M4nut contact CMMG, they have armalite buffer tubes that will work in the non-post ban. But you will need to get a new spring and buffer that is used in the collapsible stock as well. Jeff at CMMG didn't know that the BM would not work with his stock. He sent me a gratis tube in the spirit of good customer service. I didn't advise him of the spring and buffer issue since I got the tube gratis and I didn't know in advance either, you don't notice the depth until you stick the spring and buffer in and it short strokes. I couldn't have even loaded the thing, so there was no danger there. Doh! I am going to contact him and see how much for the spring and buffer.
Here is what I did, this armalite tube has a lot of meat on the end. I mean a lot. I had it cut down and redrilled the stock screw hole to make a permanently shortened stock. I reduced it down by about 3 inches. (this will make it more mobile IMHO) I will attach it sometime tomorrow and post a couple pics on www.assaultweb.net on the AR/M board.
Just a little warning, that castle nut is a bitch to take off. You will need a GOOD wrench to take it off. I ruined my cheapy wrench trying.
I wonder why nobody is making a permanent short stock for the M4? They make the short entry stock for the A2. Hope this helps.