1. Checking for "off the face":
Why does anyone believe that any break action gun needs to have the forend removed before checking to see if the barrels are off the face?
The forend iron in all conventionally designed break action guns serves multiple purposes. The most obvious purpose is as a place to attach the forend wood. Another purpose is to serve a place to either house, or in some instances, to attach, the ejector mechanism but the primary purpose of the forend iron (in a conventional design) is to stabilize the joint/pivot. It does this by pulling the barrels forward, thus ensuring full contact of the hook's concave face against the hinge pin's mating convex surface. This is what provides primary lateral support to the barrels, not the fit of the lump to the frame slot, as is commonly believed.
How does the forend pull the barrels forward you wonder? It does this by spanning the distance between the action knuckle and the barrel's forend lug with what amounts to a very slight interference fit. The forend is braced against the action knuckle and rotated into position against its ramped lug, thus putting forward pressure on the barrels as the forend seats. This is precisely why the forend iron should be in place when checking for an off-face condition. This is also why the forend iron is in place when the barrels are "blacked down" (mated to the standing breech) during the making of the gun.
Here's a scenario to ponder: The gun's barrels appear to be tight on face with the forend removed but when the forend is installed, light suddenly appears between the barrels and standing breech. Is this gun off the face? Yes it is, and it means absolutely nothing that the gap closes up when the forend is removed because the forend is a structural part of the mechanism. There are those that may want to repeat that latter part to themselves, alot.
Scenario two: The barrels are tight on face with the forend in place but there is vertical movement at the rear, where the barrels meet the standing breech. Is this gun off the face? No, the bolting mechanism is either worn or not engaging completely.
It's really that simple. In a conventionally designed break action gun, the forend iron is integral to the joint's structural integrity. The mating surfaces of the hook and pin, and the mating surfaces of the action knuckle and forend iron shoe are circle segments with different radii but they share a common axis. Anything that causes any of these to deviate from that axis (like removing the forend), even by .0005 of an inch will cause perceptible movement. This is why welding and attempting to refit the hook to a worn hinge pin that isn't round any longer is a fool's errand but I've covered that elsewhere on this blog.
If as many "experts" on forums really knew as much as they think they do (about seemingly everything), there would probably be far fewer f***ed up guns floating around.
2. Shotshell pressures (Again):
I've said it here before but it apparently bears repeating: There is absolutely zero difference in average or maximum pressures between 2 3/4" and 3" shells, be they 12 or 20 gauge. Both shell lengths operate at the same pressure levels in their appropriate, or longer, length chambers.
This means that a 3" shell averages 11.5KPSI when fired in a 3 inch chamber, as does a 2 3/4" shell when fired in either a 3 inch or 2 3/4 inch chamber. These numbers apply to 12 and 20 gauge.
Firing a 3" shell in a too-short chamber will cause an increase in chamber pressure because the hull is opening into the forcing cone. This would seem obvious, since shell length refers to the FIRED length, NOT the loaded length (another point worth repeating). The extent to which the pressure will increase over normal is dependent upon certain factors, such as hull material and thickness, forcing cone angle, payload weight and shot size and powder burn rate.
"But, muh 3" shells urr magnums!", you say? No, Cooter, they're not. Your 2 3/4 and 3 inch "magnum" shotshells operate at no greater pressure than any other shotshell. It's called marketing, the same force that is responsible for the very existence of the abominations known as 3 inch shotshells.