Originally Posted By Augee:

The easiest visual discriminator of whether you're looking at a SOPMOD equipped M4A1 or an MWS M4 is usually the identity of who is holding it.  If it's a SOF troop, it's probably a SOPMOD M4[A1].  If it's a conventional troop, it's probably an M4 MWS.

SOPMOD = SOF
MWS = Conventional

The SOPMOD program was initially fielded in 1996, and the two have not significantly converged or crossed over at all since then, and there is almost no commonality between systems used in the two.   They are separate lines of development and evolution managed by two totally different components (Army and Navy).

SOPMOD is run by NSWC-Crane to equip SOF M4A1 Carbines with accessories.  There are two distinct "blocks" as identified by NSWC-Crane PowerPoints as the primary source of information, however, there was a "transitional period" of several years in which carbines with items from both blocks were used.  Because of a PowerPoint slide that was published (IIRC?) 2006 showing a block labeled "phased replacement," it has become the colloquial term for "transitional" configurations.

"Block x" does NOT refer, in an "official" sense to a specific configuration of rifle.  It refers to specific modifications (read accessories) to the SOF M4A1 weapon system that are managed by Crane.

Colloquially we refer to M4A1s with accessories from the first block of the SOPMOD program as "Block 1," while using "Block 2" to describe carbines with the RIS II, one of the last components of the second block of the SOPMOD program to be fielded.  Carbines with the accessories/features of both blocks of the SOPMOD program, but without the RIS II, we refer to as "Phased Replacement."  It is important to note, however, that this colloquial classification is generally limited to this thread, and others both here and on other sites that feed off of it.

The important thing to understand is that the SOPMOD program is almost always limited to SOF only, and 99% of the time, the "actual" weapon in question will be an M4A1, not an M4.

The M4 is a conventional forces weapon system, and is totally separate from the SOPMOD program and SOF modifications, though they were "inspired" by the SOPMOD program, almost every single actual item is different.

The M4 is managed by the Army overall through TACOM, though the Marine Corps also manages their own programs through their own directorates for their specific incarnation of the M4.

The M4 is a conventional forces platform, and is upgraded through modular weapon system (MWS) upgrades.  There are no specific blocks, increments, or generations, and their specifics vary by time period, unit, and MTOE.

~Augee