Lol the Burst Rifle is fine without upgrades. If you are having trouble with it then thats down to you not the gun. The people who say its only good when upgraded need more practice with it. Without upgrades the Burst Rifle can down a enemy in one burst (3 bullets) it also causes swing delay without upgrades.
All three yes and 1 of those shots to the head. People who claim the Burst Rifle is only good when upgraded are people who miss those shots or just dont do it right. They just need more practice is all. Try landing your first shot on the enemies chest then let the recoil go up to the head. If you upgrade the Burst Rifle you can hit three bullets (one burst) in same area. With the upgraded Burst Rifle aim all three bullets at the enemies head. If two bullets hit it will down them, if three bullets hit its a execution. Off course you dont have to aim for the head if you dont want to. I was just explaining what happens if you do go for the head.