If you're counting Assault Cannon rounds as explosives (stated to be a "highly stable explosive rounds") for the purpose of barriers, then the Sniper Rifle could just be loaded with Explosive rounds or EX-Explosive rounds and do the same thing. They use the same terminology. As for Guass Cannons, they fire a dart, which is going to fall under "projectile" or "bullet" for DV purposes.
One of the issues with SR is that Heavy Weapons do a lot less damage than they really should. By its definition the Assault Cannon should be firing something very large compared to normal weaponry, it fires the same round as small tank cannons which should be in the 80-120mm range if I'm not mistaken. The Sniper Rifles usually top out in the .50 - 20mm range.
Personally I think that Cannons should get a Blast zone like explosives and have their DV dramatically increased.
All that in mind, Cannons do have a major advantage to Sniper Rifles. They don't require an edge test when used in non-Sniper situations (pg. 319 SR4A).