18" barreled Striker 12 question

[AN94NK]

If I buy one of these DDs would I have to file a form 1 (SBS) to shorten the barrel to 12"?

[ExecDirector]

No - it is still a DD so no additional tax is necessary. Optional: if the change is permanent you may write a notification letter changing the dimensions of barrel and overall length but no requirement to do so. If you ever transfer, the form should accurately reflect the length (which will not match what's on record now) and you can explain the change then.