I don't know how ATF looks at pinning now, but I know in the past what they said was that if you put a pin or several pins in to hold the stock open, then it was real easy to just knock the pins out and make the stock collapsible again. It was the same philosophy they used with dewatted machine guns, i.e. if you could convert them back to full auto without doing any complex machining, then they were not legal. So, as my esteemed colleague stated, using epoxy really works well, because once you have epoxied the stock open, all the drilling or beating with a rubber mallet will not get it to collapse again. So, I would go with the epoxy. I have used epoxy on a couple of collapsible stocks, and it works real well. The key is to make it impossible to get the collapsible stock to collapse again.
Charles Tatum
Alamo Professional Arms