Thanks for helping me. I have some questions about a new gas block for my AR 10 3308.What size is the gas block tube & what is a good gas block to replace the factory one?

When I changed the gas block on mine, I used the existing tube. You can just punch out the pin to remove the tube from the gas block and put it in the new one.

As far as what block to replace the factory one with, we need more information. Are you replacing the hand guard with free floating one or are you just looking for something else?