Balancing the GPMG
Well, RTFQ has blamed me for this, so I'd better kick it off; this is how it started:-
There were a goodly few beers flowing at Cutaway Hall, in anticipation of a jolly on the ranges; in this case, firing an MG3 and a few other delicacies with the Danish Home Guard. The discussion turned to rates of fire, (something like 1200 rpm from the MG3), and then wandered off to tangle with the intricacies of balancing the L7 GPMG and the reasons for having to do it at all!
I asked if it was taught at all, having heard some horrendous 'go faster' firing from the ranges near my place and RTFQ said it was,
˜Although,he said, ˜I'm not sure it gets down to the grass roots, given the scale of issue, these days.
Being an ancient old cnut, I mumbled something about having a 'Gimpy' in every rifle section, when I was a boy, and we all knew how to do it and why! In response the wretched gwar dwarf threw a beer can at me but fortunately it was empty and no beer was wasted in the needless embellishment of this tale!
How well and how much is balancing the L7 taught these days
To find the reasons why and the wherefore, we need to go back to WW1. John Moses Browning came up with the Browning Automatic Rifle, the so called '˜Trench Broom', the same BAR which served all the way through WW2, Korea and the early part of Vietnam.
When FN brought out the MAG, the L7's parent design, they used virtually the same working parts as the BAR, except that the locking link swung downwards; the overall dimensions stayed pretty much the same. This allowed the gun to be produced in any calibre that had the same cartridge head dimensions as the .30-06, with a different feed mechanism to cope with a longer or shorter cartridge.
Most MAG variants, including the L7, fire the 7.62 x 51 cartridge, which is approximately half an inch shorter than the .30-06. However, the sear, on which the working parts are held when the gun is cocked, is set up for the .30-06. This is fine, just so long as the gun is firing normally, but when it starts to become fouled or the gun is held loosely, the working parts may not reach the sear, when the trigger is released, and the gun will 'run away' or fire uncontrollably.
Tip; if this happens to you on any gun, simply grab the belt and twist it, inducing an instant stoppage.
The answer to this problem was to install a second, or 'safety' sear, half an inch in front of the main sear. With a fouled gun, the working parts would then either hook up on this sear or the feed horns wouldn't recoil far enough to engage a cartridge and the whole lot would slam forward into battery, causing an IA.
The result of this modification was the ability to balance the gun and achieve a steady cyclic rate of fire.
Why not a high cyclic rate of fire?
Consider; a high cyclic rate of fire wastes ammunition and damages the gun. Specifically, under high cyclic rates of fire the gun is more difficult to control and the barrel will wash out very rapidly.
Think of it this way; if loads of gas is being used to recock the weapon, the bolt assembly will batter off the back end of the gun. If too little gas is being used, the bolt assembly won't go back far enough, and the weapon will "run away". If just enough gas is used, the bolt will travel backwards past the sear, slow gently to a halt because of the pressure of the springs, and return forwards.
If "too much" gas causes the recoiling bolt assembly to "bounce off the back of the gun", not only does the gun fire faster, but the impact of the bolt assembly on the back of the gun will help to knock the gun off-aim more than the gentler recoiling motion of a balanced gun.
Then one should consider the recoil buffer assembly. When you strip the butt off, you will notice a round disc on the face of the butt; that is the front end of the recoil buffer. This recoil buffer is composed of a series of opposed pairs of dished steel washers. These washers will very quickly develop radial cracks if the gun is misused and will eventually collapse.
I had cause to strip one buffer and found that the buffer tube had a short longitudinal crack at about twelve o'clock, one of the washers had collapsed and a further four were cracked. This came from a fast firing, unbalanced gun.
Even in the sustained fire role, although the length of bursts may be generally longer than in the light role, the barrels are changed in strict rotation and the actual cyclic rate of fire should not be any greater than in the light role.
What you gain from a properly balance gun is the ability to make the ammunition last a lot longer and deliver it to the enemy more accurately, with minimal wear to the gun.
'Oh, come on! It's an area weapon! Don't talk about accuracy!'
Yes, it is an area weapon; but only in the sustained fire role and, even then, a steady cyclic rate is far better than a fast one. The classic leaf pattern of the beaten zone is a function of the gun recoiling on the buffers of the tripod; too fast a cyclic rate and the pattern will become diffuse and less effective. A case in point is what must be the most successful sustained fire gun of all time, the Vickers MMG; it shoots slow and steady and it still has no equal.
