Open Bolt

[Bucksnort1]

Will someone tell me the advantages of an auto weapon firing from the open bolt position like the Thompson sub? Seems to me the open bolt would lend itself to a dirty weapon.

[Scorch]

Designing a firearm to fire from an open bolt is cheaper and easier than designing a firearm to fire from a closed bolt, and it aids in cooling the firearm between strings of fire.

For full auto firearms, firing from an open bolt is cheap and very effective, the trigger is often just a flapper and the firing pin is often just a raised area in the center of the bolt to fire the round when the bolt slams home. To stop firing, just release the trigger and the fire control mechanism catches the bolt and holds it open. Dirty? Yes. Effective? Absolutely. Drawbacks? A few, including difficulty in effectively suppressing the firearm, and controllability when firing, although those are easily addressed by design elements.

[bamaranger]

Scorch has it. While we're on the subject, I was amazed when I found out that the BAR of WWII/Korea fame fired from an open bolt.

About all of the first and second generation subguns run with open bolts.

[wogpotter]

It keeps rounds from "cooking off" in the breech.
The next round is only chambered immediately before firing.
It allows for air to pass through the bore, improving cooling.

[Machineguntony]

1. By definition, on an open bolt, prior to firing, is the bolt always in the "back" position? Is an M16 an example of a closed bolt, where the bolt can be forward with the round chambered, when the round fires?

2. Is an open bolt susceptible to runaway fire? I hear about M60s running away, but I never hear about an M16 running away.

I am about to possess an M60, so I am trying to learn as much about these guns as possible before the pick up date arrives.

[Theohazard]

Machineguntony:

1. Yes and yes. In an open-bolt machine gun like the M60, the trigger simply acts as a bolt catch. When the trigger is pressed, the sear disengages and lets bolt slam home, which fires a round. As long as the trigger is pressed, the sear is disengaged and the weapon will fire continuously. When the trigger is released, the sear catches the bolt in its reward position and holds it until the trigger is pressed again. But in the M16 (and any other closed-bolt weapon) the bolt is forward and closed with a round in the chamber after you release the trigger.
   
   
2. Yes, open-bolt guns are very susceptible to runaway fire, but not for the reason many people seem to think. Many people seem to think that cook-offs cause runaway guns in open-bolt machine guns, but that's simply not possible. Like wogpotter said, an open-bolt gun is resistant to cook-offs. With an open-bolt gun there is never a round in the chamber that isn't immediately fired (unless the round fails to fire for some reason), so a hot barrel wouldn't cause a cook-off. And even if an unfired round got stuck in the chamber and then cooked-off due to heat, the sear would catch the bolt as soon as it went to the rear.
   
   In open-bolt machine guns runaway guns are not caused by cook-offs, they're caused by the sear not properly catching the bolt in its rearward travel. This happened a lot with our M240Gs when I was in the Marine Corps: The sear would get worn and all of a sudden the gun wouldn't stop firing when you released the trigger. At this point you can either just let the belt run out on its own or try to cause a stoppage (this is a good reason not to link too many rounds together at once). The feed pawls on an M240G are strong enough that you can stand on the belt and it will still feed, so pulling does nothing; you have to twist the belt and cause it to jam up when its feeding.

[Machineguntony]

Thanks, Theo.

Very good explanations that really clarify things.

The idea of a runaway machine gun, especially a 308, scares the heck out of me. Even in my m16s, if I do not employ a proper stance, I get pushed back and up by the gun. If a 308 started to runaway, and I was shooting in the standing position, that could be a very dangerous situation for the people around me.

BTW, I know you might say this is why you should shoot the M60 on the ground, but the M60 civilian owners guide states to specifically NEVER shoot using the gun mounted bipod, as, long term, it will cause damage to the trunnion. http://www.machinegunpriceguide.com/html/m60_part_2.HTM

[Theohazard]

Originally Posted by Machineguntony

BTW, I know you might say this is why you should shoot the M60 on the ground, but the M60 civilian owners guide states to specifically NEVER shoot using the gun mounted bipod, as, long term, it will cause damage to the trunnion.

