MauserRifle,

The purpose of the "clamshell" design is strength, just as H&H (and others) put "bolsters" on the sides of their actions. Think of this reinforcing web as a super bolster.

There is more stress on the action concentrated in the 90 degree angle were the standing breech meets the bars than anywhere else. This is a way to relieve or spread that stress, like a gusset in structural steel.

I can think of no disadvantage to this system, other than the high cost of machining the complex shape.

To my knowledge, and as stated in the DGJ article I previously mentioned, no rifles are known to have been made on this design since the outbreak of WW2, and probably considerably earlier.

To give you a better idea of how the action is milled, I just shot this photo of the action face with the barrels off:

Regards,
Curl