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I have a sleeved double with 2 5/8 chambers that I am wondering about using 12 gauge mini shells for. On one hand, the recoil tends to be milder, which should be kinder for the stock, and the length solves the problem of a 2 5/8 chamber, but on the other hand I worry, is this putting the highest pressure on the part of the gun that was not sleeved and may be weaker? Or am I just overthinking this?

The sleeve goes all the way back to the breech face inside the bored out portions of your original breeches.

IF the mini-shells do not develop higher pressures than 2.5” cartridges you should not have a problem. Do you know what pressures they produce?

Historically using Charles Lancaster’s Pygmy Cartridges or 2” cartridges in a standard chamber was sometimes found to produce shot balling so pattern testing would be wise.

I assume, by "mini" you mean 2". Different loads develop different pressures, just like different 2.75" shells develop different pressures. You cannot generalize to all "mini" shells. I have a collection of tested loads, if you plan to roll your own.

Shooting shorter shells in longer chambers will also reduce pressure and velocities from what would be seen in a chamber properly matched to the shell. That difference will be greater with fiber wads than with plastic.

RST 2" shells were 6500 psi, but I do not know if they are still available.

Brent

Or these?
Aguila 12ga MINISHELL 1 3/4” 5/8 oz. shot - 1,175 fps - 11,000 psi
Federal 12g SHORTY 1 3/4” 15/16 oz. shot - 1,145 fps - 9,500 psi

I always learn best with pictures

As Drew points out the 1 3/4" commercial mini shells are not low pressure. The Canadian Challenger shortys do not use a shot cup and apparently use very soft lead as they will lead the heck out your barrel. I think the original thought on these was to be able hold more shells in a combat pump gun.

Federal Shorty shells were the ones I have seen locally. Sounds like they are a no go.

Is there any factory ammunition that would be a possibility in this gun other than RST?

Gordy and and Sons and Keven's Ammo Yard list a 2 1/2" 12 ga. B&P shells at under 1200psi(11 something) 25 round boxes. They are pricy though at around 22.50 a box

Someone on a website, maybe this one, said that Federal shorty's were only 6500 psi, but I strongly doubt that..

Borderbill uses Clays, Bullseye and Titewad for his 2" loads; all rapid burning (like Red Dot and e3), with a rapid rise & fall in pressure. So the max. pressures are contained in the thick chamber. It is likely the Shorty shells use similar powders.
Pressure-distance curves for 1 1/8 oz 3 Dr.Eq. trap loads



Factory nominally 2 3/4" shells are frequently shorter. Shells change and it might be worth doing some dumpster diving at the gun club. Please let us know what you find.



B&P Comp One 7/8 oz are only 5800 psi. My batch measures a full 2 3/4" but IMHO is doesn't matter FOR 12G LOADS in a 2 5/8" chamber.
At what pressure were the barrels proved after sleeving? 3 tons would be for a service load max. of about 9,000 psi; 3 1/4 tons about 9,700 psi max.