I found the following answers in the CCI Digest 478 from last year and on a Vickers 1951 website (http://www.vickersmachinegun.org.uk/...nualpart1.html
). I'm not sure if these are the same as in WWI, but hte English don't change that much.
Hope this helps.
There's an excellent description of jams - or "stoppage's" - in belt-fed guns in appendix 1 of Norman Franks and Alan Bennetts book "The Red Baron's Last Flight" (Grub Street).
On a general level there were 4 types of stoppage designated 'Number 1' through to 'Number 4', caused by different circumstances, eg; freezing, poor quality ammunition, etc, etc...
The distinction should be noted that they relate to 'ammunition feed problems' and not 'equipment failure', ie; bits breaking, which also happened and, of course, could not be repaired in the air.
Although some stoppages could be corrected in the air (some by using a hammer to force the cocking handle forwards to chamber the round) some could not; a Number 2 stoppage for example.
Pilots realised the importance of reliable ammunition and I've read accounts that some chose to select each round themselves to make sure it was defect free.
No 1 Stoppage:
(Spent cartridge is not ejected)
* * (a) Too much weight on the fusee spring.
* * (B) Grit, or lack of oil in the working parts.
* * © Excessive packing.
* * (d) Worn barrel.
* * (e) Tight pockets.
* * (f) Friction due to frozen oil or water.
the stoppage is really due to bad maintenance.
No 2 Stoppage:
(chamber is fouled)
- the front portion of the previous case has been left in the chamber. Consequently the live round cannot enter the breech fully. When the live round is withdrawn, the separated case is telescoped over its nose.
- in this case the separated case remains in the chamber. The clearing plug will remove the separated case by expanding and gripping it firmly, thus remedying the stoppage.
- a series of separated cases is caused by ineffective sealing of the breech. There is wear somewhere between the face of the extractor and the connecting rod, and as a result the lock does not go fully forward. When the round is fired, the cartridge case breaks and leaves a portion in the breech.
No 3 Stoppage:
(a misfed round)
- a cartridge has been fed up slightly crosswise. Thus the base of the cartridge is not in line with the extractor grooves. The extractor has not been able to rise to its highest position as it has fouled the rim of the cartridge instead of accepting it smoothly in the grooves. By easing the pressure on the lock and pulling the belt, you can straighten the round and the crank handle can thus be knocked on to the check lever. Grit or lack of oil may cause friction in the lock. There is not sufficient momentum to overcome this friction and consequently the extractor cannot rise to its highest position. Oiling will prevent a recurrence.
- the base of a cartridge has fouled the mouth of the feedblock. When the slide started to move inwards, the top pawls were unable to carry the round into the feedblock. This is why the slide is out and the pawls rigid. State that the cartridge fouling the mouth of the feedblock may be due to:–
* * (a) Loose pockets.
* * (B) Liner not in line with the feedblock.
releasing the strain on the belt enables the round to be straightened. Attention to points during firing will prevent a recurrence.
No 4 Stoppage:
- the cartridge in the breech is not fired owing to a misfire. Alternatively an empty pocket in the belt, which will result in an empty breech, may cause this stoppage.
- the cap of the cartridge is not struck due to a broken or damaged firing pin or a broken lock spring.
- a recurrence of No 4 stoppages is caused by play between the extractor face and the base of the round. As a result the cap of the cartridge is not struck sufficiently hard to fire the round. A No. 1 and No. 2 washer placed on the connecting rod will take up the play and remedy this.