I have an A2 dpms. I was told that a person can install a collapsible stock on this gun. Is this correct or does it go against the rules. I read some of the ATF responses to some letters reguarding pre-ban and they said as long as no more than 10 pre-ban parts are used it is okay. I was also told the method used to install this is called pinning. If so, what is pinning? hope some help is out there.

Unless it's a verified pre-ban, and was transferred to you as a complete rifle in pre-baqn config, no.

10 foreign parts relates only to imported guns, which would otherwise be unimportable due to their configuration, and basically does not apply to 99.9% of AR-15 type rifles.