After you fully research MP5 (which should last you through the first five meteors), invest all your points into fully upgrading the three shotguns. Then get everyone equipped with the Remington and blast your way into missions! Afterwards, the SPAS-12 is a major upgrade, and the AA-12 handles better than even the M60 and costs half as much, not to mention the much cheaper ammo refills.

In this game, it is very rare that you can one shot kill the most basic zombie with any main weapon, leading to the zombies easily ganging up, especially on fix turret missions. That's why the three shotguns (Remington, SPAS-12, AA-12) are OP. They one/two shot kill the zombies, and has a wide attack arc that can save your bottom in a pinch, attacking multiple enemies at once. Even the bull only takes a couple full shotgun blasts before going down. They also have cheap ammo refills.

So don't research any main weapon other than the initial, free MP5. All the main weapons--except the M60--are trash. Don't bother with the handguns either, the AI teammates are known to fire them blindly and at a rate of fire like they have to pay out of pocket for every single bullet at a shooting range.