You need about 8 shots of the boomshot and 24 shots with the sniper.
Go up to a truck, any truck.
Shoot a bullet with your sniper or boomshot at the truck while moving.
Keep moving through the whole process.
Get an active reload (while moving) and shoot it at the truck again (while moving).
Then get an active reload BUT IT HAS TO BE IN THE GREY PART. NOT A PERFECT ACTIVE RELOAD, BUT A SUCCEEDING ONE.
Then shoot it at the truck. Do all that while moving.
You should be shooting blanks.
It takes some practice and it only works for a short time. Sometimes it doesn't work at all.