My rifle is in 300 win mag I can not shoot more than 2 or 3 shots in a row before my barrel gets super hot. I am wanting to get a bull barrel. First question is what's the thickest barrel I should get and second is where is a good place to get it from.

I have Rem 700 Police and I shoot several times a month putting 50 to 70 rounds down range per range visit. I have a bull type barrel that came stock with the police and have no proble with over heating. It get warm when I am doing a rapid fire of 3-5 round per minute. My Rem 700 Police does, with my help, dime sized 3 shoot groups at 100 yds ( I have come very close to three shots through same hole at 100yds and just over a 50 cent piece sized 3 shoot groups at 550 yds. I reload all my ammo and am shooting a Hornady 225gr BTHP being driven at 2550 FPS 71 gr H-1000.
I do not shoot the hottest loads I can to avoid barrel wear but 2550 FPS performs great to 550 yds

you can't just toss another barrel on that rifle you'll need to send it to a gunsmith to have the chamber cut and re headded