Making and Using A Chamber-Neck Gauge

Determining the precise diameter of a rifle chamber neck is important to both the gunsmith and the shooter. The handloader in particular stands to benefit by knowing the exact chamber-neck diameter, given the possibility that a thick cartridge-case neck wall will restrict bullet pull from the case mouth which can lead to dangerous chamber pressures. This problem can be eliminated by knowing the loaded cartridge-neck diameter and chamber-neck diameter, and providing proper clearance.

Sloppy case-neck-to-chamberneck tolerances can also produce erratic accuracy. For precise accuracy, it’s imperative to maintain optimal tolerances between the chamber neck and the loaded cartridge-case neck.

Unfortunately, aside from a This warning should never be overlooked and may be a definite sign that a bullet is being restricted by a tight fit between outside cartridge neck and the chamber neck wall. chamber cast or having the reamer used to make the chamber, close measurement of the chamber-neck diameter is not an easy task. But there is an easier way.

With the instructions below, you can easily make a gauging tool to accurately determine the chamber neck-wall diameter, and the tool can be made with materials that are probably already in your shop. Making the Gauge The chamber-neck gauge has a folding arm extending outward from the tip of a special shaft. The fold-out arm contacts one side of the chamber-neck wall, as the laterally opposing shaft tip makes contact with the opposite chamber-neck wall.

The arm pivots and is held in position at the grooved end of a special shaft by friction. This friction is controlled by a set screw, which is an integral part of the finished tool. The major shaft for the tool begins as a 3/8-inch-diameter brass rod approximately a foot long. Chuck one end in the lathe and turn the contours as shown in the illustrations. The head is formed to a diameter of 0.240 and

a length of 0.115 inch. A 45-degree taper is formed at the head end of the gauge to facilitate entry into the perimeter of the chamber neck. With the head shaped, the shaft is turned to 0.200-inch diameter for a length of approximately 3/4 inch. The remainder of the shaft can remain at 3/8-inch diameter. I knurled 2 inches of the distal end for a better grasp. Next, cut a slit near the end of the shaft 0.050 inch wide and approximately 1-3/4 inches long.

A one-inch-diameter steel-saw blade at or near 0.050-inch thickness does a good job, with the part properly held during the cut. The cut is made up to, but not through, the head of the gauge, which leaves the side opposite the measuring arm intact for uniform chamber-neck contact. The folding arm will fit into this This totally removes the guesswork from precisely measuring the inside diameter of a chamber neck. The diameter of a loaded cartridge neck can be predetermined from a fired case by adding two times slot.

It can be made of steel or brass to exactly the same thickness as the slot in the head of the tool. The tip of the arm is leveled to a 45-degree angle and polished for smooth entrance and contact with the chamber neck wall. With the arm in place, a hole is drilled in the body of the tool approximately an inch downward from the head of the tool. This hole is drilled through the brass body (continued on page 15 the case-neck wall thickness to the bullet diameter.

Too hot a load combined with a thickened case neck sets the stage for this avoidable occurrence. Observance of proper chamber-to-case neck-wall clearance is important for safe shooting.

and measuring arm all in one pass. A 4-48 socket-head screw is fitted and serves as the pivot point for the arm. The screw hole should be de-burred and polished for smooth movement, as the arm moves and stays in place during use, held in position through friction created by the controlled tightness of the socket-head screw. Using the Gauge Use of the chamber-neck gauge results in a precise but easily determined measurement of the rifle’s chamber neck-wall diameter.

In preparation for measuring a chamber neck, open the arm of the gauge a few thousandths more than the anticipated chamber-neck diameter. Then tighten the set screw just enough to hold the moving arm friction tight. Preliminary chamber-neck diameter can be determined by checking a reloading manual or actually measuring the outside diameter of a cartridge or— more importantly for the handloader— by adding the bullet diameter to twice the case neck wall thickness.

This later measurement is the critical one, as it clearly shows the handloader if problems are present. Case necks can then be reamed or outside turned for proper neck-wall clearance. Having set the chamber-neck gauge as described, carefully insert the tool into the chamber until contact is made. This is accomplished as the gauge head and folding arm enter the juncture of the chamber neck. At this point, gently push the gauge into the chamber about 3/16 of an inch.

At this point, you should feel the beveled head and folding arm moving into the chamber with mild resistance. Keep the main shaft of the gauge well centered in the rifle-bolt hole and in line with the chamber. After the gauge has entered the chamber, carefully withdraw it and take a measurement with a micrometer. Repeating this procedure a few times will verify an accurate measurement and give you a feel for 220 Swift rifle show excellent accuracy.

The case necks were turned to a uniform neckwall thickness and proper clearance between the case neck and chamber neck wall. This tolerance was determined by using the chamber-neck gauge and outside-turning the case necks to exact wall dimensions. the correct amount of arm tightness required on the gauge. Conclusion This easy-to-make chamber-neck gauge provides a critical measurement required by both gunsmiths and precision shooters. Increased accuracy and safer shooting are the likely results of its use. ■