With most modern shotguns even if you did slip a 3" magnum shotshell in a 2 3/4" chambered shotgun, chances are it would kick like hades but it would not blow up. Most shotguns are tested with proof loads which generate pressure way in excess of standard or magnum hunting loads.
Case in point. Many years ago while I was away in the military, my brother used my Winchester Model 1200 pump to go duck hunting. Being a neophyte hunter, he did not know that all shotguns were not chambered for 3" shotshells. My Winchester, purchased in the mid sixties only had a 2 3/4" chamber. My brother mistakenly purchased a box of 3" magnum shotshells and proceeded to go into the back yard to pattern them. He rapid fired 3 shots (it had the magazine plug in) as fast as he could pump the shotgun and other than kicking like heck he was none the worse for wear.
He told me about it when I called home afterward and through judicious questioning discovered his error in purchasing 3" shotshells. He didn't do that again and it did not harm the shotgun.
I have an 1897 Winchester, 12 gauge pump made between 1916 and 1917 I purchased to use for cowboy action shooting. The gun was so pristine I could not bring myself to cut the barrel down to 20" however I did use it for a few matches. It has a 30" fixed full choke barrel and the few matches I used it, it kicked hard. One of our members who does gunsmithing work brought a chamber gauge to the next match and I discovered it had 2 1/2" chambers (which was normal for shotshells of the period that were roll crimped.) Needless to say a couple of boxes of 2 3/4" shotshells did not harm me or the shotgun and the gunsmith friend reamed the chambers to 2 3/4".
I am not nor do not recommend shooting the wrong size shotshells in any shotgun for any reason.
