They just don't make 'em like they used to

"The 1911 was the design given by God to us through John Moses Browning that represents the epitome of what a killing tool needs to be. It was true in 1911 and is true now."
-Col. Robert J. Coates, USMC

John Moses Browning

The M1911 .45 Caliber Automatic Colt Pistol (ACP) has had an impressive life of service to the American military.  Introduced in 1911, John Browning's design has withstood the test of time and continues to serve as the oldest weapon system in today's military.  The M1911 is renowned for its reliability, durability, and most importantly, its effectiveness.

M1911 .45 Caliber Automatic Colt Pistol

Despite a major initiative to replace the M1911 in the 1980's with Beretta's M9, stalwort supporters of the M1911 held out in many pockets despite the military wide issue of the M9.  The M1911 remains in service today, largely in special operations units where commands have increased flexibility with regard to top down mandates of equipment issues.

M9 Beretta 9x19mm Parabellum NATO

The predominant complaint in transitioning from the M1911 to the M9 centers around effectiveness.  The M9 utilizes the 9x19mm Parabellum NATO cartridge with a bullet mass of 115 grains as compared to the .45 caliber ACP cartridge which is most frequently loaded with bullets of 230 grains.  The disparity in bullet mass is the single most important factor in accounting for the substantially increased stopping power of the M1911 as compared to the M9.

This issue rose in precedence during the Iraq war where urban fighting increased service members reliance upon the pistol in combat due to the proximity of enemy combatants.  While the rifle remained the primary weapon system for offensive operations in an urban setting, transitioning to the secondary weapon system (pistol) became more critical due to tactical situations which did not afford the service member the time, distance, or cover necessary to reload a rifle or correct a malfunction. 

Contrary to intuition, a pistol serves more of a defensive role in parmilitary applications where the primary offensive capability is filled by an assault rifle.  The pistol is an emergency backup offensive capability to be utilized in the event that the primary weapon system is unavailable (a malfunction in the weapon's operating cycle or an empty source of ammunition).  Transitioning to the pistol is faster than correcting an issue with the primary weapon system and therefore offers an more expediant solution when engaging hostile forces, especially in urban environments where the limited range of a pistol is less of an employment constraint.

Servicemembers who engaged enemy forces with the M9 often complained of its limited effectiveness, often requiring multiple hits before the target was incapacitated.  While shot placement is a factor, the terminal ballistics of the 9mm as compared to the .45 cal round are such that shots that impact outside of vital zones often fail to do enough damage to the affected tissues to cause a negative spike in blood pressure.  This results in an enemy's ability to continue fighting despite experiencing gunshot wounds to non-vital areas of the body.

The .45 caliber round does significantly more damage to affected tissues creating a larger permanent channel along the trajectory of its terminal ballistic path.  This inflicts a negative spike in blood pressure capable of incapacitating a target with a single impact outside of vital zones.  Ultimately making the unarguable case for the M1911's continued service in the United States military.