The Sellor & Bellot buckshot rounds are slightly longer than 2 3/4", so much that my 870 will only hold 5 rounds of S&B buckshot instead of six.
All 2 3/4" shells are 2 3/4" AFTER they are fired. Most are closer to 2 1/2" before firing, but all brands are slightly different. Any shell with a rolled crimp such as the S&B will be slightly longer and it may well effect mag capacity.
A shotgun loaded with any size of buckshot works. I'd never suggest birdshot of any size except as a last resort. Don't get me wrong, I have a shotgun set up for HD in my home, but they have been over rated for decades. The reality just can't keep up with the myth. We've all been affected by the Hollywood portrayal of them being used. They don't blow 2' diameter holes through walls and knock attackers on the other side 10' off their feet when hit. The real reason they are so popular is because they are inexpensive and with varying loads very versatile for more than defensive uses.
Inside most homes the pattern is so small they offer very little advantage over a carbine which I really prefer. You have to aim either. At very close ranges I'd rather have a handgun in one hand and be able to have 1 hand free for other purposes. Recoil is also a factor. Slugs and buckshot loads are at 300 WM levels from typical weight shotguns.
In my opinion they are most useful outdoors at ranges from 20-50 yards where they have a chance for a pattern to open up large enough to make hits more likely. Buckshot goes through building materials just as easily as rifle or handgun rounds, but their limited range is safer outdoors than rifles or handguns.