DevilWasp, you need to understand some basic science. Click on the link I posted.
You can find lots of websites talking about how force is measured for rail guns. Heres another good one.
A very small round may have some usefullness but in combat your going to want to be able to rapidly fire, and have the round to not disentigrate on
impact but penetrate objects. If the projectile vaporizes on contacting just about anything substantial it will have minimal usefullness. What about a
sandstorm, or trees what about someone with windows in front of them. ;p