The M4 is by no means a terminated weapon. But it could do with some improvements to bring it in line with advancements in the civilian AR-15 sector, and to take advantage of hard won battle experience.

Recall that the standard M4 is an AR-15 configured as follows (there are other features, but these are the salient points):

1. Barrel Length: 14.5″
2. Barrel Profile: A2 type (or “Government” type)¹
3. Revised feedramps on the upper receiver (“M4 feedramps”)
4. A 7″ (“carbine length”) gas system
5. Round plastic handguards with dual aluminum heatshields
6. A flat top upper with the MIL-STD-1913 (“Picatinny” rail) on top
7. A trigger group with capability for semiautomatic and three round burst fire
8. A four-position collapsible stock (and the necessary receiver extension and buffer system changes for this to work)

Great. Of course, time marched on. And as we became engaged in the War on Terror and deployed troops, soldiers found that they had a bunch of other stuff that they wanted to mount to their rifles. Lights, lasers, that sort of thing. So the plastic handguards were replaced with the Knights Armament Rail Interface System.² This system is a drop-in replacement for the earlier handguards. It does not require any special tools or disassembly of the upper to install. It is not free-floated however. It is also not very rigid, since it uses the same delta ring attachment method as the stock handguards. Good enough for an IR laser for use at night, but hardly for mounting more precise sights.

I should also point out that Colt rolled a bunch of improvements into some of the small parts of the M4. I will not discuss them much here, but note that they exist. One of them was moving from a “carbine weight” buffer to the “H1” buffer, which is heavier and made the M4 run more reliably.

After a number of battles, the US Army wanted to make some changes to their M4s. The result is called the M4A1. The following major changes were made:

1. Barrel profile revised to “SOCOM” profile. This is a medium-weight profile, adding mass under the handguards. It adds about 0.25 lbs of weight to the carbine.
2. A revised trigger group capable of semiautomatic and fully automatic fire.
3. A heavier buffer for better reliability

The revised, heavier profile was to make the rifle perform better when providing sustained fire. It will also mean that a shooter’s groups will not open up as much after shooting a bunch. Replacing the three round burst with fully automatic results in an improved trigger pull, even in semiautomatic mode, a more useful capability, and more predictable results. The burst cam was an awful idea, and it’s good that it’s gone.

The M4A1 PIP (also known as the M4A1+) was designed to add some more capability to the M4A1. The rifle would be as an M4A1, but with the following additional changes:

1. A new, more effective flash hider
2. A longer, free float handguard (likely the Daniel Defense M4 RIS II³)
3. ‘coyote tan’ coloring on the handguard to break up the outline of the rifle.
4. A low-profile gas block
5. A folding front sight
6. An improved (read: larger) charging handle
7. An improved trigger

The M4A1 PIP program was cancelled in June 2016 when the Army decided the upgrades were not worth the effort.

Sigh.

That is absolute nonsense. Observe the USMC moving towards getting M27s in the hands of everyone. Most of those improvements are actual, serious improvements on the M4A1 design. While #6 and #7 in the PIP changelist above might not be cost effective, the rest are very good.

A longer, free float handguard is the biggest and most obvious benefit. I finally tried a long (13″) handguard on Bridget, my competition rifle. And I really, really like it. It’s not about looking cool.⁴ The longer handguard gives you more options for where to put your hand, gives you more room for mounting accessories while still having a place for your hand, and is better to rest on a barricade than a barrel or a non-free-float handguard.

Free float handguards do provide a noticeable gain in accuracy, especially when resting the rifle on a barricade or using a sling. In other words, any time when we might be exerting some pressure on a barrel. Also, on an M4, we can’t get the longer, more useful handguard without free floating, and there isn’t much of a cost difference between free float and non-free float handguards, so there really isn’t a good reason not to go free float.

The low profile gas block makes life a lot easier, and means you don’t have to worry about burning yourself by touching the hot FSB. Daniel Defense does make a version of the RIS II that is 12″ long and has a cutout for the FSB. It’s a bit more annoying to install than one with a low-profile gas block. As long as it’s pinned in place, I’m cool with low profile for issue kit.

An improved flash hider is also welcome. The A2 ‘birdcage’ is a pretty good flash suppressor, unless you’re wearing night vision equipment. Then, it’s still pretty damn bright. So your own rifle is interfering with your vision. And people with night vision can find you easily. Not a problem while you’re fighting Taliban scum. Might be an issue if you fight a more formidable opponent.

I also really like coloring rifles not-black for issue purposes. It occurs to me that this could be done easily and cheaply with some Krylon and tape to keep paint off optics and out of muzzles, but the Army isn’t likely to go for that.

Unsurprisingly, I’m a big fan of the M4A1 PIP program, though I might like to see if other handguards are better (maybe one with mlok? It’s on the Army’s new HK CSASS). And buying new upper components and assembling them is a lot cheaper than buying M27s. And if you had a good PIP product, that would get rid of most of the reason for adopting the M27.⁵

As for the other question, is the M4A1 PIP better than the HK 416/M27, that’s likely to require testing that I can’t do here. And knowledge of what price I could get each for, which I don’t have. But within the confines of “For the US Army”, or another army that has already bought M4s, it’s almost certainly a better idea to upgrade.

Finally, some weight data if you’re interested in considering some of these rifles for your Infantry Kit Challenge.

Weights in the above table assume no carry handle is mounted and the weapon is unloaded. I used the weight for the Knights Armament 600 m rear sight since I don’t have weight numbers for the Army-issue Matech 600 m rear sight. Both fold away when not in use. ‘Basic’ means with the plastic handguards. M4 assumed to have H1 pattern buffer. Front sight on the M4A1 PIP was assumed to be the Troy folding front sight. For improved muzzle device on the M4A1 PIP, I chose the B. E. Meyers 249F, since it has tested better than the A2 birdcage. HK 416A5 and M27 weights included for reference. Note that the M27 has a 16″ barrel, not a 14.5″ barrel like all others on the chart.

1.) It’s a pencil profile that’s been thickened in front of the gas block. First introduced on the M16A2. It is a stupid profile and I don’t like it.
2.) Later replaced with the KAC Rail Adapter System (RAS).
3.) It’s a 12″ long quadrail that’s free floated, pretty sturdy, and already in the US DoD’s procurement system, so they wouldn’t need an elaborate RFP/Eval/challenge process. It also allows the easy mounting of the M203 with existing brackets, since that lacks rail mount adapters.
4.) Okay, it’s not just about looking cool.
5.) Yes, I know the HK 416/M27 has a short-stroke gas piston. While this is easier to develop, the M4’s direct impingement system has got a lot of development time already sunk in. And I don’t see actual benefits from the data when you compare the 416 to modern M4s (or Mk 18s if you want to talk properly short barrels). More on this later.