The guns I shoot the most are Lefevers. In 12ga I shoot a lot of 1oz loads even for hunting purposes & use a fairly fast powder that will give be around 1,100-1,150 fps with 7,500-8,500 psi. For reliable performance in any knid of weather I do not believe in any lower pressures.
If I were loading for high volume target shooting I would try to find a suitble load (there are plenty) at around 7,000 psi. I would still do this with a rather fast powder. Personally I have little use for using slow powders with light shot wts for "Suoer-Low" pressures. This is simply not the way these powders were designed to burn.
1 1/8 & 1¼ oz loads can be found using medium speed powders & maintain a usable velocity within the 8,000-8,500 psi range Larry recommended. Moving up to these heavier loads however the concern shifts more to one of 100 year old stock wood & especially for a sidelock with its diminished load bearing area.
I also find it extremely surprising you found a Baker of this vintage having 2½" chambers, based on the assumption it is a 12ga. Would be interested in knowing exactly how this was determined. If you inserted a well made & accurate chamber gauge & it stopped at this length it is quite possible, I would even say probable, that you have a chamber slightly undersized in dia rather than a short one. Most US guns of this era in 12ga will be found with either 2 5/8" or 2 3/4" chambers.
Even a very high percentage of the British so-called 2½" chambered guns will have an actual measured length of 2 9/16"-2 5/8" (65mm-67mm).
