Just because you can, doesn't mean you should. And the Naval Special Warfare Development Group is just that - a development group. It vetts systems for wider deployment across the force. I am pretty sure the jury still out on the MP7. Of course time will tell. By the way, only Airsoft fanboys, SOF wannabes, Keyboard Komandos call it DEVGRU anymore. It is NSWDG in professional circles and has been for some time.
Here, check this out:
http://www.dtic.mil/ndia/2007smallarms/5_9_07/Armstrong_12pm.pdf
You will notice the 5.7 was considered by Crane NSWC but was ultimately rejected. And these guys can get whatever they want.
Funny you knew what I was talking about. The day my best friend who gets there after this next deployment corrects me about it, I will call it something else.