White Oak Armory has just released a spiffy new dry fire device for the AR15. It is finally everything I want in a practice device.
(https://www.whiteoakarmament.com/xcart/images/D/4dfdfront.jpg)
To install, shotgun your rifle. Remove the BCG, but leave the T handle in place. Slide the device into the mag well from the top, close the rifle, and you're ready to go. The device is the size/shape of a 20rd magazine when in the well.
To use, squeeeeeze the trigger as you normally would. Then reset the trigger by pressing the button on the back of the mag. The reset is about 12lb and fairly gritty- I was unable to really do it with my trigger finger, so I had to use the thumb on my trigger hand. But you can completely maintain your position without having to pull your face off of the stock as you pull the T handle.
I just got mine. Awesome, I highly recommend.
https://www.whiteoakarmament.com/xcart/product.php?productid=17957&cat=&page=1 (https://www.whiteoakarmament.com/xcart/product.php?productid=17957&cat=&page=1)