Quoted: How come all 16 inch uppers are not just 20 inch uppers with 4 inches less of barrel?
Yes I saw the Bushmaster Dissipator and that is the right idea. Why was it not done like this the first time?
So for sight radius, it is a no brainer. But that is more reliable gas-wise? A 16 inch full gas system, or a 16 inch carbine gas system?
And is mid-length gas more or less reliable than a full length gas with a 16 inch barrel?
How come the military did not go with a 16 inch barrel instead of 14.5? Since 14.5 is only good to 90-140 yards, I would think 16 would be better for them. I heard it was because a bayonet would not work for some reason on a 16 inch but I don't get that. They could have done a mid or full-length gas system 16 inch and it would be better.
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The reason is because there is a history of development behind the 16" barrel.
(note this is a "quick and dirty" overview, someone else can probaly do a much better coverage of this)
Originally some military (Saudi Arabia? Somehwere in the mideast) contracted out in the early 1970's for Colt to make them some carbines, except that part of the specifications were that the carbines had to be able to mount a bayonet. Colt already had the carbine length handguards and gas system developed, so basically they extended the barrel from 11.5" until the flash hider was in the right place to mount an M7 bayonet...and that happened to be a 14.5" barrel.
Later on the US military adopted the same length barrel most likley for the same reasons- a more compact barrel that could mount a bayonet. What you have to realize is that mounting a bayonet is simply a specification requirement of a military rifle. You can argue "well that doesn't make any sense" but that really doesn't play into the argument- Uncle Sam wants a shorter barrel that mounts a bayonet, and Colt deliverd them an already developed off-the-shelf product.
However, in the United States you cannot have a barrel under 16" without NFA paperwork, so many of the commerical manufacturers just added 1.5" to that length and *poof* a non-nfa carbine barel length was born. Of course, the bayonet no longer mounted properly, but with the AWB that ended up becoming a moot point.
The midlength gas system is a very recent development, and wasn't an option back around 1990 because it simply did not exist, not even on paper! Being what we know now about the carbine length barrels and their ballistics, it would not suprise me if it been available when Uncle Sam was looking for a shorter barrel that it would have been adopted in lieu of the 14.5" barrel. then again, the military went to the newer 62gr ammunition to be in line with the NATO standard despite the fact its terminal ballistics are inferior compared to M193 55gr ammo. As you can see, many of the decisions can have less to do with "the best weapon" and more to do with politics and expediency.
As for the Bushmaster Dissipator, it DOES NOT use a full length gas system- it actually has a cut-down front sight block under the handguards, and then a second front sight block that has no purpose in the gas system and is only used to retain the handguards and hold the front sight post.
At this point there is no reason for the military to adopt a midlength barrel from a simple logisitcs standpoint- they woud have to stock THREE lengths of barrel, handguards, and gas tube.