Re: Colt 6920 or MP5 clone
If shooting as a rifle, why spend the cash (200) to get the SBR? Shoot it as a 16" and run it. If you are truly looking for a shorty, go for it, and have fun, I just can't get the cash for that. I don't know about the rules with SBR, but if you can get numerous for the same receiver, might be cool.
Sorry, no real tell, but I'd go with a 16" .223 or .22 for plinking.
Chad