Interesting. In the Marine Corps we were taught to always fire our machine guns from a tripod if possible, otherwise we used the bi-pod. Hip-firing an M240 was almost never done, it was something you would only do in an extreme close-quarters situation. And firing it from the shoulder is never done (except for fun); due to the 240's length, weight, and the location of the ejection port it's just not practical at all.

The Marine Corps had long ago stopped using the M60 by the time I joined, but I know it's easier to shoulder-fire than the M240. But the Marine Corps still used the M60 primarily from the bipod and tripod.

Originally Posted by Machineguntony

The idea of a runaway machine gun, especially a 308, scares the heck out of me. Even in my m16s, if I do not employ a proper stance, I get pushed back and up by the gun. If a 308 started to runaway, and I was shooting in the standing position, that could be a very dangerous situation for the people around me.

I don't know as much about the M60, but I know they're not as robust as the M240. But, unless your M60 is very used, I doubt it will have anywhere near as many rounds through it as our M240s did. We took good care of our guns, but we shot the heck out of them. That, combined with the Marine Corps' crappy budget before 9/11, contributed to our runaway gun problems.

Also, if you're worried about your M60 running away it's a good idea to never link together too many rounds. Once we linked together over 1000 rounds just for the heck of it, then we had a runaway gun. So our normal technique of just letting the belt run out was a bad idea. That's when I learned the technique of twisting the belt to induce a stoppage, because pulling on it did nothing.

If you have a runaway gun on your M60 and you're shooting from the standing position, you should be able to hold your stance while it runs out if you don't have too many rounds linked together. Practice shooting 100-round bursts just in case. Also, you could have a buddy stand to your left ready to twist the belt if you have a runaway gun.

[Machineguntony]

Theo,

The civilians owner's guide states that you should never apply military standard care and use to a civilian gun. The military can shoot the crap out of a gun and not care about replacing the gun due to the fact that it is easily and cheaply replaceable; NFA guns, especially the serialized part, are not replaceable. For example, the guide states that you should never shoot using the integral bipod because in the civilian realm, it will loosen the trunnion, and on M60s, the trunnion can not be replaced because it is the registered and serialized part. In the military, they can just replace the old trunnion with a new $100 trunnion.

Another thing you should never do in the civilian realm is to shoot loose belts. Shooting loose belts will cause the belt to whip and mar and scratch the top cover. I saw such a marred and scratched M60 when I was looking to buy one.

I am having my M60 reworked and refitted. I'll post some pics when it comes back from the shop. It's practically a new gun now.

I'll take your advice and not link too many rounds together.

If you are ever in central Texas, drop me a PM, let's go do some shooting.

Final question, as we are in danger of hijacking this thread...what is the best way to prevent a runaway machine gun from happening?

[Theohazard]

Too bad, I moved away from Texas a few years ago when my wife got a job here in WA. But if I find myself in your neck of the woods I'll definitely PM you!

That makes complete sense as far as the difference between a military and civilian gun; our armorers replaced parts all the time.

I never thought about not firing loose belts. We did it all the time. But we weren't worried about marring the top cover. Also, the belt usually doesn't whip around too much.

As for avoiding a runaway gun, I don't know much about the actual armorer's maintenance on a machine gun (we were just grunts, we weren't allowed actually work on them). But we were taught not to shoot short bursts because that puts extra wear on the sear. We were taught to shoot a minimum of 6-8 rounds bursts.

[Beretta686]

BTW, I know you might say this is why you should shoot the M60 on the ground, but the M60 civilian owners guide states to specifically NEVER shoot using the gun mounted bipod, as, long term, it will cause damage to the trunnion.

If you're that worried about firing it off a bipod, just get a tripod.

I've shot M240Bs off tripods quite a bit and it makes it much more accurate and controllable than with the bipod. But I still find getting up behind the gun on a bipod the most fun.

I wouldn't be too worried about a runaway gun. I've only seen it happen once with a M240C coax on an Abrams that was used as a training weapon at Ft. Knox and thus was shot much more than other weapons.

