Why do guns jam?

[Stephan Wilkinson]

First, let me say that I know little about guns. But it has always interested me that machine guns during the interwar and even WWII period so often "jammed"--stopped firing from some kind of internal blockage.

So here is my uneducated theorizing as to the cause. Let me know if you think I am right or wrong or somewhere in between.

Many of the machine guns used aboard aircraft between 1920 and the mid-1940s could trace their lineage back to the days when they were intended to be used on the ground, a stable 1G platform where they were operated upright, atop a stabilizing mount of some sort. Particularly true of the Vickers and Lewis guns favored by the RAF.

Then they began being adopted for use aboard aircraft, where they were bolted to a vibrating vehicle that operated in multi-G modes and a variety of attitudes.

I assume that machine gun breeches and receivers are constructed of finely machined parts that slide and reciprocate just fine when the gun is in a stable position. But put it into the wing, or atop the cowling, of an airplane pulling four Gs through 90-degree banks, and many parts inside the gun begin to shift and jam. These guns were never meant to fire upside-down or under heavy G stress.

Amiright?

[Dave Hadfield]

That was one of Stocky Edwards' most frequent complaints -- the guns wouldn't feed during G. He specialized in deflection shooting in turning fights, and often the guns weren't up to it.

[wendovertom]

Completely non-technical POV - my .22 LR jams when I am just hammering away at targets on occasion - and its a well cared for Winchester. Aircraft guns can't be that different - Its a mechanical thing with high tolerances (needs to be) and there are a number of variables (ammo quality, heat, g-load, FOD) that effect the precise operation. Maybe Randy can speak to modern gun applications (Eagle/Strike Eagle?)

All I know is my brother is thankful that the A-10s that supported his unit in the Afghan highlands never had a jam

my .02

Tom P.

[kalamazookid]

An excerpt from "F-8 Crusader Units of the Vietnam War," by Peter Mersky, which I recently read:

The F-8's four cannon were occasionally troublesome. Pilots started selecting the upper or lower pair of guns so that even if one set jammed, another pair was available. The jamming problem came largely from the effect of high-G on the ammunition belts. In high-G turns, the force made the linkage curl and bind, creating the possibility of a missed or jammed round.

[lucky52]

The M 60s on our slicks in Vietnam were prone to jamming when using the regular feed chutes.The common fix used by our crew chiefs and door gunners was to use a fruit can attached to where the chute clamped on and to feed the belt by hand.I was up front flying so I don't know if it was certain M60s,dirty guns, or certain fed chutes, or some combination that just didn't work.

[Spitty]

With the Spitfires 20mm canon early in the war the jamming was so bad they took them out.Was due to the thin wing flexing,having the gun mounted wrong causing the feed to jam and the grease freezing at high altitude..They solved most of it with the MkV but cold temps at height would still freeze the grease.

[gemmer]

The M 60s on our slicks in Vietnam were prone to jamming when using the regular feed chutes.The common fix used by our crew chiefs and door gunners was to use a fruit can attached to where the chute clamped on and to feed the belt by hand.I was up front flying so I don't know if it was certain M60s,dirty guns, or certain fed chutes, or some combination that just didn't work.

I never saw those feed chutes in Vietnam although I've seen photos of them. We always used a C Rat can on the gun. It fit perfectly with the clasp intended for 100 round ammo box originally used. The ammo in a mini gun can on the floor. Hand feeding the belt was not necessary.

39EB6844-9C37-4136-873A-AB1732E21800_4_5005_c.jpeg (42.14 KiB) Viewed 1714 times

[bdk]

Machine guns are a balance of forces. Internal drag, bullet weight, powder type, and quantity, headspacing, tolerances/wear, feeding, G-Forces, etc. Get any one wrong and you may get a jam.

[shrike]

First, let me say that I know little about guns. But it has always interested me that machine guns during the interwar and even WWII period so often "jammed"--stopped firing from some kind of internal blockage.

So here is my uneducated theorizing as to the cause. Let me know if you think I am right or wrong or somewhere in between.

Many of the machine guns used aboard aircraft between 1920 and the mid-1940s could trace their lineage back to the days when they were intended to be used on the ground, a stable 1G platform where they were operated upright, atop a stabilizing mount of some sort. Particularly true of the Vickers and Lewis guns favored by the RAF.

Then they began being adopted for use aboard aircraft, where they were bolted to a vibrating vehicle that operated in multi-G modes and a variety of attitudes.

I assume that machine gun breeches and receivers are constructed of finely machined parts that slide and reciprocate just fine when the gun is in a stable position. But put it into the wing, or atop the cowling, of an airplane pulling four Gs through 90-degree banks, and many parts inside the gun begin to shift and jam. These guns were never meant to fire upside-down or under heavy G stress.

Amiright?

The gun itself - bolt. breech. barrel, chamber etc, is subject to such forces that gravity or Gs is inconsequential. Much like the engine, only more so.

The feed mechanism on the other hand, is the weak spot of any repeating firearm. Rather a lot of the gun's energy budget - recoil for the most part - is spent in operating the belt or drum feed (the entire drum on a Lewis or K turns slightly with each shot). To this end electric feed motors are sometimes added to relieve some of the stress, and belt layout in the ammo box is critical, not just a matter of neatness.
Extremely low temperatures can also contribute with thickening oil either increasing friction of moving parts, or congealing in hydraulic buffers, hence electric heaters fitted to breeches, and the movie trope of 'clearing the guns' before action to get a little heat into them.

[p51]

The M 60s on our slicks in Vietnam were prone to jamming when using the regular feed chutes. The common fix used by our crew chiefs and door gunners was to use a fruit can attached to where the chute clamped on and to feed the belt by hand. I was up front flying so I don't know if it was certain M60s,dirty guns, or certain fed chutes, or some combination that just didn't work.

I'm, not surprised they jammed in the air. I used them only as an ROTC cadet (got my butter bars in 1998) and they jammed any time the belt was pulling downward, which was common given how they'd get shot. I'd try to make a point of using a bipod if I could, and shooting with my right hand and keeping the belts slightly elevated as the belt fed, with my left hand. After that, I never handled anything like that as an officer other than on ranges when we had to burn up extra ammo we didn't wanna turn back in.
In my line units, I only ever saw M249 SAWs and M2 .50 cals being used and the ammo boxes for each fed the belts in about level with the feed tray. the only jams I saw in the field or on ranges was from debris entering the mechanism. I saw a couple where the blockage caused rounds to actually punch through the barrel and would then star digging hole, near the gas tube. Beats me why that happened, as the combat arms units had this happen often and would consult with my armorer on it.
I'd never thought about G loads on belts feeding into automatic weapons, but it does make sense.