We are actually using a modified version of the Colt open bolt system for another machinegun platform. I ended up with two sets of the fire control from Tactical Innovations that have been modified for use with another firearm transferable registered receiver. We have sent a complete set in an adapter for the M10, into the FATD of the NFA Branch for determination.
The biggest difficulty of the project is the sear notch on the bottom of the carrier. I managed to find an original Colt open bolt, bolt carrier assembly. It was $1,000.00 plus shipping. We had a local EDM shop profile the notch in the bottom of some carriers. We had some 5.56X45, 7.62x39, and CMMG rotary delayed blowback carriers modified. We modified the closed bolt system to be able to be used with both closed and open bolt versions.
The other issue was the hammer release trip in the open bolt auto sear assembly. The Tactical Innovations auto sear assembly is missing the hammer release lever. It doesn't need the lever for the AM15 upper as the bolt in that system has a fixed firing pin protrusion in the face of the bolt. The Tactical Innovations auto sear only needs to release the bolt, not release the hammer. The original Colt open bolt system was designed to release the bolt carrier to strip and chamber a 5.56X45 round, then release the hammer just as the closed bolt system does it.
Since this was for another machinegun receiver, these modified fire control parts were not going to be installed in a M16 receiver. It is my understanding that the standard open bolt fire control can be very hard on the M16 receiver. Please let me know how I might be able to help you.
Scott