The M73 Tank Gun

Nathaniel F
by Nathaniel F

The period from 1945-1970 did not represent the figurative finest hour in US small arms design. From the problematic M60, to too little too late M14, to the disastrous initial fielding of the M16, US small arms design during the time seemed to simultaneously reach to far and grasp too little. One family of firearms that was a product of this period of development was a short-action armored fighting vehicle secondary machine gun design, incarnated in the M73 7.62mm and M85 .50 caliber, and later M219 7.62mm types. (EDIT: I don’t really think the M85 should be thrown in there, as it’s a substantially different design, though it shares some features with the M73 and M219. Mea culpa.)

Below is a training film covering the operation and disassembly of the “Machine Gun 7.62mm Tank M73”, hosted at the excellent channel of Jeff Quitney:

The M73, though short, got a reputation as being complex and unreliable. Why is unclear to me, as the gun generally appears to be well-thought-out for a tank MG, with some very desirable features:

1. The barrel can be removed while the gun is mounted behind armor, via the receiver swinging from either the right or the left).

2. Feed can be switched from the left to the right, a good feature to allow the gun to be reloaded easily from different positions in a cramped armored vehicle.

3. As previously mentioned, the receiver is short, reducing the space taken up inside the vehicle by the gun.

4. The weapon is charged via a chain, like a lawnmower. In the confines of a tank, this allows the gunner to charge the weapon without either having to reach all the way over to the gun or, god forbid, getting out from behind armor.

5. Cyclic rate of 450-500. This in theory should reduce wear and ammunition consumption while improving reliability, and is plenty high enough for tank MG duties.

6. Solenoid fired, to allow for a remote trigger. The guns are also provided with a manual back-up trigger.

So why was the M73/M85/M219 family so unreliable and bad? Today they have an almost legendary reputation for being stinkers in the field, but I admit I have no good theories as to why they were so unsuccessful. The fundamental designs seem sound enough, and the unique features they brought to the table really do impress.

Whatever the reason, this family of weapons was eventually replaced. The M73, by the improved but not much better M219, and then eventually by the tank variant of the now ubiquitous M240 machine gun, and the M85 by none other than the very weapon it was originally to replace, the timeless “Ma Deuce” M2 Browning heavy machine gun.

Nathaniel F
Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. He can be reached via email at

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  • Tom Currie Tom Currie on Aug 04, 2015

    The author's comments suggest that he is a historical researcher but has never served on any armored vehicle - much less any M48- or M60-series tank equipped with the M73.

    Comments, 1,3,5 & 6 are correct. Comments 2 & 4 however demonstrate his unfamiliarity with armored vehicles. The feed is NEVER switched once the weapon is on a vehicle -- the entire mounting and feeding system is designed around feeding the gun from one direction. The ability to change feed direction is useful to the government so that the same weapon could be used on different kinds of vehicles, but that "feature" is useless to the crew of any vehicle. The charging chain is not really any better than the rigid charging handle of the M37 machine gun that the M73 replaced. And it is the tank loader (NOT gunner) who charged the M73 (both for loading and for immediate action in the event of a stoppage).

    Nathaniel scratches his head and asks "So why was the M73/M85/M219 family so unreliable and bad?" Apparently he doesn't realize that the only purpose of these weapons is to fire. The M73 was a marvel of mechanical ingenuity, but it simply did not work (despite all the great features Nathaniel mentions).

    So, what really was wrong with the M73 and the M219 -- the Army never quite figured that out (as demonstrated by the fact that the M219 had exactly the same problems as the M73 it replaced); but tank crews and ASI C5 Master Gunners had plenty of opinions.

    ONE problem of both the M73 and M219 was that the feed mechanism would not reliably PULL ammunition from the stowage box mounted on the turret wall. Apparently it never dawned on the Ordinance Corps testers that this gun was being used in peacetime. In war, the ammo box is full (thousands of rounds), bringing the level of the ammo belt up about even with the gun. In peacetime, you never got more than 300 rounds -- not quite two layers of ammo in the very bottom of the ammo box, with the gun having to pull ammo up about two feet. Many crews discovered that they could build up a platform inside the ammo box so that the 200-300 rounds of ammo were at the top of the box instead of the bottom -- this did not solve the problems of this gun, but it did help A LOT.

