Wow, years later, I know, but I was browsing through the forum and just saw this. This is wildly inaccurate. When you leave an MCRD, you are a basically trained Marine, that's it. Which counts for something, but you're a far cry from a rifleman. "0311" is a PMOS assigned to someone who qualified to become an infantry rifleman at one of the Schools of Infantry, at their subordinate Infantry Training Battalion.
When you show up to SOI, you (were) funneled to either ITB, or Marine Combat Training (MCT). MCT is essentially a familiarization course that mostly teaches non-infantry Marines how to conduct themselves adequately in an engagement that is incidental to their actual occupation.
All the prospective infantry guys get tossed into one box when you first get there, and then you start filtering into either rifleman training or one of the more specialized infantry roles available to us. This filtration starts a few weeks into the course proper, where you're offered your preference for infantry occupation, and then you have everyone divvied up into their training elements for their specific infantry MOS after that.
TLDR, no one in the Marine Corps holds the 0311 PMOS except 0311s, and it's not a secondary MOS, either. The closest thing Marine recruits get to being a "rifleman" at MCRD Parris Island or San Diego is your rifle qual.
Slay bodies, sports fan.