A 16" barrel with a rifle gas system will have the gas bock at the muzzle.. Google Dissapator AR.
https://www.del-ton.com/Custom-Upper-p/cu112.htm
about the gas system, I learned from here. It seems like the carbine should be ok for a commercial rifle with 16" barrels.
https://forum.officer.com/forum/equ...83550-so-you-want-to-buy-an-ar-15-huh?t=81462
GAS SYSTEMS OF THE AR-15 FAMILY
There are three types of gas systems for the AR-15 rifle: carbine, midlength and rifle. This is referencing the gas system, not the sight radius (distance of the front sight from the rear sight.) The gas system is composed of the gas port (located under the front sight base), which the gas block covers and redirects the gas back into the upper receiver through the gas tube. The handguards cover up this tube which is just made of thin aluminum. The M16 utilizes a rifle length gas system, and has 12" handguards. The midlength uses (you guessed it) a midlength gas system (9" handguards), and the carbine (M4, CAR-15) uses (that's right) the carbine length gas system (7" handguards.)
Carbine
An intersting note in regards to gas systems, is that the carbine length system was designed to be used with 11.5" barrels (Colt Commando.) When the M4 stuck a 14.5" barrel on there (and commercial manufacturers used 16" barrels) it makes the short gas system extremely harsh on the carbine, leading to the symptom of "hard extraction." To overcome this, a bolt upgrade is needed. The bolt in a carbine should have a black insert under the extractor spring. This helps the symptom, but commercial manufacturers went further and fixed the PROBLEM, not just the symptom when they created the midlength.
Midlength
The "middy" is less harsh on the action, resulting in smoother extraction like its big brother, the rifle gas system. The middy naturally gives the shooter a longer sight radius than the carbine, but not as long as the rifle. It's an excellent compromise if you are going to run a 16" barrel, and it is even the correct dimensions (length from flash suppressor to FSB) for the USGI bayonet to fit!
Rifle
As mentioned, the rifle is the original gas system used in the M16, and is the best. However, it necessitates the use of an 18"+ barrel, and that is getting too long for most LE work. But if you're one of those rural deputies, and you have plenty of room in your squad/truck for one, go for it. The extra barrel length will increase the velocity of your bullet, and that's always a good thing. It would not fit well in my vertical rack, so I carry a 16".
CMMG is now making a 16" using the rifle-length gas system, called the M10R. This is the first one I've ever seen. I just got off the phone with Jeff at CMMG, and it turns out he did not have to do anything different to get it to work. I don't have any firsthand experience with this rifle, and I've not even seen one, yet. Until I start hearing reports of users experiencing flawless function with this new rifle, I won't be spending my dollars on one. If it does turn out to work as well as Jeff says it does, then it definitely has an edge over the mids, enjoying a softer recoil impulse (less harsh on gas system/extractor) and longer sight radius, while still having just a 16" barrel.