Something similar to 450/400 or 10.3x60R

I have some odd dies and a chamber cast for this, but sadly no cartridge. I posted about them on another forum and it was suggested that I bring it to the experts here. I was told that some folks here love a challenge. :)

The dies I acquired with a group of other dies and obviously came from an estate. They are RCBS custom dies, marked “412 M&V B.P. EXPRESS”. The assumption is that the cartridge was either a 10.3x60R or 450/400 2 3/8", in possibly a Miller & Val Greiss rifle. The chamber cast came with the dies, and I made a cerrosafe cast of the sizer die. Here is a photo of the chamber and sizer cast next to the COTW page for the 10.3x60R.

Cartridge length and neck diameter match pretty close for the 10.3mm, but the base diameter is smaller (.520 vs. .545) than what COTW shows as dimensions for either the 10.3x60R or the 450/400 2 3/8, the body appears shorter than the COTW photo, and the neck longer for the same OAL.

So, 450/400 and 10.3 are the closest rounds I can find, but not quite close enough? Someone went to a lot of expense to have some custom dies made. Any ideas?

If RCBS dies, they should be able to tell you exactly what they are for.

Your cartridge is very similar to the 10,25 x 60 R Mauser (considering that you have measured out the chamber dimensions, that are a little larger than the cartridge dimensions)

Here some dimensions:

bullet: 10,24 - 10,34 mm
neck: 10,90 - 11,06 mm
shoulder: 12,45 - 12,70 mm
head: 12,99 - 13,08 mm
rim: 14,73 - 15,00 mm
case lenght: 59,94 - 60,45 mm
neck lenght: 18,00 - 19,00 mm
rim to shoulder lenght: 35 - 36 mm

I called RCBS; they were helpful but no help. The person I talked to said she had no records of these kinds of things but would take a note and have someone more knowledgeable call me back.

Pivi- thank you for the info. I have both the chamber cast and a sizing die cast (that would be closer to what the cartridge would be, hopefully), and the dimensions you posted are much closer to what would work. I do understand that chamber dimensions can vary and the sizer dimensions are just based on this particular chamber cast, so who knows? :)

Your suggestion also put me on track to do a little research and find that apparently there were a multitude of similar sporting cartridges a century or so ago that were based on the 43 Mauser (11.15x60R). From all appearances this certainly looks to be one.

Here are the dimensions from both the chamber cast and the sizer cast:

chamber cast_______sizer cast
.412" (10.46mm)
.452" (11.48mm).428" (10.87mm)
.510" (12.95mm).503" (12.78mm)
.528" (13.41mm).520" (13.21mm)
rim:___________.607 (15.42mm)

case length:
~2.4" (60-61mm)
neck length
___~.7" (17.8mm)___same
rim to shoulder
~1.44 (36.5mm)____–

So, it makes sense now why someone had some expensive custom dies made for their rifle. It may not be a one-of-a-kind, but darn near.
Thank you for the help guys!