1: Yes there is a setting allowing both R15 and R6 in-game. If you are in Roblox Studio, press the Home tab and click on Game Settings. Now go to Avatar, and you should see that under Avatar Types there are options to choose from.
2: If you are searching for quality guns then don’t even open up the Toolbox because there is a very high chance of all the gun models having lots of bugs and errors. You could use FastCast which is designed to emulate bullet physics and hit detection.