Hello! It's my first post here, so please cut me some slack ^^
I've been doing a lot of sonar/radar projects with my Arduino Uno R3, and I've been wondering if it would be possible to do a paper plane AA project?
The basic idea would be a BB Gun mounted on a servo that would be guided by either an RCWL-0516 Doppler Radar or an MB1240-000 Ultrasonic Sensor (basically an upgrade of the HC-SR04). No clue how realistic or unrealistic it is, so any advice/tips is welcome. Thank you!
Very unrealistic, but fun to think about.
A good place to start would be to experiment with a sonar or radar module to see if (1) they can even detect a moving paper airplane and if so (2) determine its position with useful accuracy.
Totally unrealistic. I used the RCWL-0516 in my RV to turn on the lights when I came down the hall. It works, but it would spot any movement in a wide swatch and it stays on for quite a while. I have no experience with the MB1240-000 but I see Pololu carries it plus a few more. A quick look tells me it is worth checking oiut but you will need deep pockets as the cost is $45 for one and you will need to test a few as you will see.
Here is the Link
Here is the link to the manufacturer Link
It was a good thought 
I'll probably still do it for gits and shiggles. I'll give an update whenever I get around to it
I think it is doable. I didn't check, but there are many models of the basic sensor, most with much narrower fields of view. Probably need 3: one very wide angle for detection, then two more narrow fields of view to fine-tune the aim point. Maybe even use the RCWL-0516 as an early warning sensor. I know it has at least a 20 ft range and maybe much more. I mounted it inside wood cabinets, and it punched right through for 20 ft+.
The first kit you need to find are suitably rated goggles and body armour.