Interesting Machamp lead you have there. I always thought Bullet Punch was better on a LeadChamp, and here's why.

First, you'll probably see a few Azelf leads out there. They normally have a super effective Psychic, too. Not only that, they are faster than Machamp.

So normally what happens, an offensive-minded opponent will Psychic your Machamp with Azelf, rather than setting up Stealth Rock. You'll select Payback, since you'll be going second and it's Dark type (super effective).

So, here's the amount of Damage you'll take from Timid/Jolly/Naive Azelf's Psychic (assuming a different EV spread than yours. 240 HP / 248 Atk / 16 SpD / 4 Spe Machamp.

252SpAtk Azelf (Neutral) Psychic vs 240HP/16SpDef Leftovers Machamp (Neutral): 83% - 99% (320 - 380 HP). Guaranteed 2HKO.

You fire back a Payback second turn:

248Atk Machamp (+Atk) Payback vs 0HP/4Def Levitate Azelf (Neutral): 54% - 65% (160 - 190 HP). Guaranteed 2HKO.

So, he does 99% to you, and you do 65% to him. This means, you will die on your second turn without Bullet Punch.

Use this Machamp in other words:

Machamp@Lum Berry/Leftovers
Ability: No Guard
Nature: Adamant
EVs: 240 HP / 248 Atk / 16 SpD / 4 Spe
- Bullet Punch
- Payback
- Dynamic Punch
- Substitute (if Leftovers)/Stone Edge (for flyers)/Protect (for scouting, and easier prediction)