Since early in the Iraq war, Marines in combat have carried either the M16A4 rifle or M4 carbine, 5.56mm workhorses that leathernecks know well. Soon, the Marine Corps could adopt big changes to those weapons. While the Army searches for a possible new carbine, Marine officials want to overhaul both the M16A4 and M4, modernizing to make them more adjustable, accurate and comfortable. The effort to modernize the M16A4 shows a continued divergence with the Army, which continues to favor carbines and their shorter barrels. The Corps continues to stick largely with the M16A4 and its 20-inch barrel, which advocates…