Also, don't worry too much about babying your M60. Yeah it is irreplaceable, but it's still a designed to be humped around all day by someone who scored a bottom-percentile on the ASVAB and still perform. I wouldn't abuse it, but I also wouldn't stay up at night worrying about breaking it.

[44 AMP]

Will someone tell me the advantages of an auto weapon firing from the open bolt position like the Thompson sub?

The advantage to an open bolt firing system in a full auto weapon is cooling. This applies to all, whether a blow back low pressure system (like a Tommygun) or a locked breech belt fed firing rifle rounds.

Except at the actual moment of firing, the chamber is always empty. This allows air to circulate, cooling the gun (somewhat) and prevents a hot barrel from cooking off a round.

An additional benefit to submachineguns is the open bolt system can use a fixed firing pin, which means the guns can be built simpler, and cheaper (less machining needed for the mechanism).

The draw back to open bolt systems is, the open bolt allows for dirt to get in. BUT it also allows for dirt to get out, so, other than when you have to roll your weapon in the mud, tis kind of a wash...

The other drawback is that when you pull the trigger, the bolt slams home, firing the gun, which is a disturbance to precise aiming. Not a big thing with machine guns, and sub guns, usually.

All modern machine gun designs I am familiar with use an open bolt. Select fire weapons (assault rifles and some smgs-notably the HK MP5) fire from a closed bolt.

I was MOS 45B20 in the mid 70s. Small Arms Repairman. Direct & General level maint. (essentially, everything short of replacing recievers). I worked on, and inspected a LOT of machine guns for Uncle Sam, and had the pleasure of working closely with a number of NATO allies and their guns, as well.

I know they cost a TON of money for civilian ownership, so I really hope you won't take it personally, but the M60 machine gun is about the worst gun you can have, for a belt fed machinegun.

Possibly "civilian" guns are better, I really hope so, but the military M60s were as close to a piece of crap as anything the US ever used, and worse than a lot of designs.

That's part of the reason they have a reputation for "running away". It can happen on a lot of guns, but the M60, because of the way it was designed, and the way it works, causes this to happen, when wear reaches a certain point. And the way an M60 works, it chews itself up as it works.

IN other words, the useful life of the gun, without repair is less than many other designs. And with repair it is still less than some other designs.

I have heard it said that a camel is a horse, designed by a committee. The M60 is a machine gun designed by a committee. They took several good design features from other, previous designs, and put them together in one gun. Badly.

I'm happy to discuss this at length, but that might be best for another thread, or in PMs.

If you want a belt fed gun for personal play, I would seriously consider a different gun. Every gun has flaws, and issues, but the M60 is a nearly "perfect storm" of them, from the point of view of a gun owner & shooter. From a military point of view, it was "good enough" for a long time, but you will note that it has been essentially replaced in front line service today.

And fwiw, since the beginning of disintegrating link belts, we have known that twisting the belt (to the point of breaking it) is the fast way to stop a runaway gun. With a non disintegrating belt, you have to twist it, and hold it twisted until the gun jams.

The M240 was just coming into service (replacing the M219 coax gun on M60 series tanks) when I got out of the service. I got to see the gun, and see some very impressive demonstrations, but have no personal experience with it as a repairman. But I know the M60, M2, and several others well.

Also, don't worry too much about babying your M60. Yeah it is irreplaceable, but it's still a designed to be humped around all day by someone who scored a bottom-percentile on the ASVAB and still perform. I wouldn't abuse it, but I also wouldn't stay up at night worrying about breaking it.

And there's the rub. It was designed to be used by grunts and still "perform" but that design concept also included spare parts and repair services. No, it won't break if you look at it cross eyed, but it will break and wear from normal use. Parts of the gun are intended to be replaced regularly, because they don't last a long time, even when not abused.

Even proper cleaning of the original M60 design requires replacement of the lacing wires. The gun is a laundry list of things that ought to have been done differently, and could have been done differently, BUT were not, because it was not a priority to the military at the time.

As long as you know this, going in, and are willing to keep an close eye on things and replace what needs replacing when it does, the M60 will work for you. Everything but the receiver can be replaced, so don't lose sleep over that. Just know that over the shooting life of the gun, you'll have to do more to that M60 to keep it going than many other designs.

