The accepted view of involuntary pull prior to this discovery was that after the shot from the first barrel, recoil jumped the gun away from the finger, and then the shoulder rebounded the gun forward on to the stiff finger, which, being struck by the trigger, let off the second barrel. The author for some time previous to 1902 had become conscious that this explanation was open to question. However, it was not until he sat down and worked out the times of recoil and finger movement, that he felt safe in challenging so generally accepted a statement. But this calculation proved to him that, so far from rebound causing the unwished-for “let off,” the latter occurred in one-twentieth of the time occupied by the recoil backwards. However, the author’s powers of persuasion failed to convince everybody, and for this reason the editor of The County Gentleman, with the assistance of Mr. Robertson, of Boss & Co., and of the late Mr. Griffith, of the Schultze Powder Company, formed a committee of experts to test the point by chronographic examination. Results were published in The County Gentleman on December 6, 1902, and were to the effect that the second discharge came in one-fiftieth of a second after the first discharge, but that the recoil backwards, before rebound could occur, took from four different shooters respectively .32, .29, .34, and .38 of a second, or, roughly, an average of one-third of a second. So that it was demonstrated that the rebound from the shoulder had nothing whatever to do with the involuntary pull. The true and now always accepted cause was as the author had stated it to be—namely, that the recoil jumped the trigger away from the finger in spite of the muscular contraction that still continued after the let off of the first barrel; that this muscular contraction continued to act and again caught up the trigger, as soon as the pace of recoil was diminished by the added weight of the shoulder, and so the finger inflicted a heavier blow or pull on the trigger than in the first pull off. In the first pull it was finger pressure, in the next it was pressure acting over distance, and was measurable in foot-pounds, as work or energy is measured. This proved to be the correct solution.
Consequently, a good single trigger is one that prevents this finger blow from discharging the second barrel. It is impossible to prevent the blow itself, but quite easy to prevent it letting off the second lock. There are at least three principles employed for doing this.
The first is called the three-pull system; it is based on the necessity of either the voluntary second pull, or involuntary blow (as the gun may be loaded or unloaded), for intercepting the trigger connection which the subsequent release of the trigger allows a spring to place in readiness to receive the third trigger pull, and act on the second tumbler; this pull in the unloaded gun is observed to be a third pull, and in the loaded one is only observable as a second pull, because the second has been given involuntarily, and not consciously.
The double-pull actions are different in principle. Most of them are based upon a lengthening of the time between the first let off and the connections with the second lock coming into position for contact with the trigger. In other words, they are time movements, based upon the knowledge that the second pull, or impact of trigger and finger, came very quickly, and that to delay the intermediate connecting link between trigger and second lock until after this unconscious impact rendered it inoperative.
A third system is somewhat different, but is also a timer action. It is based upon having a loose or nearly loose piece, which is partly independent of the gun, and either by its lesser motion or want of movement, during the jump back of the recoiling gun, gets in the way of a further trigger movement, until the recoil of the gun is over, and the weak spring can replace the independent piece in its normal position again.
It has been said that the greatest advantage of a single trigger is the facility with which it can be removed and double triggers substituted. But this is merely what those gun-makers have said, who, being obliged to have a single-trigger action of their own for those who ask for them, have been too proud to pay a royalty for a good one, and have not felt quite safe in recommending their own to good customers.
The real advantages of a single trigger are many. First, one does not have to shift the grip of the gun for the second barrel. As explained above, recoil occupies one-third of a second, and one does not want to add to the jump of the gun during recoil by partly letting go, nor to be unready at the end of it, by still having to move the right-hand grip in changing triggers. In practice, the single trigger is also much the quicker. It is not necessary to say anything about cut fingers and their avoidance by the use of single triggers. But a wonderful advantage is in the more correct length of stock. If one’s gun-maker gave one a stock an inch too long, or short, in double triggers, he would be thought not to know his business. There is only one best length for everybody, but every double trigger has two lengths of stock, one an inch longer than the other.
The author is told that there are still some very bad single-trigger actions being made, but that is quite unnecessary when the best can be employed by paying a royalty, as some of the best gun-makers are in the habit of doing, or were, until the recent action Robertson v. Purdey was settled.
Probably it would be more correct to say that the principal advantage of a bad single trigger is that it can readily be exchanged for a good one. The author would not on his own authority speak of bad single triggers, because he has tried most of them, and had difficulty with none.