How about adding a sniper turret, High Damage, Above Average Range, Slow Firerate, average spin speed. It's upgrades would be the same as others, and add a special upgrade that it's shots have a chance to go through enemy's and hit another. also, instead of upping it's firerate, you up it's crit chance and x2 it's dmg, and instead of range, you increase. the chance of crits
Here's the quickest way (That I know) to get levels, just keep doing the "On Rails" level. You get 30+ exp per kill and it lasts for a while (because it's a Domination gamemode)
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Yeah, thought about it, but I already have some work on other types of turrets, so not sure when i will add that one