I must not understand something here, I replaced my car stock with a newer version M4 stock and I believe I've installed the spring and pin correctly but every time you pick up the rifle it comes loose and rattles around within the upper reciever. DUH!

Sounds like you don't have the new stock screwed in far enough.

When you install the Carbine stock, make sure that the buffer tube covers part of the Bufer retainer. Only a little bit of the buffer retainer should protrude and actually it is held in place by the buffer tube. Then make sure you use a spanner wrench to tighten the nut on the buffer tube to put tension on the back of the lower reciever. Once you find the right spot, back the Nut off and using Loctite, fix it in the correct postion. ONLY AFTER YOU GET IT IN THE RIGHT ORIENTATION! Then let it set for a while and in a day, it will be good to go.
If you are uncomfortable with doing this with poor instructions, I would suggest finding someone who has done it before and letting them guide you step by step in person.

Find one of us who lives near you and someone will help you.
I'm in Houston on the weekends.

I had the exact problem.  Email me.