ray guns legit for making crawlers. as you get in the higher 20s lower 30s, it kind of gets annoying. The rest of the zombies dont spawn until you kill the crawlers, so you never know when you can leave one or when they end.
As far as getting max ammos in later rounds, it just doesnt happen very often. Thats why you should save your money, not go box crazy, and make good use of every gun you get aside from the china lake. once i run out of ammo, i buy a new gun. Only gun i ever keep on empty after an upgrade is the zues cannon.