Capable of piercing armor more effectively than a standard General Purpose Rifle
An interesting weapon, it's the only one of few available that deals extra damage to Psionics or Heroics. This is valuable as the UA General Purpose Rifle-12 is the opposite, dealing less damage to Psionics. Aside from this, its fire rate is a bit faster than the UA GPR-12, but with less splash. These Rifles are invaluable during the Apollo campaign when you face large hordes of Infested Marines. Typically, players drop their UA GPR-12 when they pick up an M5. Although not a true replacement for the UA GPR-12, it performs well enough overall that most players consider it one.
The M5 is one of only a few weapons to deal extra damage to Heroic units, aka Bosses. This, combined with its high firerate, and low armor reduction make it an incredibly competent weapon to use against almost any boss.
Sadly, this weapon has been removed from the game.
On Normal/Hardened/Veteran and Nightmare, only one M5 Pulse Rifle will be available per chapter, so make good use of it. The weapon is extremely effective against Erebos. Although not as high a damage output as the Flamethrower MK-3, it is an excellent ranged weapon to use against Erebos, and outperforms the M45 Marksman Rifle.
Lastly, in place of an AER138 Laser Rifle, the Pulse Rifle will deal better damage to the Queen than either the HK420 Assault Rifle or UA General Purpose Rifle-12, so give the weapon to any class not sporting a laser rifle for the final battle.
Finding an early M5 is always helpful for the initial horde and Deimos. It's a solid weapon throughout the campaign.