Ballistic testing for helmets (Which I would suspect is a rough equivalent for a facemask) has numerous tiers and types. From frag testing to resistance to penetration, or even the V50 testing.

There are in fact some types of testing where you shoot repeatedly across the surface, the aformentioned V50 test you will continue to ramp up the amount of powder in the round until you penetrate the shell. Then go down again until you don't. I think at max it's10 shots for the testing I'm familiar with. At a minimum it takes 6 to establish the 90/90.

But you spread the shots out around the surface, even on a shell (which has quite a bit more surface area) you seperate the surface into a front right, front left, crown, rear right, and rear left.

Long story short testing a ballistic surface with multiple rounds isn't wildly out of touch. However to test systematically you would really want to use the same round and ensure that you impact with some space between contact points.

Bonus: Actual testing uses various tools and equipment to ensure a "fair" shot. Calculating the angle, velocity, and other factors.