If you bought it from a dealer on only a Form 4473, it was identified on the form as a pistol.
It was.
It was listed as:
2005 Bushmaster Type 21-S Carbon Fiber Pistol 5.56mm/.223cal, Serial #D080xx.
Lots of fun to shoot, loud as hell, and a true ‘spray and pray’ piece. Looked great with red laser, light and 100rnd BETA drum mag.
I have a Class III 5.56mm SBR/suppressed w/ 12” barrel, and it’s a hoot.