CavVet,
If your talking about the little round "nut", under the latch on the collapsible, there should be a tiny hole that extends through the "nut" and the bolt, that lil' hole takes a role pin. That roll pin stops the "nut" from spinning. I don't know the exact size of the pin, but call bBushmaster @ 1800-998-SWAT, not endorsing them, though they are good guys, its just that I have the most experience dealing w/ them. Any AR parts dealer will have the correct part. Online try Brownell's @ http://www.brownells.com/Default.aspx
They may have it online. In the meantime if you absolutely need to shoot it, go w/ a paperclip, or any small pin, you can push through the holes. A lil' blue loctite will help too. NOT, HEAVY DUTY Loctite, will be tougher to get off and isn't necessary. Lots of luck -S2.