The length of the shell is usually referred to in inches. It describes the length of the fired case—not the unfired case; this is important to be familiar with as you want to be sure the shotgun you have for home defense can accept all available shell lengths.
Common lengths for 12 gauge shells:
• 2 ¾”—tried, tested, and true option; has been around the longest and has the most choice for combination of loads for buckshot and slugs
• 3”—the ‘magnum load’ before the 3 ½” came out; longer more powerful load than the 2 ¾”; good selection of loadings
• 3 ½”—the largest of the 12 gauge shell; powerful, contains a lot of shot and lots of selection for loads
It is very important to never use a longer shell than what is marked on the barrel, but a shorter shell is always acceptable.
Bigger is not always better—the strong kick back from a 3 ½” shotgun shell makes it hard to properly make another shot right away.
Recommendation for home defense usage: use 3” chambered 12 gauge shotgun and mostly use 2 ¾” loads.