Sleek Bolt is a sniper rifle mod that will increase crit chance and higher versions will even boost the damage. Sniper: For snipers, players will usually want to look for weapon mods that will boost crit chance and accuracy, like Optics and Precision.The Utility Belt mod on boots will reduce cooldowns on frag and stem grenades, which can work nicely with the Frag Grenade Mastery skill.
It is also a viable option to equip the heavy with more armor due to its lack of mobility.
Here are some general tips for how to gear up your characters based on their class: