Re: Thumb placement?
Just as an added safety note:
If you are trying this technique and you also have a match trigger, be very conscious of trigger control / follow through.
(Press trigger, gun goes bang, release trigger, listen for the reset of the trigger)
If you do not do this, doubling is often the result due to what is essentially a bump fire.
Different grips that fit your particular hand better can often yield the same result as using the "No Thumb" grip method.
With an AR platform, trying various firing techniques to find what works best for you is often as useful as load development.
Start with solid fundamentals and fine tune as needed...