You may want to look at the AR-15 mount from Pro-Gard. Our department uses them with the 9mm SMG model, but they should also work well for longer barrelled weapons.
The mount has an electronic lock, with a backup key if you lose power to the lock. The mount has a plastic guard that covers the receiver and trigger, and is designed to block the charging handle.
This is the new and improved version of their mount. They have an 800 number, but I forgot to write it down. They are on the web at: www.pro-gard.com
We have been using an older version of their mount for about 3 years and it works great. The newer one is even better for the new features. Good luck.