What's wrong with belt-fed AR15s?

The ides of belt fed is for sustained automatic fire, as in for a "true" (military definition) machinegun. In sustained fire, there are issues with things like overheating barrels and cook-offs, so another desired feature would be a quick change barrel. Still a third would be open bolt operation. A M16 will reach cookoff temperature after 120 rounds of automatic fire at the cyclic rate with an air temp of 75 degrees F. Yes, there are/were squad support weapons like the RPK and Israeli FNFAL and even the M14 that operate from the closed bolt, but they really aren't that effective. The whole concept of a belt fed closed bolt weapon with no quick change barrel ability makes no sense at all from where I'm sitting, and even less if it doesn't have full automatic capability.

DocRock said:

A belt-fed semi-auto would be silly. I can see no real purpose or benefit to it.

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Presumably, this would be used with a registered full-automatic lower.

But, people do buy the semiautomatic FN M249S (for $8,000). There are lots of things in the gun world that are "silly."

edwardware said:

In some sense, they did, and named it the M249. The lightest/smallest practical 5.56 machine gun.

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The M249 fires from the open bolt and has a quick change barrel. Not the same critter.

Jim Watson said:

Heck, a lot of Marines have gone to the M27, a box magazine gun replacing the M249.

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The story about the HK416 and how it ended up in the USMC in very convoluted, somewhat confusing, reads like a conspiracy theory, and still isn't over.

herrwalther said:

The M249 is essentially a belt fed AR15, as far as weight and length go. It can even use AR magazines. The issue is with the gas system. The M249 wasn't reliable with the DI from the AR, which is why it is an open bolt long stroke piston action instead.

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The M249 shares the same cartridge as the AR/M16, and that is where the similarities mostly end. The AR magazine capability seemed like a good idea, but was eliminated in later versions, such as the SOCOM MK46- because it just didn't work.

zdc1775 said:

Personally I always thought that the Stoner 63 was the closest thing I have ever seen to a belt fed full auto ar-15.

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The SToner 63 was modular and had the ability to be used as an open bolt or closed bolt weapon, among other configurations. I never got to mess with one, but as I understand they were very expensive to build. The capabilities of the 63 as a squad automatic weapon lives on today in the SAW, MK46/48, Ultimax, and probably a few other weapons.

FL-NC said:

The whole concept of a belt fed closed bolt weapon with no quick change barrel ability makes no sense at all from where I'm sitting

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The classic Browning machine guns (M1917, M1919, M2) were all closed bolt w/o quick change barrels. Yet that was one of the most successful designs of all time.

FL-NC said:

The M249 shares the same cartridge as the AR/M16, and that is where the similarities mostly end. The AR magazine capability seemed like a good idea, but was eliminated in later versions, such as the SOCOM MK46- because it just didn't work.

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Anytime we used the 249, the SAW gunners would get a few AR magazines. Gave the gunners some experience with non reliable method of ammo feeding. I believe the non magazine models started filtering to line units after awhile but I was on my way out the door by then.

zdc1775 said:

Yeah the SEALs had them, the one I saw was at the Naval Special Warfare Museum, and I believe that the Marine Corps tested a version of it in the late 60's or early 70's. If I am remembering correctly they made around 4,00 of them but in field use it proved to be too fragile for general issue.

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The USMC liked the Stoner 63, and after the test in 1963-64, recommended adopting them. They tested them at Parris Island and Quantico. No mention of "fragility".

The reason HQ USMC did not adopt the XM23 was they felt that 7.62mm was going to remain standard and did not feel 5.56mm was adequate.

Remember in 1964 the XM16 was still undergoing growing pains and the Army was still planning on issuing the XM16 only to airborne and air mobile divisions.

DocRock said:

A belt-fed semi-auto would be silly. I can see no real purpose or benefit to it.

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I thought the sole purpose was to work around magazine restriction States, though I can be naive about such things. If true however, it might make for an interesting project until some politician takes it to task to further restrict rights.

Fightlite makes one ($4000) and i think kinghts armament made a civilian belt fed upper but cant find it. Magazine restrictions like skylerbone said is my only thought. But set up with a heavy barrel and a spade grip/trigger i could see it maybe having a use, would rather have

edwardware said:

I'm interested to know what one has to do to get near blowing a gas tube. Is this a high round count thing, or and extended mag(s) dump thing?

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Gas tube is like a fuse, should blow before anything else, designed to be for if things get too hot it will let loose before the barrel, if i understand correctly.

FL-NC said:

The M249 fires from the open bolt and has a quick change barrel. . .The M249 shares the same cartridge as the AR/M16, and that is where the similarities mostly end.

