Before you mail the SBR paperwork to the NFA branch, where it lists caliber, make sure you write the primary caliber you want, and also list any alternate calibers you're interested in so you'll have more options in the future with different caliber uppers. ;-) .22 LR, 45 ACP, and 9mm make better choices for suppression than 5.56 because of the sonic crack inherent to any round traveling much more than 1050 FPS.
I've heard some folks state they have listed multi or all available for caliber and still get approved, however, when submitting a form 1, I have also heard a few examiners send the paperwork back to list the specific calibers. Your mileage may vary.
Also, where it lists barrel and overall length, if you write a minimum length, will also not tie you down to only the length barrel you list on the application/form.
examples:
Caliber:
5.56x45 (primary)
6.8, .22LR, 7.62x39 (alternate calibers)
Barrel Length:
6 inches (minimum)
Overall length:
24.5 inch (minimum)