The Causes and Cures for Doubling on Krieghoff Shotgun

The term "doubling" means that two barrels of a shotgun go off in response to a single trigger pull. So, the second shot is obviously unwanted and undesirable. Doubling occurs on all kinds of shotguns including over-and-unders and side-bysides with both single and double triggers. Often the reason for this problem is a worn sear or hammer-notch edges. The sharpness of these edges ensures reliable hammer retention in the "on-cock" position under the pressure of the mainspring.

Gradual dulling of these edges in turn makes the hammer prone to slipping from the sear. In such cases, the stress created by the first shot is sufficient to "jerk" the second hammer from the sear even without a trigger pull. This malfunction is less likely on guns with parts that are precisely made out of good steel, but I have had to deal with doubling on Krieghoff’s over/under shotguns, which, ironically, are very precisely made out of very good steel.

In this article I’ll analyze the reasons for this problem and detail the methods for its repair using the fine Krieghoff sporting guns known as the models K-32, K-80, and K-20. Since 1886, Krieghoff has been a well-known German manufacturer of the highend European hunting guns including double rifles and combination guns. In America, the Krieghoff name became popular after the introduction of the competition over/under shotgun K-32 in the late 1950s.

This gun was based on the Remington 32 action with a modified Miller single trigger. The K-32 has earned a great reputation among clay-target shooters. Despite the age, this gun is still widely used. The K-80 was the second in the series, made since 1980. With the same basic design, it has a number of advantages over the K-32. Among them area better trigger mechanism, interchangeable and adjustable barrels, and improved engraving.

At the time of this writing, I strongly believe the K-80 has became the most popular over/under shotgun among competitive skeet, trap, and sporting-clays shooters. The K-20 appeared just recently. It is designed as a small-gauge competition gun. Currently it is produced in 20 and 28 gauges only. Frankly, I have not seen the doubling problem on this gun yet. However, I include it in the scope of our analysis because of its similarity to the first two guns.

In fact, the manufacturer describes it as a scaled-down K-80. From the standpoint of this article, all three actions are identical. In order to understand the reasons for doubling on these fine sporting guns, I’ll start with an analysis of the mid-pull and inertial mechanism on the double shotguns with the single-trigger action. For the sake of simplicity, the photos will be referenced as a figure and an item number.

For example, a reference to Figure 1-3 means item 3 on Figure 1, which in our case is the left sear. The Mid-Pull First, let’s accept the fact that double guns with a single trigger are normally more complicated than their counterparts with double triggers. The reason is that, with a single trigger, there has to be some way of switching automatically from one barrel to the other. With double triggers, this switch is done by the shooter by simply moving his finger from one trigger to the other.

In other words, with a single trigger, the trigger link must move automatically from one sear to another. The trigger link in any gun is the part which mechanically connects the trigger with the sear. In Krieghoff’s action, the trigger link is a pear-shaped part ( Figure 1-4 ) called a pear-stud. Keep in mind that often the action with a single trigger should also accommodate the selector for choosing which barrel will shoot first. But this is not always the case.

The major difference between singleand double-trigger actions is in the intermediate-pull arrest. This subject is seldom discussed, probably because it is a bit advanced for the everyday chat over the gunsmith’s workbench. However, we have to look at it closely in relation to Krieghoff’s doubling. Actually, our analysis also applies to any other single-trigger gun when it comes to the matter of doubling. Note that after each recoil generated by the shot, the shooter’s body generates counter-recoil.

Counter-recoil is an uncontrolled muscle reflex. It moves the gun forward’in the opposite direction to

This photo shows the position of parts on the K-80 during counter-recoil: pendulum (1), safety-post (2), left sear (3), pear-stud (4), right sear (5), hammer (6). The mid-pull is arrested by the pendulum (1) engaged with post (2). The trigger link (4) is engaged with the sear (3). This is the position of parts during counter-recoil when the trigger is released just before the mid-pull. The arrow shows the gap between the pendulum and post. recoil’t o restore the initial body position.

The surprising feature of counterrecoil is the speed of its movement. In practice, it is hardly even noticeable, just like the recoil itself. Counterrecoil is also associated with another involuntary move. This is the so-called "intermediate pull," sometimes also referred to as a third or involuntary pull. I will call it "mid-pull" here for the sake of space and simplicity. This pull is a reflexive contraction of the trigger finger muscles during counterrecoil.

