Go look at the Black Rifle II book on pages 94 and 95. What is stated in that book is that the M4 barrels were originally made thinner under the handguards. However, SOCOM troops have an odd way of exfiltrating an area under fire. Rather than use the old Vietnam CQB trick of a team exfiltrates in line with the last man in line emptying one mag, then throws a grenade and then peels off and goes to the head of the line, then the man who was next to last in line is now the last man in line so he empties one mag, throws one grenade, and then he peels off and goes to the head of the line, then the man who was next to the last man is now the last man and he does the same, So, what you are doing is leaving an area while the enemey receives a full mag of full auto fire followed by a grenade, then a full mag of full auto followed by a grenade. This provides a gradual exiting form the area but keeps your enemy pinned down. With this type of shooting you empty a mag and your gun has a cool down period before you work your way back to the end of the line and you shoot it again. Well the new SOCOM method is for everyone to stand on line or behind cover and empty mag after mag after mag with no rest in between magazines. Their theory is that they saturate the area where the enemy are suspected of being with a very high concentration of bullets. The problem was that a barrel could achieve its "tranformation" temperature which is around 1100 degrees F to 1,375 degrees F. This was done by firing 540 to 596 rounds, which had to be fired within 3 to 3 1/2 minutes. That was the equivalent of shooting 18 to 19 of the 30 round mags within 3 1/2 minutes. That is a lot of rounds and that is a lot of heat. This especially occurred in Afghanistan, where temps were around 100 degrees to 110 degrees F and this added to the heat buildup. By making the new barrels thick under the handguards, they can now withstand that amount of full auto fire without blowing a barrel. So, the thicker M4 barrels are referred to as the SOCOM M4 Barrels. I shoot full auto, but I let my weapon rest after one magazine. At worst, I might shoot two mags in a row. This is not sustained full auto fire, but bursts. Most people find that full auto is good to suppress the enemies fire until you can break out of an ambush. For example, the ememy springs the classic "L" shaped ambush on you. Everyone switches to full auto and charges the right angle of the "L" and so much lead is flying at the bad guys that they keep their heads down. But then you are back to the accurate single fire that is the most useful. I doubt any of us will abuse our M4 barrels like the SOCOM troops are doing. So, the slim barrel under the handguards M4 barrels should be fine for full auto and single shot, just not for almost 20 magazines of 30 rounds as fast as you can slap them in. Now for those who want the ultimate SHTF weapon, and want to pay the extra bucks, buy one and be the first boy on your block to be able to run that amount of ammo through your rifle without blowing it up. If I could trade my current skinny M4 barrels for SOCOM barrels, I would take them. But if I have to pay money for them, I would not do it.
Charles Tatum
Alamo Professional Arms