I just built an M4 kit from Model 1 on a DPMS lower.  This is my first build BTW!!!  Woo hoo!  Anyway, when I hold the pull the trigger and hold it back and pull the charging handle back during a function check, the bolt carrier gets stuck halfway back into battery.  It doesn't do it every time, but sometimes.  Never experienced this before.  Anyone know why it would do this?

If the disconnector is not holding the hammer back, this will happen.

I would bet your trigger/disconnector spring is not in place properly.

Take a look at this pic:

www.kevinholman.com/byor/lower/partsbig.jpg

See the trigger assy.  Do you have that coil spring properly in place?  That holds pressure on the disconnector to catch the hammer.  

If it is assembled correctly, remove the upper from the lower.  Safety off, pull the trigger, and then hold it back.  Pull the hammer back with your hand.... it should click and be held by the disconnector.  If it ever hangs up on the bolt carrier.... this is your problem area.  Either that spring is missing/improperly installed/disconnector out of spec.

Don't let the hammer free fall with the upper off! Never dry fire with the upper off! You must carefuly let the hammer down.

Quoted: If the disconnector is not holding the hammer back, this will happen. I would bet your trigger/disconnector spring is not in place properly. Take a look at this pic: www.kevinholman.com/byor/lower/partsbig.jpg See the trigger assy.  Do you have that coil spring properly in place?  That holds pressure on the disconnector to catch the hammer.   If it is assembled correctly, remove the upper from the lower.  Safety off, pull the trigger, and then hold it back.  Pull the hammer back with your hand.... it should click and be held by the disconnector.  If it ever hangs up on the bolt carrier.... this is your problem area.  Either that spring is missing/improperly installed/disconnector out of spec.

That's the problem.  I don't have the disconnector spring installed.  I wondered why I had an extra spring.