We can debate the reasons for this move but for our purposes here we’ll just accept the fact that every time the gun moves into counter-recoil the finger unconsciously pulls the trigger again. As a reflexive move, the timing and strength of this pull varies from shooter to shooter. Double-trigger shotguns are indifferent to this move, because after the shot the trigger is disabled. Single-trigger guns, however, are sensitive to it, since after the first shot the action has to be immediately ready for the next one.

If the mid-pull is not blocked and the action is ready for the next shot, then the gun doubles. Hence, something has to be incorporated into the shotgun action to disable this pull for the duration of the counter-recoil. I’ll call it the "mid-pull arrest." Inertial Device and Pendulum The search for the most efficient mechanical device for arresting the midpull has never stopped since the very first single-trigger designs.

Great gunmakers competed for the best solution, and just within the years from 1880 to 1900 dozens of single-trigger mechanisms were patented in England alone. Among the most well known of them was the famous Jones- Baker mechanical three-pull system with a shifting bar. With this design, every pull of the trigger shifts the bar to the next position.

The first pull triggers the shot and shifts the bar so that the following mid-pull shifts it again without a shot, but enables the second shot in response to the next conscious pull. Robertson implemented a somewhat similar three-position rotating drum, which disabled the mid-pull after the first turn. Falford’s action used a pneumatic cylinder for delaying the engagement of the trigger with the second sear so that the mid-pull happens when the trigger is not yet touching the sear.

Greener’s design of 1898 used an inertial block to make the switch and disable the mid-pull. This block is nothing more than a metal weight hinged at the top. Greener, therefore, called it a pendulum. Multiple designs that serve the same function often indicate there is a need for improvement. In this case, improvement meant a reduction in mechanical complexity and a simpler means for arresting the mid-pull.

Greener’s inertial design finally won the competition, and during the last century it appeared in multiple shapes and variations on many different guns. The best feature of the inertial design is that it requires fewer mechanical parts than any other single-trigger mechanism. This design utilizes the energy of the recoil or counter-recoil and the inertia of the pendulum. Inertia is the tendency of any object to resist changes in its state of motion.

In other words, if the object is moving, it resists any stopping force; and when it is resting, it resists any moving force. This is the essence of Newton’s First Law of Motion. Note that during recoil the gun moves backward and during counterrecoil it moves forward. So, when the

gun moves forward into counter-recoil or backward into recoil the pendulum tends to retain its position and therefore rotates around the hinge. The energy of this rotation can be utilized for the switch from sear to sear or the mid-pull arrest or both. The pendulum is the core of any inertial device. This device, however, normally includes a couple of other parts such as the trigger link, spring, and plunger.

While most inertial devices, including Krieghoff’s, utilize counter-recoil, some others successfully utilize recoil. The best example of such a gun is the Winchester Model 21. It locks the mid-pull during the backward motion of the gun. Practically speaking, it’s not important whether the pendulum moves forward or backward. It’s also not important whether the pendulum is hinged at the top or bottom. Important, however, is the timing of this device, which determines when and for how long the mid-pull is arrested.

Without diving into serious physics, let’s summarize the function of the pendulum in this context. Simply put, the pendulum’s timing depends on its weight, attached springs, and the speed of the gun motion. Both weight and springs have to be chosen by the manufacturer for the certain range of average speeds during gun usage. The speed of recoil, in turn, can vary depending on the shell’s power, the shooter’s body build, as well as recoil suppressors or other additions to the gun.

And there’s one more factor to account for’friction. Friction, which can be the result of rust, dirt, or dried-up oil, resists the pendulum’s motion and can disrupt its work. Anyway, we’ll assume that the action is clean and the inertial device can work properly within certain average limits of recoil. Note, however, that any significant reduction of recoil can diminish the efficiency of the inertial device and render it useless.

This is important to keep in mind since the natural tendency of any shooter is to reduce recoil by all means. This is why recoil suppressors, sub-caliber tubes, shockabsorbing stocks, and weak shells are seen with increasing frequency. Taken together, these factors often greatly affect inertial devices. One wellknown example is the Browning family of over/under shotguns.

On these guns, the pendulum moves backward, disengaging the trigger-link from the sear and placing it at the rear of the action for the whole duration of counter-recoil. Disengagement consumes some energy of the pendulum. In many practical cases, there is not enough energy for the pendulum to accomplish this disengagement. The result is that the second shot doesn’t happen, since the trigger link is still attached to the first sear rather than having been switched to the second one.

