What caliber gasser should I get and why?

You have an entire thread going. There was no need to start another thread. Additionally, why would you want someone to tell you what caliber YOU should buy???? Only you can make that decision based off your intended use. Have you researched the rounds in question on your own? Have you read the numerous threads on 6.5cm semis here on the Hide alone?

You need to put the time into researching everything yourself and I promise you will have 99% of your questions, concerns, answered. If you then have specific questions Im sure we can help.