I have AR's with a 1/7, a 1/9, and a 1/12 twist barrels and have tried a 22 LR conversion kit in the 1/12 and 1/9 guns. The 1/12 gun will shoot well enough to post good scores on the A-18 50' Smallbore rifle target, the 1/9 didn't shoot worth a hoot, so I didn't bother trying the 1/7 gun.
The old 1/12 twist "noodle" barrels off the old Colt SP-1 pop up from time to time, and if lead in your AR is your goal, this might be your best option.
John