For this reason, many of these guns have been modified to accomplish sear disengagement mechanically, without inertia, but still using inertia for the mid-pull arrest. These actions often are casually called "mechanical," ignoring the presence and the role of inertia for the mid-pull arrest. The good news here is that the efficiency of the inertial device can often be improved by adding weight to the pendulum, reducing friction, and by changing springs.

Detailed description of such improvements for the Browning-type shotguns can be found in my article "Understanding Inertial Devices on Single-Trigger Double Shotguns" (American Gunsmith, September 2000). Unlike the Browning, the Krieghoff’s action is designed to switch sears mechanically. So, it consumes the pendulum’s energy only for the mid-pull arrest as is explained below. There is another important difference between these two gun families.

While the trigger on the Browning is completely disengaged from the sears during the mid-pull, the trigger on the Krieghoff remains engaged with the second sear (Figure 1-3) via the pearstud (Figure 1-4). The movement of the trigger however is locked by the pendulum (Figure 1-1) and post (Figure 1-2).

Here’s the complete sequence of events ending up with the mid-pull arrest: After the shot, the gun first moves backward into recoil and then forward into counter-recoil; the finger releases the trigger; the pendulum rotates backward, swinging underneath the safety post; the mid-pull is now taking place; the pendulum, moved by the mid-pull upward, meets the bottom of the safety post and stops, preventing further movement of the trigger and sear.

After that, the finger releases the trigger, the sear lowers down and engages with the second sear. The only distance the trigger can move during the mid-pull is determined by the gap ( Figure 2-3 ) between the pendulum and the bottom of the post when the trigger is fully released. The same gap is determining how much the sear (Figure 1-3) can be raised above the hammer by this trigger pull. We’ll discuss this important matter in the next section.

Note again that Figure 2 shows the gap when the trigger is released during the counter-recoil just before the mid-pull and Figure 1 shows the midpull in progress right after that. Reasons for Krieghoff’s Doubling Having the trigger and sear engaged during the mid-pull presents some strict requirements to the timing and tolerances of the inertial device on Krieghoff guns. The first of these requirements is that the gap (Figure 2-3) between the pendulum (Figure 2-1) and the post (Figure 2-2) should be minuscule.

Otherwise, the mid-pull will be able to raise the sear far enough to drop the second hammer and cause a double shot. The action shown in Figure 1 has this gap enlarged and therefore the sear during the mid-pull has risen too far, almost releasing the hammer. Figure 3 shows a magnified fragment of Figure 1 with sear-hammer engagement. It is quite evident that despite the arrest of the mid-pull, the hammer has almost slipped from the sear. Now any stress can jerk the hammer and cause the double shot.

In my experience, the Krieghoff’s doubling occurs mostly because of this increased gap. Even when the gap is properly adjusted during manufacturing, it gradually increases after thousands of shots. One of the reasons is that the mid-pull presses the

This is an exploded view of the hammer-sear engagement shown in Figure 1. The Krieghoff’s hammer is almost released by the sear during the mid-pull, despite the arrest of the trigger. Here, the switch to the second sear happened before the arrest of the mid-pull. pendulum against the post at every shot. As weak as it is, this pressure rocks and loosens the post. This is especially true for the K-32, since the K-80 and K-20 have improved safety posts.

Second, the hinged joint of the pendulum wears and loosens as well. As a result, the gap between pendulum and post slowly but surely grows. Hence, the first reason for the Krieghoff’s doubling is an increased gap between the pendulum and the post. Mechanically, this is a result of either bad adjustment or wear and tear. Note that the gap has to be validated with the trigger completely released. Let’s return for a moment to inertia.

As we saw in the case with the Browning, reduction of recoil dramatically affects the efficiency of the inertial device, causing in many cases a conversion of the action to mechanical. The Krieghoff is less sensitive to recoil reduction, since it does not spend the pendulum’s energy on the sear-switch. However, even that does not make Krieghoff completely indifferent to recoil reduction. One of the important characteristics of its inertial device is how readily the pendulum moves underneath the post.

If this motion is insufficient, the midpull may not be arrested at all or it can happen prior to its arrest. Both cases will end up with a double shot. On the other hand, as we have mentioned earlier, the pendulum’s motion depends on the speed of recoil. So, everything that slows down recoil also diminishes the pendulum’s efficiency. On Krieghoffs, it happens The old safety post can be replaced with the new one.

