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Fostech does not state if the Fostech Echo AR-II Drop-In Binary AR-15 Trigger is compatible with Pistol Caliber ARs. I recommend contacting Fostech to confirm compatibility.
I have three Fostech triggers in 9mm, 556, & 7.62x39 all in ar15 pistols. You may need to fine tune buffer weights. The 7.62x39 may need loading ramps polished and a stronger firing spring if using Russian steel ammo. One thing I do not like is the trigger is not smooth making precision shots difficult. Also Fostech customer customer service is horrible to get in contact with. Issue a separate lower with a rise trigger instead of the echo trigger because for precision shooting it is so much better. Forget precision shooting in binary.(echo mode) as you can not hold the rifle securely.