So, I finally found the split washer for my XM16E1 build, and I have the correct wrench (an original USGI) but in just screwing the FH on hand tight it seems like it's going to time so the slots are off center. I rolled the washer around a bit, but it doesn't seem to help. I'm wondering if that's why they came up with the thicker washer later - to accomodate rifles that the threads might be a little off? The top slot is at about 11 o'clock maybe 11:30 now and I want it at 12 obviously. I haven't put the wrench on it because it seems clear that it isn't going to rotate that much more so the next slot is in line, or even the tine. What am I missing? I have 2 other three-prongs, maybe I'll try them, but I'm guessing what I need is the thicker, knurled washer.
ETA: Okay... so, the next three-prong I tried timed at just after 12, so it may tighten down fine. Weird. I'm going to go find my wrench...