A gun is not a bomb. It is a reasonably precise tool. If the point is to puncture a wheel, and there is one guy at NASA who isn't bloody blind, I really don't see how this becomes less safe from the tire exploding anyway.
I'm sure that there exists a real reason, but I choose to believe that it's because NASA is made of engineers, and engineers are by nature not practical people. They want to design a solution, not find a solution.
If they'd asked the technicians then they would've landed on Steve with his target rifle, or just a plate with a few spikes in it to drive the wheel over. Y'know, simple, straightforward, and absolutely no drawings required.
NIH doesn't describe this. They bought a model tank from a hobby shop and strapped a drill to it. For an organization plagued by accusations of excessive costs, this is downright cheap. NIH would involve a purpose-built rover to the tune of millions of dollars.
The point isn't to puncture the wheel, it's to measure the exact failure modes. They know if they puncture the wheel, it will burst.
A drill, for example, tells you the characteristics of exactly when and how it will lose structural integrity given a partial puncture of a given diameter and depth.