I never did up the windows I always made it a a challenge but I'd never play more than 2 games a day and in all COD games and Battlefield the guns are under powered I mean not killing someone with a .50caliber shell to the leg would still make them out of action.
I used to challenge myself too. I'd do no perks, knife only, no buying anything, no opening doors, no mystery box, prone the whole match (Got to like round 5 )
I didn't do specific weapons. Once I did explosives only. Then I ran out of grenades on like round 5, had to run in circles for them to bleed out, had to open stuff, get box, semtex, it was hard.