The 7mm Kubla Kahn, which is a 7mm 378 Wby with 35 degree conventional shoulder would be much larger then the 7mm Allen Magnum in capacity. That would be the only advantage that chambering would have over my 7mm AM.
The reason, the EXTREMELY expensive Wby brass will loose its primer pockets at relatively low pressure compared to the Lapua case. Even though it has a 10-12 grain powder capacity advantage over the 7mm Allen Magnum, the ability of the Lapua case to handle much higher pressures will even the score.
While I have not tested the 7mm AM head to head with the 7mm Kahn, I have tested the 300 AX with the 30-378 and the 338 AX to the 338-378.
In both of those cases, in same length barrels with same bullets, the top loads in the 300 AX and 338 AX will easily match or slightly exceed what is possible with the same caliber Wby chamberings.
Now keep in mind, that is with the goal of getting at least 5 firings per case. If you are ok with loading one load and toosing brass, certainly the larger case capacity will have an advantage simply because I would be scared to load the Lapua case up to the point it would loose a primer pocket on the first firing!!!
Remember that capacity is only as useful as the case that holds it. If you want good case life, you will not see much more performance then my 7mm AM.
As to actual performance of the 7mm Kahn. I personally believe there is a limit to where the velocity continues to increase as case capacity increases. Again, in my opinion, the 7mm AM is about as large of a capacity you can use effectively and consistantly. Going larger in capacity would more then likely only decrease consistancy and also decrease an already limited barrel life.
As to barrel life with the 7mm AM. Formed cases are now availiable from me for this chambering. Either in the form of new cases I provide that I form or with cases customers supply to me for forming so that will GREATLY save on limited barrel life.
ballistically, there is really no comparision to the 7mm AM but there is no arguement that its not overly barrel friendly, one reason I decided to offer this case forming service.
I would again say that I believe the big Wby case is just to much capacity for the 7mm bore even with the very large 200 gr class bullets.
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