If concerned about the breech face thrust of the 30-30 case head size, simply chamber for the standard Zipper or the standard 25-35. Their case wall taper/slip angle is apparently exactly the right size to ensure case sticking if loaded above 40-43K, and this characteristic should relieve much of the bolt thrust associated with the comparatively large case head area. I have proven this assumption to my own satisfaction by chambering a standard Zipper and testing it. Please see Frank de Haas' very informative article on the Zipper family of cartridges for more details, it was most informative to me.
Yes, the case head will move to the rear if loaded above the elastic limit of the brass, i.e. above 40-43K, but below that level it appears to me that any bolt thrust is largely mitigated by the tendency of the case to stick. AAMOF I had to polish the chamber walls just to get good extraction with this cartridge and that's something I've never had to do before.
The standard Zipper when loaded to 40-43K will slightly exceed the performance of the 222-223 family of cartridges, at a much lower pressure. Use of the LR primer instead of the SR primer as in smaller cases also means, in my experience, that bushing the firing pin is much less critical. Also I've found that necking down 30-30s provides a built-in tight neck situation for better bullet alignment, with very few or no brass issues. Please see the exceptional preliminary performance I achieved with this cartridge in a Ruger factory barrel; normally most Ruger barrels aren't worth rechambering/reusing for whatever reason but this cartridge proved the exception.
Not saying it's the the best choice but IMO it's the most performance for the least hassle, by far! Heavy emphasis on the 'least hassle' part, grin.
Regards, Joe
