I built a .450 Bushmaster upper last year with a 16" Craddock Precision carbine-length barrel.  At least I thought it was carbine-length.  I didn't like the Superlative bleed-off block that I used, so I got a new gas block and carbine-length tube, only to find that the carbine-length tube was one inch too long.  I don't know if I'm losing my mind, but I swear I put a carbine-length tube on this upper last year, and that I got a carbine-length barrel.  As far as I know, there is no gas tube size between carbine-length and pistol-length.

In the photo is a carbine-length tube (top), the tube I built with (middle), and a pistol-length tube (bottom).

Before you ask, the gas tube that I built with, when installed, fits into the BCG perfect and hits the "tick" mark on the upper perfect.  It's 8.75" long.  Carbine-length is definitely too long.

My questions for you guys, is (1) am I going crazy and (2) what the hell kind of gas tube is this?

http://i557.photobucket.com/albums/ss13/George_cle/Mobile%20Uploads/20191023_131304_zps6uugztmw.jpg

The last time I had a weird gas tube length it was for an AR10

Pistol: 6 3/4?

Carbine: 9 3/4?

Mid-length: 11 3/4?

Rifle: 15 1/4?

The top is a carbine, the bottom is pistol, as the OP stated.

There is also an intermediate length gas tube and a second size pistol gas tube that Olympic made for the OA series pistols.

I'm also confused. I have an 18" barrel that is supposed to be a mid-length system. I measured from the gas hole in the barrel to the upper just after the threads. That measurement is 9". Will 11.75 be too long and will 9.75 be too short? This is also a 450 BM.

I figured it out.  Paul Craddock got back to me.  The gas tube is a proprietary size that Craddock Precision makes themselves.  I somehow forgot that it came with the barrel I got from them.  It's a "Car -1" that Paul has found to be optimal for .450 Bushmaster and some other calibers.  He actually makes 15 different length gas systems himself.

For those looking for an excellent barrel, I can't recommend Craddock Precision enough.

Your gas tube should protrude exactly 1.5" into the upper receiver, measured from the front edge of the upper.  Your measurement should not be taken from the gas port, since a portion of the gas tube will be forward of the gas port, typically around 10 or 11/16".

Other than the highly-tuned systems of Craddock Precision, or some of the occasional "intermediate" gas systems out there, I don't know of any other deviations of the three major gas system lengths.

That is exactly what a midlength should measure.

Remember there is about an inch and a half on the inside of the upper, the length of the threads, and a bit ahead of the gas port

Have seen multiple different pistol length tubes.

I see there are multiple length gas tubes...
But are there various tube diameters?
Not like a PRI Fatboy , that has a larger diameter for a small percentage of the tube length.
But a different size tube for the entire length of the gas tube.

Or is gas tube diameter an industry standard size that no manufacturer deviates from?