In short, you gain in flexibility and forgiveness. Well, with more pellets, you can either leave the choke and maintain your pattern density at a further distance or you could go with a more open choke and gain in pattern size at close distance but still maintain density. Kind of underwhelming, right? All that increase in recoil for a mere 33% more pellets and a bit denser pattern. So when you bump up from 2.75″ to 3.5″ shells, you’re gaining about 33% more pellets by weight.
In general, 3″ and 3.5″ shells add more payload weight, but don’t usually add to velocity. Fewer pellets give a pattern that’s less dense, so we need longer shotshells if we want to maintain pattern density. Usually, that means you need to use bigger pellets and the lack of room in the shotshell means you need to use fewer of those pellets. Steel isn’t as dense and heavy as lead, so it doesn’t carry as much impact and energy. However, regulations prohibiting the use of lead when hunting migratory game birds has meant that most hunters are now using steel shot. 2 3/4″ shells have been a long standby and with lead shot, they’re very deadly at range.
When it comes to 12 gauge shotgun shells and the shotguns that fire them, there is a bit of variety out there in length of shell.