Based on tales from a friend who enlisted in 2006, went through basic training, and then got accepted to OCS (and subsequently deployed to Iraq in 2007-2008), he learned the following essential infantry weapons in basic training (he was a signals officer, so not a lot of other combat training):
[ul]
[li]M4 Carbine[/li][li]M249 SAW[/li][li]M240B machine gun (he said fun to shoot, but sucks balls to actually carry around in the field)[/li][li]M2 machine gun[/li][li]Mk-19 grenade launcher (in his opinion the coolest thing to shoot ever)[/li][li]M203 grenade launcher[/li][/ul]
I’m pretty sure he picked up the automatic pistol somewhere in OCS or BOLC; he carried a SIG P228/M11 during his deployment. He never mentioned shotguns at all- I don’t know if that’s because he was a pretty proficient user before enlisting, or because he didn’t get instructed in how to use them.