Hmph. Too much don't-try-this-at-home, let-the-experts-do-everything.
And: not enough math, and especially not enough understanding.
Consider: rock launched by lawnmower. Compare to bullet fired from .357 Magnum. Lab session with rocks and lawnmower: good; got to gather data. Range time with .357: lame; good numbers for .357 muzzle velocity for various loads are readily available.
But then: having velocity numbers, do they figure in the masses of the projectiles and calculate momentum and energy? They do not. They blather on about energy, yes, but then insist on comparing energy using a ballistic pendulum... an instrument for measuring momentum, not energy. Oh, and the term "force" also gets bandied about, meaninglessly.
C'mon, Splodey Channel: this is supposed to be educational, right? Not just "look how fun this is, and don't any of you in the audience even think about having this much fun"?
If it's educational, can't we have some proper physics, with math and explanations? And correct usage of the terms?
Additionally: if you fire a gun at a ballistic pendulum with the intention of measuring the momentum of the bullet, it may be appropriate to set it back far enough to be out of muzzle-blast range. Too close, and it's not just the bullet pushing on the pendulum. That setup looked awfully close.
Update: Wrong, wrong, wrong! It looks like the bullet embedded itself in the ballistic pendulum, as it should, but the rock didn't. If the projectile bounces off the target, it transfers extra momentum. Also, for a proper comparison, the mass of the pendulum needs to be large compared to that of the projectile, which I'm not so sure was the case here.
Further: in the actual-lawnmower experiment, having a dense pile of rocks will give suboptimal results by overloading the mower. Better to whack one rock at a time.