Shown is the unfinished rod during its fitting into the frame slot. rarely, but can result from using additional weights, full-length inserts of 28 or 410 gauge, mercury or Gra Coil recoil suppressors, and light shells. So, the second reason for Krieghoff’s doubling is inefficient motion of the pendulum. Mechanically, this results from slowing and reducing recoil. The third reason for Krieghoff’s doubling is related to the timing of the trigger-link switch from the first sear to the second.

Particularly important is the position of the pendulum at the moment of this switch. Let’s follow the dynamics of the action during the switch, starting with the first shot. First, the pear-stud (Figure 1-4) is

engaged with the first sear (Figure 1-5). In the photos, the first sear is on the right side of the action. After the first shot, the finger starts releasing the trigger. It lowers the pearstud and the first sear until the pear-stud swings underneath the second sear. This situation is shown in Figure 1. As soon as this has happened, the gun is ready for the second shot. The mid-pull is an individual reaction and therefore may happen microseconds earlier or later.

If it happens before the switch, it has no affect at all, since it raises the sear which does not hold the hammer anymore. If it happens after the switch, then the pendulum should be already under the post. Otherwise, the mid-pull is not arrested and will cause a double shot. However, in some practical cases, the switch happens when the pendulum is still not under the post. This situation is shown in Figure 4.

Here, the pendulum still has not swung under the post and therefore the trigger is not blocked; however, the switch from the first sear to the second has already happened since the pear-stud is under the left sear after the first shot on the right side. The mid-pull at this moment will cause doubling. My observation is that this particular problem mostly happens after replacing the pear-stud without proper adjustment of its height. Such replacement is quite often required, since the pear-stud is prone to breakage.

So, the third reason for Krieghoff’s doubling is improper adjustment of the pear-stud height. Repairs For the first case (increased gap between the pendulum and post), we have to adjust the post. Note that in these situations the post is most prob- Here, anew heavier pendulum (1) was ordered to replace the old pendulum (2) on a K-32. Solder can be added to the flat surfaces of the pendulum to increase its weight. ably wobbling. So, the fix has to stabilize the post and also adjust its length.

I’ve found that making anew post may be more efficient than dealing with the old one. It boils down to increasing the diameter of the post for a tight fit into the frame hole, providing anew pin for locking the post in place, and adjusting its length for a minimal gap. The process of replacing the old post with anew one is shown in Figure 4. Clearly, the new post shown here still has to be shaped and adjusted in length.

The method of installing this post varies between the K-32, K-80, and K-20, but is quite straightforward (see Figure 5 ). The looseness in the Krieghoff’s pendulum joint has to be eliminated by replacing its shaft or ordering anew pendulum or both. The second reason for doubling is reduced recoil, which diminishes the efficiency of the inertial device. There are a number of ways to increase recoil and make the pendulum work properly.

However, this is undesirable, since it reverses any previous efforts for reducing recoil. The problem should be solved mechanically for a given level of recoil. As I mentioned above, behavior of the pendulum depends on its moment of inertia. The moment of inertia, in turn, is proportional to the weight of the pendulum and the speed of the gun motion. Since we are not going to change the recoil, the speed of the gun motion is not changed.

However, we can increase the weight of the pendulum, which will increase the moment of inertia. Figure 6-1 shows a heavier pendulum, which can be installed instead of the old one (Figure 6-2) on the Krieghoff K-32 suffering from doubling. Replacing the pendulum is not always necessary. Instead you could solder a small amount of lead-tin compound on the flat surfaces of the old pendulum. This is shown in Figure 7. The third and final fix is a bit advanced.

The pear-stud height should be adjusted to allow searswitch only when the pendulum is able to swing underneath the post (Figure 1-2) when the trigger is completely released. The standard pearstud has two cuts for the pin by which it is held in place. The cuts on the pear are at different distances from its top. This allows installing the pear with one of two heights.

Normally, this is enough to properly adjust the stud; however, in doing this job you have to be extremely careful, since the pearstud is hardened to a very fragile condition. I

AGGDBI7 AG DBI – 2 pg. AD, NEW (coming directly from Belvoir)

AGGDBI7 AG DBI – 2 pg. AD, NEW (coming directly from Belvoir)