What Causes Shotgun Choke Tubes To Lock Up Or Freeze In The Barrel?
Stuck Choke Tubes From the Use of Steel Shot
Part 2 of 2
Choke tubes frozen in place from expansion is different from rust or residue build up. However rust, residue build up and choke expansion can all occur at the same time.
Without getting to deeply involved in the science of metals you need to know that when a shot charge passes through a choke tube it exerts a force in two different directions. Force is applied to the choke tube when the shot charge strikes the choke forcing cone.
The first direction is longitudinal. The force in this direction simply tries to push the choke out of the barrel. This is prevented from happening by the interlocked threads on both the choke and the internal threads of the barrel. If enough force is exerted then the threads in the barrel and or the threads on the choke shear and out it goes. I have never seen a choke fail from thread shear.
The second direction is at approximately right angles to the bore. In this direction the applied force tries to make the choke expand. As the shot column moves through the choke tube forcing cone, the pellets are in constant movement to rearrange themselves so that the shot column becomes smaller in diameter and elongated. This generates a force that wants to expand the choke tube.
• Steel is elastic to some degree.
• If enough force is applied the choke tube will expand by a small amount and when the force is removed it will contract to its original size and shape.
• One of the ways we measure the strength of steel is by “Yield Strength”.
• For our purpose we can define yield strength as the maximum amount of force that is applied that does not cause any permanent deformation of the steel.
• If we increase the level of force past the yield strength of the steel used in that choke, it will expand past its elastic limit and stay in this expanded size.
• If you continue to shoot this load in this choke tube it will expand a small amount each time and at some point it will be solidly locked into the barrel.
• This force reaches its peak nominally at the intersection of the choke forcing cone and the parallel section. This is the area where choke expansion will occur.
• Choke tubes in this condition cannot be removed by normal methods.
The actual force is generated from a number of combined factors.
1. Size of steel shot- The larger the diameter of the shot the more force is created
2. Weight of the shot charge- Heavier payloads cause higher forces
3. Choke constriction- More constriction causes higher forces
4. Velocity- the higher the velocity the more force is created
Very high velocity steel shot shells with large diameter (particularly size B and larger pellets) through tighter (typically full or tighter) constriction chokes are usually the culprit causing choke expansion.
One of the best ways to prevent choke expansion is to follow the choke manufacturer’s recommendations.
• Some choke manufacturers will mark the choke “no steel” if it is not rated for steel.
• Some mark the choke “approved for steel”
• All factory chokes, as a rule, are rated for steel shot loads from cylinder bore through modified constriction, at least all that I have seen.
• If you use a factory choke tighter than modified that is not marked for use with steel, I would check it on a regular basis for choke expansion.
• If you find a choke tube that gets progressively harder to remove and replace each time you do so, examine it carefully as it is probably failing from expansion.
• If in doubt whether a choke is rated for steel, contact the manufacturer
In my experience with steel shot I have found that if a choke does not expand with a given steel load after 25 shots it will never expand. Don’t take that as a fact as nothing is 100% when it comes to shotguns. However to date, this rule of thumb has never failed me.
While this is certainly not intended to be a true technical paper, hopefully it has given you a basic understanding of why choke tubes can and do freeze in the barrel.