I say unique but I could just be ignorant, hence I am here asking those who might know. I got a hold of a Frankenstein AR10 with an unknown (unmarked) 16" barrel. Noticed the gas tube was slightly warped so I went to replace it. At first glance it looked to be a standard 11-3/4" mid length tube, but a closer look put it at 12-1/4". Thought that was a little strange but I changed it out with the true mid length thinking it was a mistake. Sure enough the mid length isn't long enough, barely protruding beyond the gas tube channel in the upper. An intermediate length tube at 13-3/4" would be way too long. The tube looks like it could have been special made for the rifle. Wondering what others might have to say about it??