's see. We want 4" MAX COL, and as much oomph as we can get. The 50 Spotter would be a contender, but the rim size/base diameter precludes use of most actions.

Why not take the 585 Nyati (or the 585 AHR) and shorten it and then neck it. If you take the COL and case length of the 50 BMG as your reference point, then you can have a 2.365" long case, and still get 4.000" COL.

We already have the dies for the .510 Phalanx, which is the .585 Nyati shortened to 2.000" and necked to .510. Water capacity to case mouth is more along the lines of 120 gr IIRC. Adding the .35" should get you the desired 165 gr water.

Assuming 45,000 psi, 32" barrel, 2.35" case, 165 gr water and the Speer 647 gr military bullet at 4.00 COL, max MV is 2400 fps.

Ironically, according to QL, the .500 Jeffery has a max COL of 3.6", max pressure of 46,000 psi and water capacity of 165.26 gr ...... case length is 2.7-ish.

That's right along the lines of what I'm talking about Marty. I suppose if we're starting with a .8" dia bolt, might as well use the biggest casehead available.

(The difference in this wildcat and the Jeffery would be obvious once the bullet is in place.)

Not sure how good the AHR brass is; do they manufacture it themselves?

Maybe the BMG case is a good reference. The neck-bottom to COL is ~2.2", and the 750g Hornady is around 2.35" or so, so 2.2" for 650g might be perfect. Anyone know lengths of good, light-weight BMG target bullets?

To do it right, I need to get ahold of prospect bullets and to measure out the case body. Then figure how fast they'd need to go to stay supersonic at reasonable altitudes out to 1km and 1 mi, then I'd have an idea of whether or not to pursue it.

There's always the option of a 2.8" Nyati case in single-shot mode. That should give about 155g of water UNDER the bullet. With 4.3" COL, this number would give something like 2650 fps with the 750g out of 30"!! Maybe 2450 at 4"?

I scaled a picture of an M33, and got about 2.13", with a .35" long BT, and .315" tail dia. This, loaded out to 4.45" comes out around 2850 fps. At 4" COL, 2700 fps with "only" 142g!
This would be a mean shooter in a 12 lb rifle. Maybe around 150 ft lb of recoil?

Until someone corrects me on the length of the 650g, looks like 2.5" would be just right for a 4" COL. That, with minimum taper (.005" per inch) to minimize boltthrust, would give water capacities of about 164g and 154g for the Nyati and Gibbs cases, resp. The 408 C-T may end up with 10g less, since the case is thicker; can't say.
My guesswork brings the two rounds to about 2680 and 2600 fps from 30" bbl and about 135g. 120-125 ft lb of recoil in a 14# gun.
Hmmm....
Not exactly shoulder-fired. Loading it "down" (?) to 2400 gives 100 ft lb; I'm thinking more like 70-80 would be barely doable.