Open Bolt

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.
 
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?
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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
 
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What supports the round against the impact of the closing bolt/firing pin before it headspaces against the chamber mouth?
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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.
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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.
 
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.
 
Yeah, those MM E series guns are kinda cool. I guess. I want the 7.62x51 mag feed conversion for mine.

One of the NFA guys at the funstore down the way claims to have one of those M249 from US Machinegun but I have never seen it.
 
Im trying to figure out how I can get one without having to find a new wife.

I cant really blame her when she says its excessive.

Maybe if I had it delivered in a brand new car that was for her. Im not above bribery.

Have you heard reports on how the 249 works?
 
Interestingly, one machine gun that can be switched to closed bolt firing (single shot) with a light trigger pull and which is quite accurate in that mode, is the old .50 caliber.

The .50 HB M2 is capable of hitting an individual target at 600 yards and up and even at 1000 yards a hit will definitely get the attention of the victim.

Jim
 
The FN SCAR HAMR IAR was said to automatically switch from closed bolt to open bolt operation when the barrel heated up. I have never found out what mechanism caused this to happen or at what temperature the change from closed to open happened.
 
Interestingly, one machine gun that can be switched to closed bolt firing (single shot) with a light trigger pull and which is quite accurate in that mode, is the old .50 caliber.
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Actually, the M2HB is switched to the open bolt mode, in order to fire single shots, accurately. Now, this sounds like a contradiction, but its not.

The M2HB has a number of features that are NOT found on other machineguns, and they operate differently (and in some cases opposite to) the ones found on more modern designs.

The trigger only fires the gun. It does not hold, or release the bolt. There is a separate button for this. It is inbetween the two "wings" of the butterfly trigger.

There are three ways to operate the gun. The usual way is with the bolt catch locked down (not holding the bolt open). There is a hook on a sleeve over the buffer disc tube for this. With the catch locked down, the bolt runs back and forth, firing as long as the trigger is depressed, and the ammo lasts. Closed bolt operation.

OR one can depress the catch and the trigger at the same time, allowing the bolt to run and fire the same way.

OR, you can, with the bolt open, trip the catch, closing the bolt, and then press the trigger firing the round. When the bolt comes back, it will be held open.

This is how you shoot single aimed shots, "from the open bolt". Unlike other designs, where the trigger alone releases and fires the gun, with the M2 you can release the bolt (chambering a round) and then later, fire it. Having the gun in "open bolt" mode means only the chambered round will be fired.
 
I am redacting this post to make it G rated.

Point of post was to say that the M60 trigger is to not be pulled gently like a rifle trigger, but to pull it quickly.

That's just what I read. I have no personal experience with an M60.
 
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Nice pics Tony! Besides the different stock furniture and rails, I can see several changes from the M60s that I worked with.

Looks like they took several of the complaints to heart. The carry handle is now on the barrel, and the bipod now on the gun. Good move there!

The gas piston has a much different look, and hopefully the redesign will prevent it from vibrating loose during firing, as it was prone to do with the original M60 (which is why the gas piston was tied with lacing wire back then).

The leaf spring holding the pin for the triggerguard appears to be gone, which is another improvement. Not certain about the change to a pusbutton safety, its probably an improvement (or at least somebody thinks it is).

I also see the pattern of the bandolier hanger is different. I couldn't tell from the pics but if the hanger is still riveted to the feed tray, that is a point to watch out for. The original tray was light stamped metal, and the riveted on hanger would get loose in a fairly short time. Not loose enough to fail right then, but loose enough to see the rivets "wiggle", which made it unserviceable under the GI specs. If its still made that way, keep an eye on it, and maybe get a spare (or two?).

If they kept the original cam angles on the bolt and op rod, (hopefully they did not, as these were known to be a problem, and they did fix other known problems as far as I can see) you are going to have issues with them. If they changed them, it ought to run better & last longer.

Good luck with your new gun, and if I can be of any help or insight, just ask.
 
In the aftermath of WW2, the Norwegian Army had a large assortment of open bolt guns. MP40s, MP35s, Thompsons and Soumi KP31to mention a few... A lot of people experienced the runaway gun phenomenon when loading weak pistol ammo into these guns, who were intended to be used with "MP"-ammo. A dude I know with a collectors permit and an open bolt UZI demonstrated this with his own home made 9mm reloads.

Fun fact; the MG34 will chamber and fire 6,5x55 reliably, but not accurately ;)
 
A lot of people experienced the runaway gun phenomenon when loading weak pistol ammo into these guns, who were intended to be used with "MP"-ammo.
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You get a runaway SMG (open bolt type) when, on firing, the bolt comes back enough to strip the next round from the magazine, but NOT far enough to be caught and held by the sear. Its a very small difference, but sometimes, the stars line up, and the entrails are favorable, so it can happen.
 
The Thompson submachine gun, and the M3/M3A1 submachine guns, were designed around standard US military hardball ammo -- 230-gr. bullet at roughly 890 fps.

The US never produced any special "submachine gun" ammunition for use in these guns.
 
While I have never heard of the problem with US guns or ammo, I have heard of it before, with European 9mm guns and ammo. Not common, but possible.

More common is "hot" SMG 9mm being found identified only as 9mm in batches of surplus ammo. I have personally seen a Browning HiPower that was damaged from shooting such ammo, the owner had no idea such a thing was even possible. The gun was repairable, and the owner got an education that all 9mm ammo, and guns are not suited to each other.

There is another kind of "runaway" on open bolt SMGs although usually when it happens only a single round is fired. This can happen when the bolt is forward on an empty chamber, and the gun is dropped, or shaken so that the bolt slides back, just short of being caught by the sear, but enough to strip a round from the magazine, chamber and fire it. On firing the bolt is then (normally) caught by the sear and held back.

The US M3/M3A1 guns had a fixture in the cover, which prevented bolt movement when the cover was closed, acting as a safety.
 
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