Shotgun Headspace Regulation

Gunsmiths normally deal with headspace in rifles rather than shotguns. This is understandable since the shotgun is not so critical to accuracy and has lower chamber pressures. Normally, shotgun headspace is completely disregarded when diagnosing a misfiring gun. Three things a gunsmith needs to identify in a failing shotgun are insufficient firing pin strike, bad shotshells, and if there is excessive distance between the primer and the breechface of the gun.

For this article we’ll assume the shotgun has a firing pin with normal length (1.5-1.8mm) that strikes hard enough and that the ammunition is within specifications. That leaves us with excessive distance between primer and breechface, which is an excessive headspace. The sketch here shows the rim recess and relation between rear edge of the chamber and the face of the action.

In the United States, chamber rim recess dimensions are specified by SAAMI as the American National Far His the depth of the rim recess, measured from line A to the edge of the chamber. Sis the possible "off face" distance, increasing headspace, later assumed S=0. The angle of 55 degrees is a standard tapering of the front portion of the rim recess. This number is indicated on all reaming tools. Standard SAAMI Z299.2–2015 (R- 2019).

This standard defines the headspace as distance Hfrom the central line A to the rear end of the chamber. For the 12 gauge this distance Hhas to be between 0.0624 (1.585) minimum and 0.0764 (1.941) maximum. The standard also assumes that the distance Sbetween the chamber edge and face is 0, non existent. If this distance S > 0, then the actual headspace is H+Sand can be too long for igniting the primer, even with a sufficient firing pin. In these cases we are saying that the gun is "off face".

Putting the gun back to face (when S=0) is a repair procedure, which is beyond our scope here. For this article, if the gun is off face it must be rectified prior to any other modifications of headspace. It has to be put back to face so that S=0. Headspace is totally defined by rim recess and distance H. At different times and different countries the specifications for the chambers

1&2, 20 and 12 gauge. 3, a high quality German reamer creating minimal chattering. 4&5, 20 and 12 gauge rings. 6&7, GO and NO-GO gauges. were quite different and not always consistent with current standards of shotshells. Some older European guns may have smaller depth of recess H, making the gun difficult to close as shell heads are sticking out. More often, the gun has an H dimension greater than specifications and the shell head is too deep below the chamber edge.

We are leaving aside the reasons why it is so—different specs, manufacturing problems, previous modifica tions, or wear. Instead, we look into the way of correcting this problem. We need to reduce the depth H of the rim recess. Normally, it is done by soldering a ring into the chamber and then reaming it to specs of the rim recess. The required tools and rings are shown here.

The first step is clearing space for the ring in the chamber with one of the rim recess cutting tools (reamer 1 or 2) after removing the ejectors. The reaming chamber for the ring can be done manually or on lathe; I prefer the later. The rings are about 0.6" wide and can be obtained at Brownells, however, I often Top Center The depth of the recess for the ring ideally is so that a fully inserted ring is just a bit below the chamber edge. This drop below should be quite less than the headspace.

The ring itself and the chamber recess should be thoroughly degreased before soldering. I cover both ring and the chamber recess with solder flux paste. I leave excess of the solder all around since all empties between recess and the ring have to be filled up. Remove the solder drips and start heating up the chamber. As always, keep the flame away from the solder itbefore soldering. Alternatively, any other reliable way of soldering can be used. There is no special concerns about the strength of this joint.

I use excess solder all around since all empties between the recess and the ring have to be filled up.

Far This excess can be removed before heating. The can be started on a lathe but the last depth adjustments need to be done manually. self and heat the heavy chamber around the ring. When solder melts make sure that the ring is completely seated all the way by lightly tapping it down. The last step is done with a standard chamber reamer. It cleans the surface of the chamber, smoothing the ring step and also provides a standard rim recess.

You can start in a lathe, but the last cuts of headspace adjustment have to be done manually. During these adjustments we need to use GO gauge and finish the reaming when this gauge is flush with the chamber edge. This way we can be sure that the standard headspace is achieved. The alternative way of adjusting headspace is to use the GO gauge in the assembled gun and stop reaming when the gauge just lets the gun completely lock the barrels.

After that is done, ensure the NO- GO gauge is sticking out beyond the chamber edge and that it does not Below far It is sticking out beyond the chamber edge, which is well visible. Above far allow the gun to completely close and lock the barrels. Remove the part of the ring in the ejector recess and install the ejectors in place. Normally, no actions require modifying ejectors or extractors after the rim-recess adjustments.

Finally, you’ll be able to see howa standard shotshell will sit in the chamber after adjustment comparatively to pre-adjustment. Conclusion Regulation of the shotgun headspace may be needed for some older foreign guns, guns with non-standard chambers, guns with previous modifications, or those that are worn out or misused. This procedure involves special reamers and rings, which can be obtained at Brownells or made.

It is not difficult but requires correct diagnostic of the misfires, which is a major reason for concern about headspace. If regulation is required, it boils down to changing the geometry of the rim recess with the tools shown here. Some accuracy is required, especially on the final steps of reaming the rim recess. AG