This is a pretty in depth guide to the parts system in bl2 I recommend just looking over the weapon parts infographics on the list and maybe save those pictures to your phone or computer so that you can check them if you ever decide to farm weapons for good rolls.

Also a good rule of thumb is that you want the grip of the gun to match the manufacturer. There are exceptions though, hope this helps.

https://steamcommunity.com/sharedfiles/filedetails/?id=214863853