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You just illustrated my point. Those gaps between the M249 and M16/M4 are "why no belt fed M4?"

Bump firing would be fun, but like a detachable box, bump firing gets old and boring. So what’s wrong with them? Cost, weight, portability. Belt fed guns have been around almost as long as a box magazine, and it only survived as feasible in a military application. Could one be made to work? Absolutely, and with reliability and rate of fire capable to keep up with a mans finger. But it is inherently heavy (more moving parts, heavier ammunition supply) and bulky (ammo can hangers and whatnot to feed it, and a bipod/tripod to support the weight) this less maneuverable and less suitable for any practical purposes. Really cool as a movie prop, or as a conversation piece, but for the expense and hassle I want more enjoyment... from a used Harley or pontoon boat.

FL-NC said:

The M249 shares the same cartridge as the AR/M16, and that is where the similarities mostly end. The AR magazine capability seemed like a good idea, but was eliminated in later versions, such as the SOCOM MK46 - because it just didn't work.

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Israelis have an answer for the "legendary unreliability of the SAW when feeding from magazines", and it's just a 2-position gas regulator:



Russians had to go the other way when they modded DP to feed from belts as RP. They upgraded the gas system of DP considerably, as evident just by comparing their gas cylinders and pistons. Of course RP feeds in 2 stages, too.

For an AR15/M4 to be successfully turned into a belt fed weapon would require a lot of changes to the design. It would probably need to be converted to some sort of piston system and would certainly need to be converted to fire from an open bolt. It would need a heavier barrel and would benefit from some sort of quick change barrel system. And I do mean quick change. Not unscrewing allen keys and changing barrels in a minute. Flipping a switch, grabbing a handle, and switching barrels in five seconds. The bolt would benefit from fewer smaller lugs. The location of the standard charging handle would probably interfere with a top cover operation, so either you'd have to do something funky to load the belts, or you'd have to move the charging handle to the side of the receiver. All of this could be done, and has been, with varying degrees of success. But at some point you have to ask yourself, why change literally every defining feature of a system rather than designing a new system from the ground up?

herrwalther said:

The M249 is essentially a belt fed AR15, as far as weight and length go. It can even use AR magazines. The issue is with the gas system. The M249 wasn't reliable with the DI from the AR, which is why it is an open bolt long stroke piston action instead.

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The M249 has functionally nothing to do with the AR15, on any level. About the only way to make an M249 malfunction is to feed it with STANAG magazines. It will run like a sowing machine and you can write your name with it if you feed it from a belt. You can even clean the thing with sand. The M249 fires weighs ~20 lbs. It is accurate for a belt fed weapon and is controllable, for sure. But it is over weight for its cartridge and under powered for a support weapon. This is why the Marines went with the M27 IAR, which is basically a Western M4 equivalent of the Soviet RPK. It fires from an open bolt on automatic fire and a closed bolt in semi-auto, giving it the ability to sustain automatic fire better than a normal closed bolt M4, but still retaining the accuracy of a rifle in semi-auto. Fed from 60 round Surefire magazines, it has a decent amount of firepower available, but it is still lightweight and maneuverable enough to be used in fast and dynamic situations. The M27 lacks the M249's raw firepower, but makes up for it by being a more useful all around system. Lugging a 20 pound poodle shooter around rarely makes sense. With the M249, you're carrying the weight of a support weapon without the power and range.

And that is the problem with the entire concept of belt fed 5.56s. It is a lightweight carbine round. Putting the 5.56 in a dedicated support weapon is like putting a 4 cylinder engine in a 3/4 ton pickup.

MTMilitiaman said:

The M249 has functionally nothing to do with the AR15, on any level.

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The M249 is essentially a belt fed AR15, as far as weight and length go.

Emphasis added for the reading impaired.

edwardware said:

I'm interested to know what one has to do to get near blowing a gas tube. Is this a high round count thing, or and extended mag(s) dump thing?

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Eugene Stoner intentionally designed the gas tube so that it would come apart at the gas block before the barrel got hot enough to ruin it. The idea being, it is cheaper and easier to replace a gas block and/or gas tube than to replace a barrel. So the gas tube is the weak link by design so that sustained automatic fire doesn't ruin larger or more expensive critical components.

Iraq Veteran 8888 on Youtube has some fantastic melt down videos where he just does mag drop after mag drop, full auto, until a rifle fails. Even a cheap PSA upper will go through more rounds than most people can carry before it fails. But the fact is that the AR/M4 was intended to be a lightweight infantry rifle, not a support weapon.

herrwalther said:

The M249 is essentially a belt fed AR15, as far as weight and length go.

