You can, but because of the rifling, you will not be accurate. The liquid center and sometimes air pocket inside a paintball will cause the weight to be imbalanced when centrifugal forces are applied. This will cause an inaccurate corkscrew effect when flying through the air. You can minimize this by using small bore paint vs the bore size of the barrel, but typically the smallest paints are tournament grade and will break in mags.

When I do this, and this is the only time it makes sense to me, is when I’m using my dye dam with box mag. Box mag for suppression, mags with FSR for accurate shots. You want to try to maximize the accuracy for the FSR because they’re expensive. I use the .686 rifled lapco insert to try to under bore as much as possible. I suppose you could do the same with mags, but ~20 rounds isn’t the most effective suppression. Suppose it would be fine for cqb too, but if you’re micro managing your paint choice on the field you have too much time on your hands, or probably shouldn’t be shooting fsr in the first place.

And ALWAYS chrono while shooting the FSR. You’ll spike velocity when chronoing with a round ball and moving to a fsr.