We talked a little bit about the M4A1 PIP before. Let’s compare it to the popular and successful HK 416A5.

The 416A5 is the latest variant of HK’s version of the M4. It’s available in a number of barrel lengths, but to try to keep this comparison as objective and direct as possible, we’re going to compare carbines with the same barrel length: 14.5 inches.

While the 416A5 is a production weapon, the M4A1 PIP isn’t. So I’ll have to make some guesses as to what it might have looked like in an approved format. In the above article, I made some guesses:

1. “SOCOM” profile (a medium profile) barrel
2. Safe/Semi/Full Auto Trigger Group
3. Daniel Defense M4 RIS II (12″)
4. Cut down pinned gasblock (as on Colt 6920-OEM2)
5. Knight’s Armament folding front and rear backup iron sights
6. H2-weight buffer
7. B. E. Meyers 249F flash suppressor

Pretty simple. I chose those parts because all of those have NSNs, and are already-approved accessories, and Colt already makes guns with that particular low-profile gas block. I keep going back and forth on the notion of changing the stock, and settled on not changing it mostly to keep things simple. The obvious stock alternative is the SOPMOD stock, however I couldn’t find anything on stock changes, so I opted to be simple and leave the existing stock. The SOPMOD stock weighs 11.5 ounces, which is about 4.4 ounces (0.275 lbs) more than the regular stock.

Let’s compare them. Both carbines are capable of semiautomatic and fully automatic fire. Both carbines do not have a burst feature. Both carbines have collapsible stocks. Both carbines offer quadrail handguards. The M4A1 PIP has a 3″ longer handguard (12″ compared to the 416A5’s 9″ handguard), but both are longer than the 7″ RAS/RIS handguard currently present on the M4.¹ Both fire the same 5.56×45 mm round.

Doing a weight comparison, the M4A1 PIP weighs 6.99 lbs as above, unloaded and without optic. The 416A5 weighs 7.68 lbs unloaded and without optic, which comes to a weight difference of 0.69 lbs. Where does the weight difference come from?

Both rifles have medium-profile, 14.5″ barrels. The 416 also has a short-stroke gas piston system and a heavy handguard, both of which add weight. The stock of the 416 is also a bit heavier.

Carrying more weight sucks. But weight can also bring advantages. Bearing in mind the following destructive tests had a sample size of one, let’s see what weight helps with.

On a standard M4, firing 140 rounds rapidly and continuously will raise the temperature of the barrel to the cook-off point. At this temperature, any live round remaining in the chamber for any reason may cook-off (detonate) in as little as 10 seconds.

What if you keep going cyclic? What if you’re desperate? Colt tested this in 1996, and discovered that the barrel on a standard M4 (with full auto trigger group) will burst after 596 rounds fired cyclic. That’s just under twenty magazines worth of rounds. That is a lot.

The M16A2 (also with a full auto trigger group) was also tested to destruction. Its barrel burst after 491 rounds. That’s also a lot, though less than the M4.

The M4A1 with the heavier SOCOM-profile barrel will fail after it has fired 840 rounds cyclic. In this case, the barrel won’t burst, but the gas tube will fail. If this was a problem, you could conceivably use a beefier gas tube. Or accept that this is good enough, since a soldier’s basic load is 210 rounds.

Unfortunately, I can’t find much in the way of good data on when the HK416 (or the M27 for that matter) fails if it’s run cyclic until it chokes. I do know that it’s “more than 900 rounds” but that’s the best I’ve got. This makes sense: the barrels are similarly beefy, and eventually the op-rod/piston will fail. Or the barrel will. Waste heat sucks.

So what do I think? Well, a good part would depend on what price you could get both guns for. But if you could get the M4A1 PIP as a package from Colt for any price that isn’t exorbitantly over that of the 416A5, I’d probably go with the M4A1 PIP. I like less weight, and I really don’t think the piston system gets you all that much for your trouble.

That goes triple if you already have M4s/M16s in your procurement system, since you can just swap uppers (and trigger groups if you have that infernal burst mechanism).