The Colt SOCOM barrel is a medium contour barrel which was produced to bridge a gap in the M4 Carbine’s ability to handle full auto fire. The thicker Socom profile handles longer strings of full auto fire than its skinny predecessor. (from Lothan, as posted in The New Rifleman, December 1, 2017)
In the early 1980s, the US Army sought a shorter and more manueverable rifle than the 20" M16 with the A2 buttstock, and over time the M4 developed with a collapsible stock and carbine gas system. The US Army Armament and Research Center determined that 14-1/2" was the optimal barrel length for a carbine weapon system. By 1985 Picatinny Arsenal was tasked with developing what would become the M4 carbine, starting with modifications to improve reliability of the M16, to include the M4 feed ramps and the carbine buffer system.
The SOCOM barrel features a medium contour underneath the handguards, distinguishing it from heavy barrel profiles commonly associated with the M4. This design choice allows for a balance between weight and sustained fire capability (See M4Carbine.net post). Notably, the barrel has an increased diameter between the receiver and front sight compared to standard M4 barrels.
The SOPMOD Block 1 rifles were developed as early as 1997 with an M4 Government profile barrel, but not produced in enough volme for the beginning of the War on Terror in Afghanistan invasion in 2001, where the M16 was the primary infantry rifle. Some began to see action in the evasion of Iraq in 2003, and then back to Afghanistan, slowly ramping up to be fieled enmass around 2005 to 2007, and had the front sight post.
As the Global War on Terror ensued in the 2001 and 2002, the M4 was introduced and massive production began by Colt, but saw overheating issues, as the design was not optimized for heavy fire duty. The Special Operations Command (SOCOM) later developed the heavy duty barrel design for use by Special Forces. Colt maintained the exclusive contract for the M4A1 and the barrels until 2013, when FN was awarded a contract for M16s and M4 rifles.
By late 2002, operators of the M4 carbine with the Government (thin profile barrel) started to report issues with overheating. 34% of users said the handguards rattled and became excessively hot when firing (see article by Antony Williams, April, 2013). The 2004 adoption of the SOCOM medium-heavy barrel changed the M4A1 for almost two decades.
The Block II program then focused on component improvements to the M4A1, with the adoption of the heavier "SOCOM profile" barrel in 2004 and free-float Rail Interface System II (RIS-II) handguard from Daniel Defense in 2008, initially in the FSB configuration. It was not until around 2012 that the Daniel Defense RIS-II with a flat surface and no front sight post became part of the SOPMOD Block 2 program, and this newer slick SOCOM barrel without the front sight post was issued by Colt and the Mk12 low profile gas block came into service under the RIS-II rail.
The SOCOM barrel is designed to be thicker and absorb more heat from rapid fire, and thus avoid the warping that was starting to be seen in the M4 Government profile barrels in fully automatic fire. We cannot find that that the thicker barrel was meant to be more accurate, but the fact is that its heft gives it strong resistances to what is known as barrel whip, which is a harmonic effect of the high pressure gas wave occurring to propel the bullet out the chamber, and as a result gives a wider range of point of impact of the downrange bullet. The lack of such whip results in a more predictable (accurate) outcome with the SOCOM barrel.
The Colt SOCOM barrel has a history of highly accurate fire at medium distances. Over the years, the SOCOM barrel has shot just under 1 MOA, which is outstanding for a military grade chrome-lined barrel. AR reviewer Molon has documented accuracy of Colt barrels, and found the SOCOM barrel to be 0.90 to 0.93 MOA on several tests, see SOCOM barrel test results here.
Molon conducted tests on both a Colt 14.5" SOCOM barrel and a popular Colt 6920 Government profile barrel, and while many might expect better results from a longer barrel, he found sub-MOA performance with the Colt SOCOM and 1.50 MOA with the 16" Chrome-Lined barrel:
A barrel is a hollow beam held in place at one end by the receiver. There is a pressure gradient starting at over 50k PSI at the chamber which rapidly drops as the bullet progresses down the bore. When bullet exits the bore, the pressure instanteously drops to the ambient external pressurs. The behavior of the barrel as these forces act upon it is known as barrel whip or barrel “harmonics”. Heat is also a factor. Rapid fire heats the steel, makes it more pliable, and causes it to expand, and thus move around more fluidly. This was the case in the Government profile barrel. Stiffness, the result of both a cooler metal and a thicker diameter, results in less harmonic movement.
Teledyne Technologies has experimented with barrel profiles, and one of the easiest properties to change is the stiffness of the barrel.
For starters, the increased stiffness negates the accuracy loss from barrel whip. Everyone who has seen high-speed video of a Ar15 firing has seen the barrel dance and whip as pressure is built up and released in the propulsion and expulsion (uncorking) of the projectile. Skinnier barrels will have more whip, and heavier barrels, being stiffer, will have less whip at the cost of weight. When a barrel whips, there is the potential for the barrel to release the projectile on a slightly different X,Y axis with each shot. If you eliminate whip, then you are eliminating a variable that will get in the way of accuracy. Ideally, a traditional barrel with the correct “tuned load” will release the bullet on a consistent manner with each shot. (See article on barrel performance review with Teludyne Technologies)
Charlie's offers barrel pinning service, with your choice of muzzle device. You can also get pinned and welded by a local gunsmith. I will tell you that our pin and weld service is the best in the business, because we searched long and hard and found the best shop, and we send our barrels out with muzzle devices to be pinned and welded so that your barrel is street legal at 16.1" or more. Our pin and weld service is also a little expensive, as it includes shipping to our pin and weld shop and return.
Charlie's Custom Clones will also attach your choice of low profile gas block for a modest fee, but we need your barrel nut sent to us, or buy a rail with barrel nut from us.
Note: The unpinned barrel is subject to all NFA laws and local regulatory oversight for a short-barreled rifle. Be sure you are familiar with all laws and that you can own and possess a short rifle barrel, or short barreled rifle.