....uhhhh
First off
the Imperium
second, the UNSC would ROLFSTOMP the reapers. A supermac cannon is SEVERAL orders of magnitude more powerful than anything the alliance has and more than capable to one shot a reaper capital ship. That is not to mention the fact that the Covies have weapons that bypass kinetic barriers so they would curbstomp the reapers too
as a comparison an Everest-class dreadnought accelerates one to 1.3 percent of light speed. It impacts with the force of a 38-kiloton bomb.
UNSC Orbital Defense Platforms typically mount larger and more powerful versions of the standard Magnetic Accelerator Cannon, nicknamed "super" MACs or "the big stick."
These cannons fire a 3000-ton ferric-tungsten round at 12,000 kilometers per second, or 4% of the speed of light, impacting with a massive amount of kinetic energy which (assuming the mass is in U.S short tons—even though the standard MAC calculation above uses metric tons) is equal to 5.392531072x1021 joules using standard kinetic energy calculations, 0.5 x (3,000,000 x (0.04x2997924582) )= 5.392531072x1021</ref>.
Against Covenant shield technology, the rounds possess enough kinetic energy to punch through shields, cut through the ship, and, upon exit, still retain enough energy to destroy a second ship, and cripple a third ship.
It is theorized that if a ship's armor or shields were to absorb all the kinetic energy of a super MAC round, the release of thermodynamic energy would still vaporize the ship
so they fire a slug one million time heavier at 3 times the speed (3000 tons = 3.000.000 kilos). Do the math.
oh and a regular cruiser?
The standard ship-mounted Mk. II MAC fires a 600-ton ferric-tungsten projectile with a depleted uranium core at 30 kilometers per second[2][4] The large amount of energy needed to fire the weapon is particularly onerous on a warship, and the extended recharge time is a significant factor in combat against Covenant warships as multiple MAC rounds are required to penetrate Covenant shields. At 2.7e14 Joules per shot (KE = .5 x 600,000 kg x 30 km/s x 30 km/s), it has a standard destructive output of 64.53 kilotons TNT-equivalent.
still more impact force than a dreadnought main gun from ME