[Theohazard]

Originally Posted by 44 AMP

And fwiw, since the beginning of disintegrating link belts, we have known that twisting the belt (to the point of breaking it) is the fast way to stop a runaway gun. With a non disintegrating belt, you have to twist it, and hold it twisted until the gun jams.

Good point. It's been almost 14 years since I fired a 240, but I distinctly remember doing both. Maybe I didn't always twist it hard enough and it didn't always break, and instead it just jammed up. I honestly can't remember exactly.

Originally Posted by 44 AMP

The M240 was just coming into service (replacing the M219 coax gun on M60 series tanks) when I got out of the service. I got to see the gun, and see some very impressive demonstrations, but have no personal experience with it as a repairman. But I know the M60, M2, and several others well.

The 240 is a great gun. My only hands-on experience with the M60 was using one of the SeaBee's M60s with blanks during a training exercise (we were the aggressors during a SeaBee training exercise). But our 240s seemed a lot better made and a lot more solid. The 240 is a lot heavier and longer though.

Our 240s were extremely reliable and robust, except for the sears breaking occasionally. And I don't blame that on the guns, I blame that on the fact that the armorers didn't seem to do any actual preventative scheduled maintenance on our guns, instead it seemed like they just fixed them when they broke (and any machine gun will break when you put enough rounds through it). And that might have been because the Marine Corps has always been tighter with money than any other service, and our budget was pretty crappy in the 90s.

[James K]

"The sear would get worn and all of a sudden the gun wouldn't stop firing when you released the trigger."

Off topic a bit, but I once worked on a project in which the AF Test and Eval people at Kirtland were involved. One Tech Sergeant kept asking how we could be sure that a corrected problem was really fixed. He kept going on and on about how when they tested some new machinegun the sear kept failing in spite of the contractor's assurances that the problem was fixed. At one point he said that "obviously" I knew nothing about sears. I didn't bother to tell him that I knew quite a bit about sears, and I never did convince him that fixing a software problem was not really in the same category. Anyway, AFOTEC prepared a nice test plan and we adopted most of it. Then the whole thing got cancelled, so it all went to naught. I enjoyed Albuquerque, though, especially that great restaurant on Sandia Mountain.

Jim

[Machineguntony]

44AMP,

Do you have any experience or knowledge of the new M60E6 design? Allegedly, they fixed many of the issues bugging the design of the old M60s.

[Rob228]

Theo,

I wasn't a 31 so you might be able to shine some light on this. Always heard the reason for the runaway problems on 240's was racking the bolt to the rear with the weapon on safe. Apparently it would keep the sear up with the bolt dragging across it wearing it prematurely.

[Theohazard]

Rob: That rings a bell, but I honestly can't remember exactly. It's been a while, and being a machine gunner -- like anything else -- is a perishable skill. Semper Fi, my friend.

[44 AMP]

Machineguntony,
Sorry, I don't have any experience with the M60E6, they came along many years after my hands on time with the M60 was over. The designation E6 means they have been through many design changes. In our system, the first major modification to a design get the letter A designation. Minor changes to the "A" model get numbers. example; tommygun (commercial M1928 Thompson) became the M1 submachine gun after some major design changes, to ease production. Smaller changes to the M1 made it the M1A1.

Now, an "E" model is either the 5th major revision to the design, OR it is shorthand for the "E"series of minor change, example: Sherman tank M4 (welded hull) M4A1 (cast hull), by the end of the war, the M4A3E8 was the state of the art Sherman.

M4 (base vehicle) A3 (third revision of first major design change) E8 (eighth revision of lesser design change "E""

I still remember the M60 pretty well, and I have some old books around here somewhere. If you know the M60E6, I'd be happy to compare notes. I can tell you there were a lot of things in the M60 design that should have been fixed. And also there are some things that are not "flaws" but that ought to have been done differently, and would have resulted in lower cost, and better performance in the field had they been done differently.

Top of my list for those are the bipod & carry handle. The bipod is attached to the barrel. This is quite common on LMGs that are fixed barrels (like the BAR), but on a quick change barrel gun, the bipod should be attached to the gun, NOT the barrel.

