Robby - did you know the low brass 8's develop identical brech pressure to 2 3/4" 1 1/2 ounce magnums? The heavier loads do kick more, but the breech pressure is identical, whether the round is any of the 2 3/4", 3" or newer 3 1/2". Gauge doesn't matter either as far as pressure is concerned - each gauge including the 10 bore down to .410 calibre shotguns, produce approximately 11,500PSI with factory ammo.
It was common for people years ago, to think the low brass meant low pressure - it is a common mistake and one that most people think is true today.
The big trouble with older hammer guns, is that most were originally chambered for 2 1/2" cases, not 2 3/4", no matter the gauge. Having to squeeze down the shot and wads an extra 1/10" raises breach pressure considerably. That 1/4" over length opened up on the forcing cone and the shot and wads had to be squeezed down the thickness of the case - all the way around for that length. Some of the guns didn't like that and shot-loose quite quickly. Some, like many of the cheap, but ornate Belgium guns shot loose anyway due to poor workmanship. Guns labled W.W. Greaner, W.W. Grener, H.H. Greener ,Wesly Richards, Wesly Rickards, and-etc. were especially prone to being wobbly;.