USMC Infantry Automatic Rifle submissions

[icyplanetnhc (Steve)]

As many are aware, a variant of the Heckler & Koch HK416 with a 16.5” heavy barrel was adopted to become the M27 Infantry Automatic Rifle (IAR) in 2010, and was more recently adopted as the standard issue rifle for line company squad leaders and below in 2017, while the M4 and M4A1 are used by platoon leadership and above and by MSOC.

However, some of the IAR candidates were a bit more novel, and I believe LWRC M6A4 and FN HAMR (and maybe Colt?) submissions for the IAR were able to fire from an open bolt when fully automatic and closed bolt when semi. Has there been anything published on the method for accomplishing this? For comparison, the current M27 fires from a closed bolt for both semi and full automatic.

As a side note, I’m personally not very fond of the M27 as a standard issue infantry rifle, due to the sheer weight of the system once all enabling attachments (i.e. PEQ, SCO, suppressor) are mounted, and the standard fighting load isn’t enough to sustain automatic fire.

 

[Scott Kenny]

As many are aware, a variant of the Heckler & Koch HK416 with a 16.5” heavy barrel was adopted to become the M27 Infantry Automatic Rifle (IAR) in 2010, and was more recently adopted as the standard issue rifle for line company squad leaders and below in 2017, while M4s are used by platoon leadership and above and by MSOC.

However, some of the IAR candidates were a bit more novel, and I believe LWRC M6A4 and FN HAMR (and maybe Colt?) submissions for the IAR were able to fire from an open bolt when fully automatic and closed bolt when semi. Has there been anything published on the method for accomplishing this? For comparison, the current M27 fires from a closed bolt for both semi and full automatic.

I'm guessing that they did it like the Lewis Gun and FG-42 did, and the M-60 MG would have if the Army had kept the select fire ability:
The bolt enters the locked position while the bolt carrier still has at least 1/8" of forward travel but is caught by the semi-auto sear, and the firing pin is attached to the bolt carrier. Pull the trigger in semi and the bolt slams forward under much spring pressure which slams the firing pin into the primer, fires the cartridge, bolt goes back, cycles the belt, bolt carrier comes forward to lock the bolt but is stopped from going all the way forward by the semi auto sear. There's a separate sear cut on the bolt carrier for Full Auto.

Ah, hell. Have you seen the Forgotten Weapons video on how the FG-42 works, or on an EM1 prototype bullpup?

As a side note, I’m personally not very fond of the M27 as a standard issue infantry rifle, due to the sheer weight of the system once all enabling attachments (i.e. PEQ, SCO, suppressor) are mounted, and the standard fighting load isn’t enough to sustain automatic fire.

And yes, that's the complaint I keep hearing about the HK416/M27/M38/HK417/M110A1. Too heavy.

 

[icyplanetnhc (Steve)]

I'm guessing that they did it like the Lewis Gun and FG-42 did, and the M-60 MG would have if the Army had kept the select fire ability:
The bolt enters the locked position while the bolt carrier still has at least 1/8" of forward travel but is caught by the semi-auto sear, and the firing pin is attached to the bolt carrier. Pull the trigger in semi and the bolt slams forward under much spring pressure which slams the firing pin into the primer, fires the cartridge, bolt goes back, cycles the belt, bolt carrier comes forward to lock the bolt but is stopped from going all the way forward by the semi auto sear. There's a separate sear cut on the bolt carrier for Full Auto.

Ah, hell. Have you seen the Forgotten Weapons video on how the FG-42 works, or on an EM1 prototype bullpup?

I have not seen that video, so I'll have to check it out. Also, it appears that the M1941 Johnson LMG also fired from an open bolt when in full automatic mode. It will be interesting to see how the lock times of such a mechanism compares to a conventional hammer strike.

In any case, a weapon that can fire from an open bolt would have better sustained automatic fire rates. Doctrinally, the sustained rates of fire is 12-15 rounds per minute for an M4 or M16, 36 rounds per minute for an M27, and 100 rounds per minute for a true machinegun like the M240 and the M249.

And yes, that's the complaint I keep hearing about the HK416/M27/M38/HK417/M110A1. Too heavy.

Back in TBS, I carried the M27 for a patrol exercise and it wasn't pleasant, particularly with how front-heavy it is due to the barrel profile and external short-stroke gas piston.

 

[Scott Kenny]

Back in TBS, I carried the M27 for a patrol exercise and it wasn't pleasant, particularly with how front-heavy it is due to the barrel profile and external short-stroke gas piston.

Yeah, I'd much rather deal with a Mk12SPR than an HK416. Still heavy forward, but not as terrible.

 

[njiiaf]

I have not seen that video, so I'll have to check it out. Also, it appears that the M1941 Johnson LMG also fired from an open bolt when in full automatic mode. It will be interesting to see how the lock times of such a mechanism compares to a conventional hammer strike.

In any case, a weapon that can fire from an open bolt would have better sustained automatic fire rates. Doctrinally, the sustained rates of fire is 12-15 rounds per minute for an M4 or M16, 36 rounds per minute for an M27, and 100 rounds per minute for a true machinegun like the M240 and the M249.


