The OP didnt mention which version he was looking at, but since he was speaking of registering the launcher as a DD I guessed it was one that would fire a standard 12 ga shotshell. There are several companies selling different types of inserts, adapters, or whatever the proper name is... You can check out the link below, and see a difference in the description because they sell one for a 12 ga flare, and another for a 12 ga shotshell. They do not describe the difference in construction though, and both do look similar. Also note that from that site,
Originally Posted by American Specialty Ammo website
Combining this adapter with an un-registered flare launcher turns your flare launcher into an un-regestered destructive device. The penalty for possessing an un-registered destructive device is 10 years in prison and a $250,000 fine.
If you need help registering your launcher please contact us. We will send you the correct forms and answer any questions you may have
If we suspect you do not own a registered class 3 launcher we will cancel your order.
Also note ATF ruling 95-3
Originally Posted by ATF
...ATF has previously held that devices designed for expelling tear gas or pyrotechnic signals are not weapons and are exempt from the destructive device definition. However, ammunition designed to be used against individuals is available for these 37/38 mm devices. This “anti-personnel” ammunition consists of cartridges containing wood pellets, rubber pellets or balls, and bean bags. When a gas/flare gun is possessed with “anti-personnel” type ammunition, it clearly becomes an instrument of offensive or defensive combat and is capable of use as a weapon. Since these gas/flare guns have a bore diameter greater than one-half inch, fire a projectile by means or an explosive, and, when possessed with “anti-personnel” ammunition, are capable of use as weapons, the combination of the gas/flare gun and “anti-personnel” ammunition is a destructive as defined in the GCA and the NFA. As a result, registration as a destructive device is required.
Though this rulling discusses anti-personnel ammunition such as containing wood pellets, rubber pellets or balls, and bean bags, it could be hard to argue that 12 ga shotshell loaded with lead shot fired from a 37mm launcher would not be anti-personnel.
Also, to Willie Sutton,
The Form 1 should be done for the launcher as I understand it to become a Destructive Device. SBS has nothing to do with it from my understanding because the Destructive Device does not have a required barrel length, so hence it can not be "short-barrelled"
Also, just to note, that the smaller, pistol type flare launchers with an insert are considered an AOW per the ATF website, except for the 25 mm to 12 GA which is legal unless modified, but it gives no further description.