Question: What is an active target?
Answer: An active target moves on it's own. Re-active targets are already quite common in the world. The simplest version being a single plate of steel that, of course, makes a *ping* noise and moves when you shoot them. By contrast the active target will move into and out of sight to make shooting that target harder
Question: Why would you build one?
Answer: I'm guessing that if the targets at the range also move it will make me a more reactive shooter and be a better game to play. The problem is that the only active target I've seen for sale is LaRue's ~$1500 popup targets.
Question: How can you build it better?
Answer: The 'professional' versions here use a big steel plate that you shoot at. This means you can't use large weapons like a 50BMG. This also means you need a big engine to move it up and down quickly. And then you need a big steel plate to protect all the machinery that moves it up and down. And a big battery to power that motor. Etc. Etc.
Instead, I'm going to have a simple paper or wood target that isn't going to try and resist bullets. Similarly, instead of having the target lying down and pop-up, it'll be cheaper from a power perspective to have the target turn on an axis.
