1. 3000x2000 for a AT gun is too much. 1024x1024 should do the trick, and at most, 2048x2048. For both length and width, these resolutions MUST be in powers of 2. 2. Are you planning for everything to be clean green metal?
I'm pretty confused over your workflow. 1. What resolution issues are you having? 2. Why did you choose a texture resolution that is neither powers of two, nor a square?