Yep...it's definitely Deadly Weapon.
I used to own it on VHS but it disappeared years ago.
It does have some good moments and the backpack and gun are pretty neat looking.
But it does have some really bad acting here & there.
And it has the late Kim Walker who played Heather #1 in Heathers
The best scene is when the kid blasts his dad in the face with the death ray and turns him into a burnt skeleton.