Low pressure for hammer gun

[gman]

I was wondering if there is a big if any pressure difference between 2" and 2
1/2 " shot shells for a 12 gauge hammer gun. I acquired an 1880s #2 grade and proofed it to with 2" game bore 1 oz rolled shells. I want to go low pressure and will let the gun lay until my Manson WTG arrives.
I want to go soft on this gun. RST still lists both 2" and 2 1/2 " loads and I was wondering with the same shot weight whether one would throw less PSI than the other. I don't assume that that 2" is less because the hull is shorter. It could in fact be a little hotter.? Could not get RST on the phone
Thanks