Anyone ever built one before it gonna be my summer project and I could use some pics or blueprints thanks

Be sure to design it so the plates are easily replaced.  Dimpled plates at typical plate machine distances are a serious safety hazard.  You may not believe this but you will when people are getting hit with schrapnel every time the machine is used.  I have seen people bleeding and picking bullet jacket material out of their foreheads.

Be sure to design a sloped deflector plate to protect the mechanism.

If you leave it out, it will get abused - rifle rounds at 25 yards will destroy even armour plate in short order.

Having said all that, plate machines are one of my favorite target systems.