I took this idea from Jake Kempton now at JP Enterprises and formerly of Accuracy Speaks. On his 3-Gun AR, he disabled the gas system completely via taking out the adjustment valve on the JP Gas Block. Then, he threaded a bolt handle onto the bolt carrier.
Once done, the rifle would fire one shot and the rifle wouldn't cycle as all the gas bled out. From there you would rack the bolt (and carrier). As you pushed forward the next round came up out of the mag. Tah dah.
That's one way. I did it on a dedicated build for my SR-25. It's the only way it worked to bring it into the state prior to joining the Army chambered in .260 Remington and another upper 6XC. Another is a project that I'm currently working on, and is a little hush-hush. Once that's complete I'll post pics. I'm working out the magazine load feature on it for different calibers. All this while in Korea...so progress is a little slow.
SPC Richard A. White, Senior Medic
249th MP Detachment (EACF)
Camp Humphreys, ROK