I'm hoping someone can give me an idea of what pressure the "brass" (actually plated steel in my shells) will begin to stick in the chamber.

The reason is I'm experimenting with loads for which there is no load data so I'm having to nervously test shells myself with only a cheap cronograph. I'm primarily interested with the Lyman 525gr slug (it looks like a giant air gun pellet) in my Mossberg "835". So far my highest velocity is 2156fps and the brass does not stick, but in my opinion this is an incredible abount of power for a 12ga (5400ftlbs) and I'm curious about pressure.
Thanks, Corey

There is no way to determine pressure in a shotgun without test equipment. That is why shotgun reloading manuals always specify using exact recipes rather than working up loads.

There is an enormous amount of shotgun data available. Surely you can find a recipe that will suit your needs.

Working up loads and looking for pressure signs in a shotgun is like poking a rattlesnake. Sooner or later it will bite.

The action may fail before the brass sticks, i hope your life insurance policy is paid up to date.Stop what you are doing NOW.

Thanks for your replies and concern for safety but I'm at little or no risk myself, when testing I hold the gun at arms length and wear protective gear. I'm not concerned about ruining the gun either, it wasn't very expensive.
I'm my experience the highest pressure loads in load books are on the edge of sticking the real brass shells, but I'm not sure about the steel ones. My favorite 2-3/4" shot load by Lyman will stick if you follow their recipe that calls for a brass shell.
Using brass and primers works as a guideline for revolvers and rifles, and is common practice for testing wildcats. But I can't get Lyman or gun, shell, power mfg's to give me any info at all.
In reference to the slug load I listed earlier, remember I'm using a 3-1/2" shell, and as everyone knows, the bigger the case the more potential power. Also the 3-1/2" is rated at 20% more pressure, and slugs create less chamber pressure than shot allowing for still more power. So the potential is there and so far I don't believe I'm endangering the gun, I'm just looking for as much info as I can find before I push it farther.

243winxb: Yes my policy is good, as is my will.