Start with a class with One Man Army and a Tactical Insertion.

Hit Y to bring up OMA class selection. Simultaneously hit A to choose a class and the RB button. If done correctly you should be lighting your TI as your class changes.

If so, you will now be holding your new classes' equipment and when you fire your 'equip-gun' it will shoot like whatever gun that class has as a primary weapon.

Running, climbing and other activities might stop the glitch (you pull out your regular gun).