In light of The New Rifleman’s post about 20″ barrels, I felt this subject was fitting. One of the great mysteries of the firearms world is how the US military decided on the M16A2 barrel profile, or what has come to be known as the Government Profile. To most observers, this profile seems backwards. You would typically want more mass around the chamber and first several inches of the barrel to help with heat an stiffness. The government profile is thin in this area, but widens up at the muzzle.
When the M16 was first adopted, and all the way through the years of the M16A1, it had what was called a standard profile. The standard profile of the day was what we now call a lightweight, or pencil, profile.
This barrel profile proved to be quite good for a general issue combat weapon. It was lightweight and less cumbersome when walking on long dismounted patrols. Combat weapons, generally, are carried a lot and fired relatively little. When fired, the barrel was still accurate enough for combat purposes, especially when compared to the competing AK-47 design of the time.
During the product improvement efforts of the M16A2, the barrel profile was changed. The new barrel kept the same lightweight profile between the receiver and the front sight base, but increased the diameter of the barrel from the gas block forward. The general internet buzz is that this was done in response to GIs using their rifles as pry bars to open ammunition crates and rations, thus bending the thinly profiled barrel in the process. Another line of thinking is that bayonet drills led to a lot of bent barrels. If that was the case, it would seem counterintuitive to only beef up the diameter of the barrel forward of the bayonet lug, since force would be applied to the entire length of the barrel and not just the forward end.
I came across a post concerning the M16A2 development effort:
Since The M16A2 Product Improvement Program (1980-1983) was my program, this is the down & dirty on the barrel thickness issue.
We (Marines) were replacing a lot of “bent” barrels that were determined to be “bent” because the Armorer’s Bore Drop Gauge would not freely pass through some barrels during Ordnance Inspections (LTI’s). So the Logisitcs people had “Barrels Bending” on their list of “M16A1” things to “Improve” right after listing “Handguards Breaking.”
We “experts” thought this bending was from rough handling like during bayonet drills, etc., as an absence of any mid-barrel handguard damage in these rifles made one assume the fulcrum of such bending was the bayonet lug. So we made that part of the barrel thicker because we did not want the excess weight of a full length heavy barrel.
In testing using the bayonet lug as a fulcrum, and applying calibrated mechanical pressure to the muzzle, the new barrel was about 9 times more resistant to bend and take a set than an M16A1 profile. So we went with this “improvement.”
However, soon after I started using a bore scope with a video recorder and monitor to inspect “bent” barrels. What I found was a mound of bullet jacket material at their gas ports. This build up was caused by a burr left from drilling/reaming the gas port. This was where the Armorer’s Drop Gauge was geting stuck. When we removed this “mound”, the barrels would all pass the Drop Gauge.
We let Colt know what we had deduced, and that is one reason they kept models of “A2’s” in their line-up with A1 profile barrels. However, the A2 profile was already down the road for the US Military. So about the only advantage of the A2 profile was to give the rifle a little more muzzle hang. This was noted by most all the Operational Test participants, especially when they fired the standing/off-hand leg of our rifle qualification course.
So my advice to military armorers is to never replace a bent barrel until you visually check the gas port, or at least scrub the hell out of the gas port area with a new bore brush and an electric drill. And thank God for chrome bores!
Edit: Reader Daniel Watters informed me that the author of this passage, who goes by the name “coldblue” on AR15.com is, in fact LTC Dave Lutz (USMC-Ret), the former VP for Military Operations at Knight’s Armament Company. Given what I have come to know of the military acquisition and project management process, especially during the late 70’s and early 80’s (ever seen the movie The Pentagon Wars? Watch it!), it seems entirely plausible. The product improvement contract was already written and things were already being manufactured, it’s hard to stop that kind of institutional inertia and go back to the previous profile.
Now, there isn’t that much of a weight difference between the two profiles. The A2 weighs about 4.8 ounces more. That weight is also all out front, which does provide a bit more “hang” when shooting from certain positions (as noted in the above quote). If you aren’t familiar, “hang” refers to a slightly forward balanced rifle that is easier to settle on to target and jumps less after the shot breaks. My estimation is that some groups of shooters will tout the benefits of this hang over the expense of 4.8 ounces. But, if I had my way, we would go back to the standard pencil profile for general issue. These rifles were never meant for extraordinarily high volumes of fire, that’s what light machine guns are for. Malfunctions and accuracy loss due to increased rates of fire beyond the designed specification are another issue entirely.