It may depend on what exactly the “pepperball gun” is.
I personally recommend against rubber balls or rubber bullets. They rely on pain compliance which is unreliable, and depending on the gun may or may not run afoul of no-gun restrictions.
Pepper Spray.
If you really want to be sure you’re going to, IMO, want to reach out to a lawyer. You also may consider reaching out to the manufacturer of the thing
From Byrna.
No. Byrna Less-Lethal Self-Defense Pistols and Rifles are powered by compressed CO2 cartridges, just like a paintball or airsoft gun.
And that’s useful. Federally, that means to me (not a lawyer!) there is no convicted felon issue in possessing it.
IDK about state law though and if it might be different…?
Yes, I would call WI. State Police and mention the ATF ruling. 
Then ask where I could find the WI. rule so I have it.
Hello and welcome @Lenje_L
Listen to NATHAN 57 is A VERY VERY . Good idea . I TEXAS A GUN IS A GUN IS A GUN . I believe that after you do your time !!! U. Should be . THE SAME AS BEFORE IF . You did not did not do a violent crime … AND THEN . You have problems. I never would recommend . Any vileness . But if you just have to tune up . An azz hole 
DO NOT USE A WEAPON. .just stick your finger in up to . Your knuckle. And that’s will produce. Great .RESULTS!!! I Have found. . It’s pretty easy . And reliable!! I. Have never had any complaints. 
I sprayed pepper spray and every one ran out of the Unit 
Pepperball guns and tasers are not NFA items, and therefore do not require 4473’s or any other Federal intervention. Local laws, however, may apply (as these items do not fall under 2A).
Agree, local laws may see some trouble.
For instance, my city, (suburb on the north side of Denver) air guns are legal, as long as the projectiles stay on my property.
However in my sisters city (suburb on east side of Denver) shooting a slingshot in your own basement could get you charges of discharging a weapon within city limits.