Because one of your squad members starts with an Ithaca model 37 shotgun, 12 gauge shells are going to be one of the ammunition types you need to snap up. They're actually more common in the very first mission compared to all the rest, but you should be able to keep an ample supply of them all the way to Vault 0. They aren't the best rounds for the shotgun, but because flechette, slug, and EMP shells are very rare, they're going to be the ones you use the most.
Shotgun ammunition, 12 gauge anti-personnel. Unlike standard shotgun rounds, which fire a dispersing cloud of round-ball shot, flechette rounds are designed specifically for antipersonnel applications against unarmored or armored targets. Although the primary propellant charge and wadding scheme remain the same, the round-ball shot is replaced with a cluster of small darts, typically forged from carbon steel or a harder metal. Armor-piercing flechettes can be made from tungsten, osmium, or even depleted uranium for large weapons.
Shotgun shell designed to deliver a concentrated EMP burst to a small area. Very effective against robotic enemies. Against personnel, however, EMP ammunition is almost ineffective. Firing an EMP-loaded shotgun at a human or animal foe is nothing more than a waste of valuable ammunition. EMP shells do not become available until late in the game and tends to be somewhat more costly than most other forms of shotgun ammunition.
Solid-slug shotgun ammunition. Damage is increased against individual, armored targets, but burst shots will rarely hit multiple targets. With a projectile weight of one ounce and a projectile diameter of 0.729" (effectively a .73 caliber weapon), the shotgun slug is a devastating projectile at short to medium ranges. However, its minimally-aerodynamic architecture limits its range, so a shotgun slug possesses superior ballistic characteristics over short ranges, but lacks the long-range lethality of a rifle round.