Hello all, can anyone tell me how long a mid-length gas system (5.56mm) is? This sounds like a moronically stupid question, but you can’t believe how little information is out there on it. Either that or my google-foo is weak. I’ve tried every possible way of asking the question, read up on countless forums and websites, and nowhere can I find what I’m actually looking for. I’ve found every other dimension imaginable, except the simple measurement of the legth of a mid-length gas system beyond the receiver.
To make it simple, I’m asking for the length of the exposed gas tube…or said another way, the length of the barrel from the end of the upper reciever to the beginning of the gas block. By guesswork, I think it must be right around 9.2", but I don’t want to guess. I’d like to know exactly. I’m in the market for a new hand-guard, and I’m debating between 9, 9.5, 10, and 11 inch guards. If I know the measurement of the mid-length gas system it’ll take the guesswork out of which one I want to order. I get that the gas blocks themselves vary in length. But I’m pretty sure the length to the start of the gas block is standardized. I honestly have no idea why this measurement is so hard to find online.
Thanks!
9 3/8" by my measurement. Sample of one. Centurion gas block, BCM mid-length gas tube, BCM upper receiver (in the event that it matters).
same, 9 3/8" sample of two BCM’s with FSB
My P-SWAG is that there may be 1/8 inch or so variation depending on how well the gas block is centered fore/aft on the gas port. I could be wrong.
I’m not really sure why you are so obsessed about the length though. You can view various pics on the internet and see what will be exposed and the length. If you want the gas block covered, you need a 10" rail.
Tactical Mark and Patrolman, thanks very much! I appreciate you taking the time to measure and doing it so quickly. You guys rock.
Your response, Iraqgunz, is what the internet is full of…people telling how long or how short a rail needs to be in order to cover or not cover. That’s not a very accurate way of answering the question, especially considering gas blocks vary in length. But to answer your (sort of) asked question, I don’t want the gas block covered. That’s why I asked the question I did. I want the shortest handguard I can get away with that only covers the tube, not the block. Hence my question, and it sounds like a 9.5" guard is the answer.
I’m curious, why do you want the gas block exposed?
I’m wanting this to be a minimalist set-up is part of the reason, in both weight and size. I don’t hang tons of stuff at the end of a rail (BUIS and sometimes a flashlight at most), and I don’t use a far-out grip. I prefer my support hand to be quite close to the upper, in fact the 7" handguard on my 516 is comfortable enough for me. The other reason is aesthetics. I understand that it’s vain, but I don’t really care for the looks of a long guard with a skinny barrel sticking out. Having the gas block exposed, even partially, makes a more pleasing transition from the bulky handguard to the skinny barrel.
I know lots of folks suggest the slightly added length and weight of say an 11" guard to improve the iron sight radius. But this upper will be used with my ACOG the vast majority of the time, and the iron sights will truly be an emergency back-up type thing. Short answer…I had an 11" NSR on a similar upper and I want to go shorter.
The “look” of a rifle is a shallow reason not to do something that has real world benefits. Covering the GB reduces the likelihood of being burned on the GB (which gets hot faster than the barrel). And covering the GB increases your sight radius, as you pointed out yourself. Even if you are running an ACOG you may need to remove it if it becomes damaged. Sight radius is important.
I don’t think “looks” rises to the level sufficient to justify keeping the GB uncovered. JMHO - To each his own.
You’re certainly welcome to your opinion Scott. “Looks” was one of three reasons I listed, but if that’s the one you get hung up on, who am I to stop you. I don’t want to sound like an a$$, but Tactical Mark was kind enough to help me and I figured the least I could do was answer his question. I’m not looking for permission, validation, or acceptance from anyone. 
We can just close this then since you have the answer.