[Astronomy]

I carried a couple different XM-177s while serving in 2/75 (Old Scroll Pre-Regiment Days).

As previously mentioned, the main purpose of those long "CAR-15" flash hiders was to dampen the fireball that comes with unburnt powder exiting the muzzle of short barrels. For that, they worked pretty well, making the muzzle signature about the same as our more widely issued M16A1s. Between the muzzle device and the flash retardant additives already present in USGI ammo (like M193), muzzle flash was not a problem with the short carbines.

The other thing they did was dampen audible blast a little bit, ensuring that the shorties at least weren't (subjectively speaking) any louder than a standard 20" rifle. My completely unscientific field observations were decibel levels about the same as with an M16A1. I'm sure they were measurably slightly louder under formal lab measuring, but the average rifleman would never notice while outdoors.

While the Feds may have classified the devices as "suppressors", they weren't remotely capable of making a weapon Hollywood quiet. Rather, they just somewhat protected the user from ruptured eardrums. I don't remember them being more loud (or quiet) than an M16A1 with birdcage.

As parts stocks of those repatriated Vietnam War weapons began to wear out (including the long muzzle devices), some of them wound up with armorer/depot replaced A1 birdcages. So equipped, THOSE shorty guns would deafen and blind you.

The 5.5" flash hider was a wartime developmental solution to the pitfalls of early SBR length select fire combat weapons. They weren't there to look cool; they were there to keep ferocious short barrel muzzle blast & fireball to a manageable level. Especially during conduct of full-auto break contact drills. It wasn't there as a nice-to-have option; it was more of a necessity.

When the M4A1 came along, that added length of barrel accomplished pretty much the same thing in a better engineered envelope. Anecdotally, those early 177E1s & E2s (that everyone called "CAR-15s") were mechanically unable to match M16A1 accuracy at distance. The M4A1 with SOCOM profile barrels put paid to that problem and would easily keep up with an A2 out to practical extended ranges. And sometimes surpass 20" A1 performance.

Many of our 177s were shot out when they were delivered to Battalion. One day I was handed a "new" one and went out to zero the gun. It was basically a smoothbore with the slightest remaining traces of lands & grooves. The gun keyholed every single round at 25 meters and would not consistently group all rounds on an E-Type silhouette at that distance. It would literally miss the entire human sized target with some bullets at that range. Needless to say, I immediately DXed it and got a serviceable replacement after wasting a day at the range.

Those guns came to us in a circuitous manner. Left behind in SEA (for allied use), they were captured after S. Vietnam & Laos fell. Then sold (in bulk) by the Communists victors on the world small arms market. Whereupon they were bought back by certain US middleman procurement agents to meet the needs of certain US SOF/Intelligence elements during the late 1970s. Made in the USA, paid for twice, and many of them very hard used across years of combat. Others were nearly pristine (depot warehoused in SEA) and apparently never issued during those conflicts.

I used to reflect upon the likelihood that my Dad's 1967 issued Vietnam weapon wound up in our hands years later. It was entirely within the realm of possibility.

The M4A1s we eventually issued to all SOCOM elements were (and still are) far superior guns in terms of accuracy, reliability, and updated engineering. Too bad we first had to endure a few years of carrying silly-assed interim M16A2s.

Absent a need to routinely mount a can, I have almost zero use for any AR barrel shorter than 14.5 inches (and that length equipped with only a standard A2 birdcage). That length is a good balance of reliable ballistic performance & close quarter dimensional compactness (vehicle employment, CQB, stowage inside of CCW carry containers).

IMHO, the whole need for shorty "compactness" concept is overblown & overwrought. More of a cool factor thing than a usefully practical advantage for most folks. Giving up effective range, velocity, and reliable FMJ fragmentation thresholds simply to gain 2-5 inches of shortened barrel length. Short barrel length gain that often gets partially cancelled out by folks adding after-market longer muzzle devices. Devices that they now really need because the shorter SBR/AR pistol barrels are too short to burn all the powder of 5.56/.223 loads. WTF? Like a snake chasing its tail.

Frankly, most after market flashhiders & muzzle breaks are just fishing lures. Designed to catch fishermen... not fish.

As always...YMMV.