Mass Effect 2 | How can I make my sentinel use assault rifles and sniper rifles?

In Mass Effect 2 I picked the sentinel not knowing it could only use machine pistols, heavy pistols and heavy weapons. I want to use assault rifles and sniper rifles, can I train him to use other weapons or possibly switch classes? Im halfway through the game and don't want to start over.Mass Effect 2 | How can I make my sentinel use assault rifles and sniper rifles?
There's a mission where you board an enemy ship. On that mission you can choose to either get a new weapon from a weapon class you already know, or you can train to use a new weapon class. As a sentinel you could then train assault rifles, snipers or shotguns, but never two or more. If you do a new playthrough with the same character you don't get to pick an extra weapon class.