Wow, you really need to do some more reading before posting ignorant comments. Shorter barrels are inherently more accurate. What the vast majority of uneducated folks can't seem to grasp is that what they perceive as "accuracy vs barrel length" is actually the relationship between barrel length and VELOCITY!! Two separate aspects, two different factors. A shorter barrel, propelling a bullet at the SAME velocity as a longer barrel will experience LESS vibration or harmonic oscillations which equates to a more STABLE barrel and thus a more consistent level of accuracy. Velocity really isn't a concern at nominal distances and when competitive targets are mentioned. Yes, a shorter barrel will produce a LOWER velocity and thus results in a more dramatic parabolic arc, also known as trajectory. Without known data or come ups, accurately PLACED shots from the shorter barrel will be somewhat more difficult. However, a short barrelled rifle with known come ups compared to a longer counterpart will produce a group with less deviation at nominal ranges. However, that all being said, a longer barrel as we discussed above will produce higher velocities, which translates into faster flight times and thus less influence from wind on your groups. As range increases and velocities decrease, the slower bullets from the short barrel will suffer greater drift from wind and produce larger more inconsistent groups. What does this all mean? Short barrels are inherently more accurate than longer barrels but suffer from reduced velocities which translates into less long range consistency. Its a fine balance between length and rigidity..
As for the SDM-R, it's what the Army(and Marine Corps) put together with CURRENT ISSUE 20" M16 rifles. It is far from ideal, and is not definitive evidence to support your claim. In addition, the US Military does not "make" the SDM-R, they procure them from Colt and or FN, the two companies with contracts to provide rifles to the DOD. They are modified by their armorers. Feel free to read up on ideal barrel lengths in a precision AR platform, in fact I will make it easy for you, its 18". KevinB on this forum happens to work for Knights Armament, and he(and others on CGN) will surely tell you that a 20" barrel is not ideal, and that yes, shorter barrels due to their rigidity are inherently more accurate than longer barrels. Have a look at Knights website, they don't offer a 20" 5.56x45mm barrel. The Mk12 Mod X series rifles used by the USN and other SOF units sports a wait for it....... An 18" barrel.
TDC