Back in TBS, I carried the M27 for a patrol exercise and it wasn't pleasant, particularly with how front-heavy it is due to the barrel profile and external short-stroke gas piston.

What makes the sustained rate of fire for the m27 so much higher than the m4/m16?

 

[Scott Kenny]

What makes the sustained rate of fire for the m27 so much higher than the m4/m16?

Doesn't heat up as quickly due to the short stroke gas piston instead of Stoner's direct impingement/inline gas piston in the bolt.

 

[MihoshiK]

Doesn't heat up as quickly due to the short stroke gas piston instead of Stoner's direct impingement/inline gas piston in the bolt.

And a heavy barrel profile.

 

[icyplanetnhc (Steve)]

It also makes wonder that if another IAR submission was chosen, would it be adapted into a DMR like the current M38? Although I would say that the M38 is rather redundant at this point, given that the standard-issue optic on M27s and M4s is the M83 SCO (military designation of the VCOG 1-8x28), which has the similar top end magnification as the M38’s Leupold scope, which I believe was directly pulled from the Mk. 12 SPR.

I would also argue that the M27 is not well adapted to the current-issue suppressor in infantry units, the KAC NT4 QDSS. Not only is the gas block not adjustable, but the can itself is it heavy and quite a high back-pressure design compared to more modern flowthrough ones like the HuxWrx Flow 556k, Sig SLX, or SureFire RC3. I do believe MarSysCom is looking at a flowthrough design, but I’d wager it would be quite a while before those hit the fleet as the NT4 procurement just finished.

 

[Colonial-Marine]

I'm of the opinion the USMC selected the HK416-based entry because they wanted a new assault rifle to replace the M4/M16 and not just for the IAR role. If you're looking at something for purely that role some of the other entries seemed more capable.

As it is I've got to question the wisdom of issuing a heavy barrel squad automatic rifle variant like that as standard issue. It's heavier than it needs to be and that's before they attach everything in the armory and the kitchen sink to it. Does every Marine with one have a bipod attached too? Perhaps too much of a focus on having one rifle do everything. I can't say I think the desire to issue suppressors to everyone and not just specialist units is a great decision either. The M27 with suppressor is probably not much shorter than the M16A4. If the benefits really outweigh the negatives it would probably make more sense to specify an integral suppressor for future designs.

For use in the IAR role I'm still surprised they haven't procured a higher capacity magazine. Something like a 50-60 round quad-stack magazine that the squad automatic riflemen could carry half a dozen or so of for when they need to lay down as much fire as possible for the rest of the squad. It wouldn't be nearly as cumbersome as drum magazines.

 

[Kat Tsun]

The Surefire 60s were tested back in 2010 when the IAR was being finalized and failed. I doubt they're much better now. The current hot idea is a 60 round drum magazine called the D60.

 

[Moose]

I got to handle an M27 once and the USMC armorer showing it to me was of the opinion that no high cap magazine solution was likely to be "marine proof" enough to get bought in large numbers.

 

[Scott Kenny]

I'm of the opinion the USMC selected the HK416-based entry because they wanted a new assault rifle to replace the M4/M16 and not just for the IAR role. If you're looking at something for purely that role some of the other entries seemed more capable.

That definitely hasn't been admitted, but sure seems to be the case based on subsequent behavior.

I can't say I think the desire to issue suppressors to everyone and not just specialist units is a great decision either. The M27 with suppressor is probably not much shorter than the M16A4. If the benefits really outweigh the negatives it would probably make more sense to specify an integral suppressor for future designs.

The various branches have run wargames when every troop had a suppressor, and the uniform response was "Holy (expletives deleted), just not having the concussion blasting troops was a huge improvement! Why didn't we do this sooner?!?!?!?"

For use in the IAR role I'm still surprised they haven't procured a higher capacity magazine. Something like a 50-60 round quad-stack magazine that the squad automatic riflemen could carry half a dozen or so of for when they need to lay down as much fire as possible for the rest of the squad. It wouldn't be nearly as cumbersome as drum magazines.

I have a Schmiesser/American Tactical 60rd quadstack, it's a pain to load. Those springs are way stiffer than any other STANAG mag I own. So bad that I actually had to buy a loading helper!

 

[icyplanetnhc (Steve)]

The M27 with suppressor is probably not much shorter than the M16A4.

It’s actually about the same, or longer with the stock extended. It wouldn’t be as bad if we used a suppressor that’s not as long or heavy as the QDSS NT4, which SOCOM phased out for the SureFire SOCOM RC2 a decade ago. A modern flowthrough suppressor would be even better (personally looking forward to see how the RC3 performs, will try to buy one for my DDM4). Goes to show how outdated the technology that was just adopted and still being procured by a major service branch can be.

 

[Kat Tsun]

I got to handle an M27 once and the USMC armorer showing it to me was of the opinion that no high cap magazine solution was likely to be "marine proof" enough to get bought in large numbers.

He's probably right unless someone comes up with a truly novel magazine.

The only people using Surefire 60s and 100s are special warfare units atm.