I'm not so sure that the problem of ringing chambers in rifles has much to do with the difference in chamber pressures vs. shotguns, because each is designed to handle normal chamber pressure with a margin of safety built in.
It could be that it might not be so easy to replicate a rifle cartridge type void in a shot shell?
craigd makes a good point here, and a compressible plastic wad might replicate the function of fillers such as dacron in nitro for black rifle loads.
I'm using 13 grains of dacron with 4198 smokeless loads in my .450 Black powder express, with no sign of chamber ringing so far. But I have been strongly considering just switching to around 120 grains of 2Fg black powder because of warnings that it is only a matter of time before I ring the chambers.
I've wondered if it could happen because of not using enough dacron filler. It is a pain in the ass to stuff all that dacron in before seating the bullet, and I can imagine that some might want to use less than what is recommended. Yet it now occurs to me that I have also been shooting various published .45-70 loads of smokeless in my Marlin 1895 and other rifles without any filler whatsoever. And these loads are quite a bit higher chamber pressure than standard .45-70 SAAMI loads. There are many sources that say it is a must to use dacron or some other filler in double rifles when loading nitro for black, but nobody even mentions it when shooting cartridges like the .45-70 in Marlins, Siamese Mausers, Ruger no. 1's, etc. I do understand that there is a lot more air space in a 3 1/4" .450 BPE than in a .45-70 loaded to similar velocities. I suppose the chambers could be much thicker in these rifles than in typical double rifles. But this does raise some questions.
