What I would like to know is exactly how far from the true muzzle is the port located. This means without any mount or flash hider attached.
Thank you.
Here’s why… 11.5" Bravo Comapany LW with AAC 51T and SPR-M4
if I understand your question correctly, gas port location has nothing to do with muzzle (barrel) length. it is the distance to the breech. barrel length is beyond the the gas port.
typically, an 11.5" will use a carbine length gas system. I don’t know the exact measurements for this but carbine uses a 7" handguard/rail. add 2" for standard front sight base and you have 2.5" beyond it to the muzzle.
hope this helps…
It should be approximately 2" from the muzzle crown. From my understanding, 14.5" barrels have 5" of barrel in front of the gas port if I remember correctly, so an 11.5" barrel would have ~2" of barrel in front of the gas port with a carbine length gas system if that math is accurate.
I think that estimate might be for a middy. The carbine gas system is somewhere around 7+ inches from the chamber, therefore I am expecting 4 something inches from the true muzzle without a brake, mount or flash hider attached.
Thanks for the reply, hoping someone has a barrel laying around not yet built and a micrometer!
I would not use the muzzle. Generally the overall length of a barrel has a much larger tolerance allowed than the length of the gas system.
OK, from the bolt face or the barrel extension, if the total length of the barrel from either is given, thanks.
Appreciate Todd, but this question is not relevant to locating a hole for building, just a basic stupid question, please indulge me.
Thank you
3.8 inches
I knew that.
Well, I do now… Thank you! 
To show the finished product
Below is a link to a diagram with measurements for a 16 inch barrel with carbine length gas system. An 5.56mm 11.5 inch barrel should use a carbine length gas system.
http://images.ebsco.com/pob/greenmountain/catalog/GM-M4%20Detail%20Print.jpg
Thanks for this added bit of reference.