I recently had a 404J built on this same commercial FN action/bottom metal that had, ironically, been a 300 win mag previously. I had been warned about issues others had with getting a 404 to feed properly and holding true to that mine did indeed pop out the cartridges straight up and out upon the bolt cycle with the first round. I can’t remember the exact individual that I talked with about the issue but they sent me the calculation Paul Mauser came up with for optimal magazine box size. I ran the numbers for the win mag and the 404J and to my surprise found my existing mag box to be too narrow both front and back. I had heard that smith Dennis Olson (in MT) was a 404J conversation guru and contacted him about the issue. Being a man of few words he simply said “send it to me”. So I emailed the Mauser calculation and sent the rifle. When all was said and done the problem was solved- he didn’t touch the rails, all he had to do was widen the magazine box as calculated for the 404J cartridge. I’m anticipating that this will also be the only work needed if this action becomes a 400HH (granted the rail shape would be the exact same as my previous 300 win mag action- we will see here shortly).
I’ve also given thought to a .458 African.
I do not remember the specifics, but I did post the Mauser magazine cartridges geometry drawing and formula several times at the request of members. For all I know we may already have discussed this
, or not, as others too have posted similar info
Here it is for other members' benefit:
Cartridges geometry in Mauser magazines. This can only be achieved if the width and back to front taper of the magazine box are directly based on the diameter and taper of the specific cartridge to be housed.
Different lengths also mean different shoulders location and characteristics in bottleneck cartridge designs. The formula is: cartridge diameter x 0.866 + cartridge diameter = box width. The calculation must be run for both head and shoulder.
- The 8x57 case head measures 11.95mm, therefore: 0.866 x 11.95 = 10.35 +11.95 = 22.3mm. Add 0.1 mm for tolerances in cartridge dimensions and field damage. The magazine box must be 22.4mm at the case head.
- The 8x57 case measures 10.95mm at the shoulder, therefore: 0.866 x 10.95 = 9.48 + 10.95 = 20.4mm. Add 0.1 mm for tolerances in cartridge dimensions and field damage. The magazine box must be 20.5mm at the shoulder.
- The 9.3x62 case head measures 12.1mm across the rim, therefore: 0.866 x 12.1 = 10.48 + 12.1 = 22.6mm. Add 0.1 mm for tolerances in cartridge dimensions and field damage. The magazine box must be 22.7mm at the case head.
- The 9.3x62 case measures 11.45mm at the shoulder, therefore: 0.866 x 11.45 = 9.92 + 11.45 = 21.4mm. Add 0.1 mm for tolerances in cartridge dimensions and field damage. The magazine box must be 21.5mm at the shoulder.
- The 10.75x68 case head measures 12.6mm across the rim, therefore: 0.866 x 12.6 = 10.9 + 12.6 = 23.5mm. Add 0.1 mm for tolerances in cartridge dimensions and field damage. The magazine box must be 23.6mm at the case head.
- The 10.75x68 case measures 12.2mm at the shoulder, therefore: 0.866 x 12.2 = 10.6 + 11.45 = 22.8mm. Add 0.1 mm for tolerances in cartridge dimensions and field damage. The magazine box must be 22.9mm at the shoulder.
In consequence:
- A magazine box for the 8x57 must taper from 22.4mm to 20.5mm, and the rail relief must be located at the proper location.
- A magazine box for the 9.3x62 must taper from 22.7mm to 21.5mm, and the rail relief must be located at the proper location.
- A magazine box for the 10.75x68 must taper from 23.6mm to 22.9mm, and the rail relief must be located at the proper location.
While I do not think that a difference between 8x57 and 9.3x62 of 0.3mm in magazine width at the head would make a huge difference (although it would have bothered Paul Mauser), a difference of 1mm in magazine width at the shoulder begins to be meaningful, and a difference of 5.6mm in the position of the shoulder (46.2mm from head for 8x57 and 51,8mm from head for 9.3x62) is meaningful for the cut of the relief in the rails.
So, the difference of 1.6mm in the two cartridges overall lengths is likely meaningless, and they fit in the same action length, but they ideally need two different magazines and rails geometry. Granted, the differences are small in this scenario, and the 9.3x62 will likely work in an untouched 8x57 action, but modifying the magazine and rails per the above calculations is what makes the difference between a rustic re-barrel job, and the refined conversion job that translates into 100% reliable and butter smooth feeding.
When it comes to moving from 8x57 to 10.75x68, the difference of 1.2mm in magazine width at the head and 2.4mm at the shoulder will make a significant difference and a difference of 7.2mm in the position of the shoulder (46.2mm from head for 8x57 and 53,4mm from head for 9.3x62) will be important for the cut of the relief in the rails.
This is why converting the standard M98 to .416 Rigby was such a delicate process, because in addition to having to cut 13mm (!?!?!) off the feeding ramp (and lower lug recoil shoulder), the magazine and rails had to be widened 5.5mm (!?!?!?), not to mention opening the bolt face to its almost full diameter...
Folks may believe that we are splitting hairs here, but we are no, and magazine and feeding rails geometry (one might as well add feeding ramp length and angle, as they too are part of the equation as they changes with cartridge length) were considered so important by Mauser that the Oberndorf factory produced no less than 20 different commercial action variations, each designed for a specific cartridge. No wonder period rifles built on the appropriate action and chambered in the appropriate caliber feed like a dream and are bomb-proof reliable
I am really glad things worked well for your .404 J and this may be the case indeed for the .400 H&H. The one additional variable vs. the 404 J is the belt, and this may or may not play depending on the state of the action. It is always a bit of a gamble - as we all know - because we never know with used and often rebarreled actions who did what before. A one-owner Supreme coming straight from the shop of a genuine Liege master gunsmith is a sure bet, one coming from unknown lineage out off an obscure self proclaimed small continental town "bubba smith" is a different story
Anyway, the magazine geometry
is critical, and so
is the feeding rails geometry, and so
is the feeding ramp geometry, and these
are of course inter dependent. Sincere best wishes for the .400 H&H. I find it a fascinating caliber and I often wonder what history would have been, had H&H released it in 1913, one year after the .375, instead of 2003. But admittedly, the 1905 .404 Jeffery and the 1912 .416 Rigby, not to forget the1909 .425 Westley Richards, likely already cornered the .400 market, who knows... and possibly too H&H would have considered it redundant with the .375 in happy times when folks had the common sense to go around 0.1, up or down between their calibers... (e.g. .375 H&H to .500/465 H&H; or .350 Rigby to .450 NE; or one of .400 to one of the .500, etc.)