With the bipod on the barrel, each barrel needs its own bipod, adding to the bulk of the unit, and the COST. Great if you are building and selling them to Uncle Sam, not so good for the grunts that have to hump extra weight and bulk.

The carry handle. Fine. Very good, useful handle, BUT, if it had been put on the barrel (a small increase in bulk and cost, compared to the bipod), it could have been used to change the hot barrel. The gunner would NOT need the asbestos mit (supplied with the spare barrel assy), one more thing to hump, and keep track of in a combat situation.

There are several other things also, and I would be interested in learing how the E6 variant addresses them as well.

Runaway guns due to worn sears, or worn op rods can happen in many designs, the sear takes a serious shock stopping the heavy bolt/op rod group. The M60 can be fired in such a way that the sear could rub (and wear) on the op rod.

While I can no longer remember the gun, I do recall that there is at least one machine gun design that uses a "secondary sear" to catch the bolt group, saving the primary sear that shock and wear (the primary sear will catch the bolt, if the secondary fails). Just an example of what could have been done, in the design, to extend the service life of the parts. But they didn't do that. Either it wasn't recognized that it would be a problem, or more likely, wasn't considered to be enough of a problem to warrant the added expenses and effort of designing it differently.

When I was in, the grunts called the M60 "the pig" because they thought it heavy (spoiled kids, try humping a Browning 1917)

Those of us in the repair shops called it "the pig" for a number of other reasons. One of which was the greedy way it ate parts.

[Machineguntony]

The M60E6 is supposed to have resolved some of the problems of the old M60. For example, as you mentioned about the lack of carrying handle on the barrel, that issue has been fixed. Apparently, the M60E6 is good enough that the Danish military picked it as their GPMG.

I don't know too much about the E6, but I have one coming in. When it arrives, I will study it and compare notes about the old M60 design, and I'll take some pics of it and post them.

Just curious, from your experience, how many rounds would the op rods on the original M60 last, before the op rod needed to be replaced? I was told that the new op rods on the M60E6 last much longer.

[James K]

A big advantage to an open bolt blowback gun, like the STEN or TSMG, is that it can use advanced primer ignitiion (API). That means that the firing pin touches off the primer just an instant before the bolt fully stops. The cartridge fires in a couple of milliseconds, so the pressure within the cartridge case has to push back against the remaining energy of the forward-moving bolt, not just against its stationary inertia.

In practical terms, that allows the bolt to be a lot lighter and have a lighter spring than a closed bolt gun. That is very evident in the original TSMG vs the closed bolt clones sold by Kahr. The originals can be cocked easily with even the little finger, where the closed bolt guns require considerable force to cock them.

Jim

[Jim Watson]

I understand the principle of API but am not clear on how it works in a simple SMG with fixed firing pin. The open bolt strips a round from the magazine and feeds it into the chamber. OK. But the round is moving ahead of the bolt as fast as the bolt. What supports the round against the impact of the closing bolt/firing pin before it headspaces against the chamber mouth?

I know there is a timing function in the WWII era Oerlikon 20mm API blowback and assume that the hammer in a 1928 or M1 Thompson has a similar effect. That drives the separate firing pin faster than the bolt is pushing the round and pops the primer against inertia of the round.

[barnbwt]

One benefit of OB I didn't see mentioned is that ignition is more reliable (or at least, more authoritative). Either a fixed-pin SMG slamming home, or the full weight of the bolt carrier smacking a floating firing pin at the end of its travel. As far as failure modes, an open bolt fails to firing, a closed bolt fails to hammer/striker follow, misfire, or out of battery ignition. Fixed pin SMGs also tend to have OOBD issues when rounds miss the chamber, but are limited in their consequences by cartridge choice.

In some belt feds, you also just need to pull a starter tab through the feed tray before firing if the weapon is cocked. Closed bolts require at least one (usually two, IIRC) pumps of the charging mechanism to get a cartridge into the chamber.

The main benefit is that OB's are so much easier to build. So simple in fact, there is virtually nothing separating them from machineguns, so the ATF has banned their ownership, apart from SOTs or grandfathered/registered guns.

