Getting ready to build a ar-15 based on the a4 platform but, with the a1 Vietnam style triangle hand guards. My question is will those hand guards fit a 16" mid length? Or do I need a 20" standard length. The hand guards measure a foot long.
There are some barrels out there that are shorter than 20", but still have full-length hand-guards (and gas tubes that run underneath)... I think this variation is what prompted TSU45's response.
Bushmaster makes these. They call them "Dissipators". A 16" barrel, carbine length gas system with a low profile gas block under the full length A2 style handguards, and a standard sight out near the front of the barrel. I understand this is done mostly to get the full rifle length sight radius.
Similarly, people also use low profile gas blocks to run longer handguards or rails. For example, I have a seen a fair number of guns with a 12" (rifle length) rails on a 16" barrel gun with a low profile gas block under the rail. An added advantage is that the gas block is covered by the handguard.
However, what the original poster is referring to is the "mid-length" gas system... The gas tube is approximately 9" long as opposed to 7" in a carbine or 12" in a rifle gas system. Many think that for 16" barrels that length gas system is ideal, as it puts less stress on the operating components as opposed to the carbine length gas system. Many think this also looks better cosmetically, as the carbine length gas system, originally designed for 14.5" barrels, leaves too much barrel protruding out the front when used with 16" barrel.
Mid-length non-railed handguards are less common, the only ones I can find are standard M4 style.