This is how I switch my weapons fast. See attached picture. You just have to practise it a few times.
Also I have my insta cannon skills on my bar without having to switch my bars. I use my cannon skills with my mouse instead of keyboard. Mostly I try using it when I used dazzling fire, with the 3 seconds stun. You will have plenty of time to switch to cannon and shoot some insta cannon skills. Also i equipped my Cannon with a 50% movement reduction godstone. I also equipped the stigma Juggernaut cannon, where the targets magic suppresion decreases by 350, and decreases recovery magic bij 50% for 18 seconds. When u use this timely opponent like clerics will have a much harder time healing themselves if they didnt dispel themself first.
I strongely recommend all gunners to use their cannon in PvP. You just have to practise it a bit to switch from pistols to cannon.