The light role gun in defence is sited to provide enfilade or mutually supporting fire across the front of neighbouring positions. Assaulting troops will tend to be fast moving point targets and short, accurate bursts will be more appropriate.
When the light role gun is used in the advance to contact, initial contacts will again tend to be point targets, unless the contact develops into something more substantial.
I don't pretend to be up on current section battle drills but when I was a lad the GPMG represented a good two thirds of the section's firepower and ammunition usage was a crucial factor in any developing contact; what you carry is what you've got and you can't afford to waste it.
My generation saw the introduction of the L7 at section level and, over time, we developed a battle drill for the gun which I have advocated ever since.
As the rifle group moves off the gun acts as advertised, maintaining fire superiority over the enemy; a two or three round burst every few seconds or so, no more. (I know the book says three to five round bursts, for observation of fire, but the gun is easier to control with two to three round bursts and it makes best use of the available ammunition).
As the rifle group moves round to the flank they may be unsighted to the enemy and here the gun controller is important. He orders the gunner to gradually increase the tempo of bursts, acting as an indicator.
As the rifle group closes up to the assault line, the gunner increases the tempo of bursts further but still firing only two or three round bursts.
Here the physical control of the gun is absolutely paramount and the gunner's ability to maintain the correct grip on the gun, through the burst, is crucial. He must always allow the gun to settle back onto the point of aim before squeezing off the next burst in a fast rhythm. This is where a well balanced, steady cycling gun wins every time and it is much more controllable for the average gunner.
The ex-Korean War veteran who demonstrated this rhythmic technique, to me and my peers, called it 'stroking the trigger'. It takes a little practice but it is far superior to the 'pray and spray' method of long, fast cycling bursts. This technique is not just confined to the L7, it can be used with any gun; Minimi, M60 or even an MG3, if your grip and trigger control is good enough!
By the time that the rifle group has reached the assault line the gun should be pouring in suppressive fire in rapid bursts. This has two effects; one, a psychological boost to the attackers, like a bugle call, gearing them up to fight through the objective; two, it keeps the enemy's head firmly down, demoralising him if it hasn't killed him!
With accurate, controlled bursts, suppressive fire can be maintained on the target for far longer and allow the rifle group to get much closer into the objective before switching.
For those who have read the pamphlet, don't ignore the next bit because the pamphlet doesn't always give you the full low-down.
Balancing the gun seems to be a 'once in a blue moon' task to a lot of folk.
Balancing the gun is part of Preparation for Battle and should be adhered to scrupulously, so those responsible for running ranges should always build it in as part of the range procedure as well; what becomes second nature in peacetime, won't be missed out in wartime.
If you find this is too difficult to arrange or you can't be bothered, don't be too surprised when things f*ck up!
In the light role balancing should be carried out, not only by the gunner, who is the prime user, but also by the No.2, the person most likely to take over the gun. In the sustained fire role the gun should be balanced to the tripod.
First, take a freshly cleaned gun. Remove the barrel and screw the gas regulator up as tight as it will go. This is important; the indicator should come to rest at '0'. If it doesn't, take the gun to the unit armourer and he will re-set the gas regulator. Whatever you do, don't try to do this yourself; there is a tab washer that invariably breaks at the wrong time and a small key piece which always goes absent.
Once you are satisfied that all the barrels that you have are properly set, open them out to '6', repair to the range, put some Warmers into the bank and commence balancing.
Place a short belt on the gun, fire a short burst and apply the safety catch. Check that the cocking handle slide comes all the way back to the short white line, painted on the right side of the gun. This indicates the position of the main sear.
Open the gas regulator by two clicks and fire another burst. Repeat this procedure until the cocking slide stops about half an inch short of the white line. The working parts are now being held by the second or 'safety' sear.
Close the gas regulator by four clicks, cock the gun back onto the main sear and fire another burst. The cocking slide should now pull back to the white line.
Close the gas regulator all the way back to '0', counting the clicks as you go and make a note of it. Re-open the regulator by that number of clicks.
The gun is now balanced:- to that barrel. Repeat the procedure for each barrel.
Remember, the L7 uses an 'Exhaust to Atmosphere' gas system. This means that the more you open (unscrew) the gas regulator, the more gas escapes out of the gun and less gas is used to work the action; therefore there is less fouling as a result.
The result is a controllable gun, with the maximum amount of gas action available and one which makes the best use of your ammunition.