Like with anything else BATFE, registering a Short Barreled Rifle takes a lot of time, patience and paperwork. Bob Boyd has been going through the process himself and has blogged about the process, along with links to all the requisite forms

Before beginning, I urge everyone to check their state and local laws to make sure you’re in compliance with all regulations.

The process is more time-consuming and aggravating than difficult. Regardless, you must have a BATFE-approved Form 1 in your possession prior to attaching the stock. To accomplish this, you’ll need the following:

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Read it here.