EDIT: Using the Due now. Adding a WeatherShield, accelerometer, magnetometer, thin film thermistor and TFT touchscreen and the software is spinning out of control. yay! Moving targets and design specs should not be the same thing.
I'm a novice in that I haven't done this stuff in 20 years and forgot a lot of it and what I did do was pretty simple stuff. Almost all of it has faded to one degree or another but the theory is still there and I'm still deeply involved in general computing in a professional capacity so I should be a quick study.
The emitter/sensor pack is going to prove troublesome due to a couple factors. 1, the distance between sensors cannot exceed 2.5" so the detection speed capability of those components will need to be very high. The sensor pack sits in a shaded area of very limited diameter and modest length (1/2" x 5") so interference from ambient sources should be minimal. The sensor/emitter pack also needs to be fairly blast/heat resistant. I have a way of shielding the components directly but the more blast shielding there is the more I have to limit sizes of the detector and emitter elements. They'll need to operate with high precision. The sensors will have about 5us of signal interruption to trip on and the time of flight between sensors could be as small as 5us depending on how far I end up having to space them, I'm trying to put them smack dab next to each other at abou 3/16" sensor-sensor distance. Ideally I'd like to be able to use 3 sensors total so that more complex functions like ballistic coefficient calculations can be done. That also requires either external data input or that I put temperature, pressure and humidity sensors on board and program for those which I don't plan on, so we'll stick with the idea of the 3rd sensor as an option not a requirement.
Another issue is which arduino to use. I bought the Due because I wanted to fiddle with it anyway but it's going to be a bit big for the final product unless I break it out into a wired up separate module. The electronics package for the final version has to be small, tiny, itty bitty. I'm wondering if the arduino nano might be capable of handling the requirements but from my math I don't know that it has the ability to run that fast.
The display seems to be an easy enough affair to deal with other than taking so many bloody inputs. Suggestions for minimizing that would be neat. I have a 20x4 standard backlit 44780 display that will be wire connected (about 3ft) to the controller. The controller will ideally reside in a housing that looks a lot like the evil child of a sound suppressor (silencer) and a muzzle brake which will also contain the sensors. It won't be a suppressor, it'll actually make the rifle seem louder. That said, if I can't get the nano size controller to work then it'll have to be done with a larger unit connected to the sensor housing via a cable (probably mini or micro USB or something similar).
I've bought several types of PIC microcontrollers and all sorts of bits and pieces and breadboards and have been toying around building blinking lights boards and other junk to get used to working with the stuff again. I've also seen lots of chronographs based on PIC's and that was my first option but I wanted to try it out with the arduino's as well... might be easier to package since I don't do PCB fab and would otherwise have had to have one made.
So, thoughts on component selections, microcontroller/board options, etc... Code isn't a worry for right now. I have a long feature list to build so I'll deal with that separately.
The velocities it'll have to measure will be anything from 5fps to 5000fps with a projectile from .4" to 2.5" in length of which 90% or greater is sufficient in size to obscure the emitter from the sensor as packed in the intended housing.