Typical rangefinders for hunting would suffice, and will be needed to calculate trajectory.
“Typical rangefinders for hunting would suffice, and will be needed to calculate trajectory.”
Hunting rangefinders don’t have enough range.
And range measurement is only one part of the problem.
Other data must be collected (target speed/direction etc), and updates are needed at high rates. Predicting future target position, estimating trajectory (no mathematics exist to predict it fully), and generating gunlaying commands is the work of a computer. Back in the day, only the very best gunners were capable of doing it all manually, and they needed constant practice to keep their edge. They also needed machine guns.
A gun firing a projectile large enough to do enough damage at a realistic range will be rather large and powerful, hence none too portable and disagreeably costly. Not to mention heavily regulated today.
Rifle caliber machine guns (think 30-06 size) went obsolete in the 1930s for anti-air and air-to-air use; even were such items not mostly illegal today, few would want to pay for one (and its fodder).
Rockets can have the necessary range and power, but are no cheaper shot for shot, and suffer from nightmarishly poor accuracy. They would have to be launched in literal clouds to attain even a modest chance of hitting.
Adding a seeker, guidance system, and control surface to turn a rocket into a guided missile (Stinger or SA-7 type) would increase chances of a hit, but adds rather a lot to cost.