Testing Trigger Pulls

It may frequendy happen that a test is necessary to determine the weight of the trigger pull of some weapon, and consequently a knowledge of the correct method of making such a test is essential.

The commonest method is to use a trigger tester which is made in the form of a spring balance. Such testers are light, portable and convenient, and are consequently used largely by amateurs as well as by gunmakers. They serve their purpose well enough if an approximate reading is all that is required, but for an accurate reading they are valueless.

The only accurate method of testing a trigger pull is by means of a dead weight, and special trigger testers with variable weights form an indispensable item in the equipment of every armourer s shop, as well as of the leading gunmakers.

The futility of the spring tester is so well known that anyone who produced one at Bisley or any Small Bore Rifle Competition would be greeted with derision. In all competitions no test other than a dead weight is ever accepted. Consequently any evidence based on a spring balance which is put forward in a capital charge can only reflect very badly on the conduct of the case.

When testing for trigger pull the weapon should be cocked and the "finger" of the tester hooked on to the trigger. The weapon should then be held at such an angle that when the weight hangs vertically it will exact a pressure on the trigger at the same angle as that adopted yy the finger when firing in the ordinary way.

The weapon should then be raised slowly until the weight is lifted off the ground. If the lock is not fired the test can be repeated with additional weights until the lock is fired. This gives the correct weight of trigger pull provided the weights are increased by not more than 4 ounces at a time. Several repeats should be made to check the value.

The angle at which the barrel of the weapon is held when making the test is of vital importance, and this varies with the type of action employed.

The following values for the angle of the barrel with the vertical may be taken as a guide when testing different types of weapons-

Angle of Barrel

Weapon with Vertical.

Ordinary double shotguns and rifles (right trigger) . 15 degrees Ordinary double shotguns and rifles (left trigger) . 25 „ Bolt-action military rifles 25 „

Martini-action rifles and most single-barrelled shotguns 25 „ Winchester Model 52 rifle Barrel vertical

Revolvers 45 degrees

Single-shot pistols 45 „

It is impossible to lay down any definite angle for self-loading pistols as this angle must obviously depend on the relative positions of barrel and butt; and in self-loading pistols there is great variation in the inclination of the grip to the barrel. It is, therefore, probable that the best plan to adopt is to make the test with the butt horizontal.

The actual weights of trigger pulls in normal use do not vary greatly in the case of good-quality weapons, but in the case of cheap weapons they are generally somewhat heavier. This is because it is not possible to give the same care and finish to the locks of cheap weapons with the result that these locks lack the stability of better mechanisms, and consequently the pull is increased in weight so as to provide the same margin of safety normally found in a good-quality lock.

The following weights of trigger pulls may be taken as a guide to usual custom in high-grade weapons, and any trigger pull much lighter than that quoted for a particular type of weapon should be considered as being potentially dangerous from the point of view of liability to accidental discharge. At the same time it should be realised that a heavier pull does not necessarily confer additional safety, as this depends on the actual finishing and adjusting of the limbs of the lock, as will have been appreciated from the description of the gun in the case which has been described.

Normal Trigger

Weapon. Pull.

The weight of pull given for double-action revolvers may, at first sight, seem terrific. But it must be remembered that this is quite a different type of pull to an ordinary trigger pull, as the trigger is gradually pulled back through a long traverse in order to cock the hammer and rotate the cylinder. The actual force required to pull the trigger is, therefore, distributed throughout a prolonged movement, and consequently does not appear to be intense at any particular moment during that movement. This is an experiment which anyone can try for themselves by snapping an empty revolver.

In cheap weapons the pulls are frequently from one to two pounds heavier.

Forensic Ballistics TestingPictures Fired Cartridge Cases MarksForensic Ballistics

(A) A Cross Section of a Cartridge fired by a Self-loading PiStol The pressure was normal and the depth of the striker indentation, which is indicated by an arrow, is also normal

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Responses

  • KRISTIN VILLEGAS
    Is there a forensics test on who pulls the trigger?
    6 years ago

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