AVR-Chat

I need some way of measuring/displaying velocity of a projectile.  
This is for my coil gun experiment.  Heres how I think I might go 
about it.  Two matched pair IRdiode/phototransitor sensors, placed in 
a tube one inch apart.  This tube is then slipped over the end of the 
barrel. Use a AVR to start timing after first LED beam is broken and 
stop timing after second is broken.  Then make some formula to calc 
feet/second and display the result by either LCD or LED display.  Any 
coments on what the code might look like.  Also which AVR Should I 
use?  Could The Tiny serries handle this?