This might end up as a long thread, but it shouldn't be too long.  It's a lecture, or a sermon, depending on how you look at it, and I'm not looking for feedback on what I'm about to say.  I'm publishing information.   In reality, it's pretty damned simple, but manufacturers are making it difficult for us. Gas tube length is pretty much set, and it's standard - for the AR15 variants. The gas tube should END in the middle of the cam pin cutout, in the upper receiver.  Done.  That's it.  Ther