25 gr. of Herco is a light load. Muck depends ont eh case, of course, and the wad column.
With the Lyman 525gr. (sabot that fits 12 bore plastic shot wads) The slugs I have, are actually 14 bore, ie: .69" and cast from what appears to be WW alloy- at 506gr.

I purchased Lyman's new shotshell loading book, just for the slug and round ball loads, but have not experimented much as of yet. Plans ensue.

In Fed Gold Medal hulls w/ Win.209 and WAA12F114 wad - folded crimp - 32.0gr. herco - 1,390fps.

Increasing the speed might bring your barrels together- or not but unless you experiment you'll never know how YOUR gun behaves.
As 12's go, Herco is fairly slow.

I started too low for my wads and hulls and got hangfires and blupers. I was using Fed hulls WAA12R(RED) and CCI 209 primers with 28gr.

Depends a LOT on the hull. With Rem Prem STS plastics, a WAA12R + W209 - and the 525gr., they used only 25.0gr. and got only 1,249fps at11,300PSI.

Rem plastic cases with 1/4" plastic base wad, W209 + one BPGS + 1/2" fiber + three .125" cards under the slug and roll crimped with 36.0gr. Herco, they got 1,533fpas at 9,400 PSI.

Same hull, Rem 209P - WAA12F114 + 525gr. slug, folded crimp and 33.5gr. Herco for 1,426fps at 10,200
PSI.

When playing with shotshell loads, one must be careful.

There are also a bunch of AA STEEL loads in the new 5th edition manual.