Magpul have developed a device called the B.A.D (Battery Assist Device) which allows quick bolt release on an AR-15 using just the trigger finger – no more fumbling around to find the bolt release.
The BAD attaches to the bolt release button. Its lever extends through the trigger guard which allows the bolt to be released with only the trigger finger .
Incidently, Magpul designed the Remington ACR / Magpul Masada rifle so that the bolt could be released with the trigger finger.
The price on the Magpul website is $29.95.
Hat Tip: Solider Systems