Emphasis added for the reading impaired.

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The M249 weight 3x as much as the M4/AR15, emphasis added for those who don't understand numbers.

FL-NC had the answer, the barrel, is the weak link.
You can’t do this with an M16.



Obviously an exercise in swapping them but shows how, even people learning how to do it can do it fairly quickly to have the ability to sustain fire.

MTMilitiaman said:

The M249 weight 3x as much as the M4/AR15, emphasis added for those who don't understand numbers.

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I have carried every version of the M249. The longest and shortest. The short versions are not 3x heavier. The DOD has gone to great lengths to make weapons lighter in recent years. Even the newest 240L feels more like a SAW today than it did even 5 years ago.

herrwalther said:

I have carried every version of the M249. The longest and shortest. The short versions are not 3x heavier. The DOD has gone to great lengths to make weapons lighter in recent years. Even the newest 240L feels more like a SAW today than it did even 5 years ago.

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I have tens of thousands of rounds through multiple versions of the SAW. The lightest SAW I was ever issued was 18 pounds, unloaded. The last version I was issued had a tri rail handguard and a M4 style collapsible stock with a buffer and weighed 22 pounds empty, which yes, is about 3x as much as an M4A4. The weight of the SAW compared to its range and power is the reason the Marines went with the IAR, and the reason the Army is replacing it with a new system running a larger, high pressure 6.8mm round.

Belt fed AR 15 / M-16 would be just fine with a water cooled barrel.

Boattale said:

Belt fed AR 15 / M-16 would be just fine with a water cooled barrel.

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Nope. The whole point of going to the 5.56 round to begin with was a lighter weight weapon, with lighter ammo for the common foot soldier. You put on some water and a pump you are taking away the very weight savings you sought to eliminate. I believe the M1917 was the last water cooled weapon we will see for a long time.

MTMilitiaman said:

22 pounds empty, which yes, is about 3x as much as an M4A4

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No such thing as M4A4. Counter-Strike doesn't count...

MTMilitiaman said:

The weight of the SAW compared to its range and power is the reason the Marines went with the IAR, and the reason the Army is replacing it with a new system running a larger, high pressure 6.8mm round.

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The rumored 6.8 is approximately .270 Weatherby Magnum equivalent.
How heavy do the rifle and MG have to be to make that manageable?
Is that worth humping to get more power?

herrwalther said:

Nope. The whole point of going to the 5.56 round to begin with was a lighter weight weapon, with lighter ammo for the common foot soldier. You put on some water and a pump you are taking away the very weight savings you sought to eliminate. I believe the M1917 was the last water cooled weapon we will see for a long time.

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Really? I never thought of that....

The M16 was supposed to be a light weight rifle that facilitated the carrying of more ammunition. When you start adding feed mechanisms and heavier barrels, etc. etc. you've blown the original design concept. It was not supposed to be a SAW.

My issue with the IAR is that I don't see it as having a major advantage compared to the M16 or the M4 in any situation other than a static defensive position. The M16 and M4 both can run full auto, and a 60 round surefire mag works just fine in each. Yes you will burn out barrels and have a risk of cooking off rounds in both platforms, but again other than a static defensive situation I don't see where the potential increase in barrel life in the IAR is too much of an advantage. As has been discussed regarding the SAW a full auto platform in 5.56 is not optimal as a support weapon, regardless of barrel life or being mag/belt fed. Getting rid of the 3 round burst feature on the M16A2/M16A4, and replacing it with full auto would seem to accomplish the same thing as the IAR with equipment all ready in the inventory.

Back to the issue at hand: If we're discussing a military usage, the ship has sailed with the advent of the surefire 45 and 60 round mags. Yes a belt can be longer, but is far more cumbersome to deal with. The soft bag "nut sack" for the SAW holds 200 rds, and unless you are running cyclic or rapid fire, you should be able to swap a new magazine to keep your ROF at least at sustained if not faster. Additionally I know of no belt fed system where you can get a new belt running in the gun as fast as you can do a mag change in the M16/M4/AR platform.

In a civilian context, using a heavy/bull barrel, free float, highly vented handguards, it would allow for a significant increase in onboard ammunition and sustained fire when you don't have 7 other guys in your squad putting down rounds.

herrwalther said:

You put on some water and a pump you are taking away the very weight savings you sought to eliminate.

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Water cooled guns normally don't require pumps. The natural convection of the hot fluids, and condensing can take care of keeping the water moving, and the water jacket full. Unless you're talking pallets of ammunition like in WW1.