Just got the GSG-5 brand stock in yesterday.  I put it on, and it seems funky.  The sliding stock will slide all the way off?  I've seperated the slide from the rear cap, and for the life of me I don't see how it should work??

Yes it will slid all the way off. Put it back on then hold the release lever and pull it out a little. Let go of release and pull it the rest of the way and it will lock into position. I think there are 3 positions.

Really, it will always slide all the way off?  That seems pretty odd.  Seem like you could do a set screw or something to keep it from coming all the way off.

Do all the third party ones do this also?

I've had the Classic Army, as well as the GSG, and both of them do it.

My gsg brand is no better or worse than my $30 airsoft.  I contacted gsg and a new one is on the way.  Way sloppy when retracted.

MIne doesn't lock at any position.  The toggle switch on the end cap is all the way to the right (from the back of the gun) on the ejection port side.  It will rotate to the left when it gets to the preset indents on the stock slides, but ends up all the way back to the right (ejection port side)?

What a POS?  I'm thinking that the round part on the inside of the cap, the two prongs that stick out aren't engaging what they should?

My family says I have a touch like a Gorilla, but this would never stand up to any real use.  Is the folding stock better?

FMCDH

Take the stock off and check  screw on the inside of the endcap to see if it's loose or way too tight. If the endcap goes on too tight, (like a lot of tapping) it could also cause binding.