Here is a nice handy little CAR-15 type carbine with a little bit of retro style tossed in to make it just a little different. All mechanicals are modern,...but the builder liked the aesthetic of the early Vietnam XM177 carbines that used the early triangular shorty handguards. 

The bolt-carrier is the heavier full-auto variant. The rifling twist rate is 1-in-9. There is a 45* degree rail mount on the right side so you can have a red dot or scope, yet instantly switch to the iron sights, or vice versa.