    ANOTHER problem of both the M73 and M219 was that the gun simply jammed frequently even when the feeding problem was mostly handled. We never were able to determine exactly why these guns jammed but my own pet theory had to do with uneven heating and expansion of the different metals used in the mechanism. The M73 and M219 had all the simplicity of a Chinese copy of a Swiss watch. One thing crews found was that (once the feeding issue was handled) the gun would fire just fine cold -- and it would continue to fire as it heated up -- but if you stopped firing for more than just a few minutes (but not long enough for the gun to fully cool) then started again the gun would usually jam quite quickly. Both the location of the gun and the required "Immediate Action" procedures prevented crews from trying to diagnose the exact nature of the stoppage, but there was no doubt among any experienced tanker that something made these guns jam much too often to be acceptable.

    Nathaniel lumped the M85 in with the M73 and M219, but the M85 was a completely different animal. It was an awkward beast in many ways but those were mostly issues with how the Army manged to stuff it into the limited space in a tank cupola (in some vehicles it was actually mounted upside-down!). The M85 worked quite well as a machine gun, but was still unloved by most tankers due to a variety of reasons unrelated to the gun or its performance.

    First, the M85 was a .50 cal machine gun that did not use the same ammo as the rest of the .50 cal machine guns in the Army. Well, actually it fired the same rounds, but they had to be linked with a completely different link from those used in the M2 (Browning) machine gun. The M85 used ammo with an M15 link where the gun pushed the round forward through the link (much like the M13 links on NATO 7.62mm machine guns) instead of pulling the round out the back of the link as the M2 did. At least in the early years of the M85, you could expect to receive the wrong ammo at least a quarter of the time you received ammo. (Later on, in tank battalions, the few vehicles that had M2 machine guns faced the reverse of this problem).

    The M85 had two selectable rates of fire -- Low rate was around 500 rpm, high rate nearly twice that -- you were only supposed to use high rate to engage aircraft by simply firing steadily into the air hoping the aircraft would fly into the stream of fire (which was effectively useless due to the small amount of ammo available in the Cupola). The M85 fired from the open bolt. Low rate of fire (which was how you always used the gun) was achieved by a mechanism that caught and held the bolt to the rear then released it a fraction of a second later. This mechanism was contained in the backplate of the machine gun - the bolt would come to the rear - hit a stud on the front of the backplate which would cause a drum to turn winding up a spring - while the drum was turned a mechanism held the bolt to the rear - then when the spring unwound and returned the drum to its rest position, that released the bolt letting it go forward to strip and fire the next round. Surprisingly this Rube Goldberg contraption actually worked -- at least as long as the rate selector lever was properly locked in place and the timing spring didn't break.

    Like the M73 & M219, the M85 used a chain instead of a charging handle. Like the M73 and M219, the M85 also had a manual trigger in case the electrical solenoid failed. BUT unlike the M73 and M219, you couldn't actually reach the manual trigger on the M85, so firing it manually was accomplished by pulling a handle on the end of a chain -- yes, an chain and handle that were identical to the chain and handle used to charge the gun except that the charging handle had a black plastic coating and the manual trigger handle had a red plastic coating -- of course they were out of sight from the shooter's position and hanging side by side (if not tangled together).

    So, bottom line: The M73 and M219 were hated because they would not shoot reliably. The M85 did shoot reliably but it did introduce a number of headaches that some people found bothersome.

    • See 1 previous
    • Nathaniel F. Nathaniel F. on Aug 08, 2015

      @Tom Currie You're right that I probably was not right to lump the M85 in. It was designed by AAI, I think.

  • John W Faulconbridge John W Faulconbridge on Aug 05, 2015

    I have to agree with the folks who are defending the M73 (and M219). You neeed to replace the drive spring before (or immediately after) a gunnery cycle but if you took care of the gun it would take care of you. M85 was also a good gun. switching back to the M2 wasn't done because it was a bad gun, it was due to the redesign of the TC station.

    • Tom Currie Tom Currie on Aug 05, 2015

      @John W Faulconbridge You might want to consider replacing BOTH drive springs -- not that this would really help either of those pieces of crap, but at least it would make the comment more believable.

      As for the return from the M85 to the M2, that was NOT due to the redesign of the CWS -- after all, a new CWS could have been designed for ANY weapon. The major reasons the Army dropped the M85 were financial. The gun itself cost more than the M2 and maintaining two incompatible kinds of .50cal machine gun ammunition in the supply system was a significant cost and had repeatedly proved to be a problem for the logistics system. When the M85 was first adopted, the Army's intent was to use it as a test bed for a new .50 cal machine gun family that some hoped would replace the "obsolete" M2 entirely in about a decade. No other NATO or SEATO country showed any interest in such a change and no flexible mount version was ever fielded. Although the M85 worked well enough, it never achieved the popularity of the Ma Deuce and eventually the Army realized it was time to return to the proven workhorse heavy machine gun.