What supports the round against the impact of the closing bolt/firing pin before it headspaces against the chamber mouth?

Supposedly some designs have shallow/narrow chambers that slow the round enough for the bolt to ignite the primer, but still have enough "give" when swaging the round in that the bolt is not completely arrested before the powder goes. It's a pretty small effect, if there is one, and in any case all blowbacks have extra mass to keep the rate of fire down (well, except for MACs and mini-Uzi's, I guess, which are notorious for their stupid-high rates of fire)

The Oerlikon has a very special case head design to ensure the bolt still has a full head of steam when the primer blows; the rim is rebated a ton so the bolt head can extend into the chamber like a piston, and the extractor has a death grip on the case (I think it might be a T-slot bolt face, actually) to make certain the primer is hit hard. I suppose a mini-Oerlikon in 50AE could be done because of its similar case head design.

TCB

[44 AMP]

What supports the round against the impact of the closing bolt/firing pin before it headspaces against the chamber mouth?

It is a very small amount of distance, but because of the construction of the bolt and barrel, the bolt is still moving forward when the round stops against the chamber mouth. Its that last little bit of bolt movement that fires the round.

..assume that the hammer in a 1928 or M1 Thompson has a similar effect. That drives the separate firing pin faster than the bolt is pushing the round and pops the primer against inertia of the round.

The "hammer" of the 1928 Thompson is a simple pivot lever that strikes the frame of the gun, as the bolt shuts, and pivots, the other end transferring force to the firing pin.

The semi auto Thompson is a completely different gun inside. In order to obtain BATF approval, the entire inside mechanism of the gun was redesigned, and is approx. 1/4" "off in dimension from the SMG, so one cannot simply put SMG parts in the semi and convert it.

This resulted in a gun that needs almost gorilla strength to cock the bolt, compared to the easy "finger cocking" of the SMG. All hail and thanks to our wonderous Treasury Dept!

As to the M60 op rod, I'm sorry, but in the shop, we never got a round count on the small arms. Mortars and Recoilless Rifles had logbooks, with round counts, but not the small arms. I can tell you the time period was 75-78, and about half that time I was in Europe, so the round count on the guns was from training, not combat, generally.

I can tell you that we tried not to replace the op rod, unless there wasn't another choice. Many times, we would just stone the op rod back to smoothness and send the gun back out. And, it wasn't the sear engagement that gave the trouble. It was the back of the upright portion of the op rod, where the bolt is cammed open and shut. The bolts would tear up the op rod at this point. Bad design? or bad execution? I can't say, but I can say that it happened, and happened regularly. NO gun, particularly an automatic should chew itself up like those did.

I understand someone finally got a clue and changed the cam angles to reduce this occurrence, I hope that's not just an urban legend, as it really needed done.

[Mike Irwin]

In submachine guns, the primary advantage of open bolt isn't cooling.

It's unlikely that anyone is ever going to be able to burn enough rounds through a sub gun to make a cook-off likely. Quite a few successful submachine guns have been designed over the years to fire from a closed bolt, most notably the HK MP-5 series.

The primary advantage is simplicity of manufacture and reduced cost. By making the gun essentially slamfire, you remove at least half a dozen different parts from the action and greatly reduce the amount of machining that is needed, especially in the bolt..

Interestingly enough, there have light machine guns that can be switched between single shot and automatic fire. In single shot they fire from a closed bolt, but when firing in automatic, the bolt is held open when the trigger is released to allow for chamber and barrel cooling.

The Johnson M1941 LMG was so designed, and I believe that the FG-42 also operated that way.

[5.56RifleGuy]

Http://www.usord.com/weapons/m60e6

These gentlemen do conversions on m60s to bring them to the E6 spec. There is a handy list of the improvements in the E6 model.

I was seriously looking into the M60, but changed my mind after looking into all the replacement parts and maintance I would be required to do.

All about one of these now (or both)

http://michaelsmachines.com/mm_23e.htm

The 23 or 21 really

Or these

http://www.machinegunarmory.com/civilian_firearms.html