UNDERSTAND THE CONDITION

CIP, SAAMI and other specifications allow manufacturers to make cartridges and to cut rifle chambers that conform to rigid specifications.  The specifications for chambers set a minimum size with an upwards tolerance. The specifications for cartridges set a maximum size with a downwards tolerance. This is to ensure that the smallest chamber for a given cartridge, will always accept the largest allowed cartridge.  The manufacturers of reloading dies also follow these specifications.

When the smallest allowed cartridge is fired from the largest allowed chamber, accuracy will not be good. The critical dimensions for accuracy are the freebore and neck areas.

The maximum allowed difference will allow a clearance of 0.060mm (0.002").  The closer to zero clearance one goes, the better the alignment will be and the easier accuracy will become.

When the cartridge is fired, any clearance between the cartridge neck and the wall of the chamber will become the clearance between the inside of the case neck and the bullet. The case expands away from the bullet until it contacts the chamber wall, before the bullet has moved much.

The more clearance there is between the case and the bullet, the more the long axis of the bullet can turn away from the centerline of the barrel. This causes the bullet to enter the rifling at different angles from shot to shot and results in different points of impact.

Excessive clearance can be caused by under-size bullets, outside cutting of the case necks or an over-size chamber neck area. 

When the condition of excessive clearance exists, the rifle will shoot more accurately the more easily the bullet deforms.  Best accuracy will be with thin jacketed, soft lead-core bullets, like the bullet on the right in the picture.

This is because, when pressure builds behind a bullet that deforms easily, the bullet will shorten, expand in diameter and take up some or all of the available space around it.  Where the bullet is loaded close to the rifling, there is then less space and time for the long axis of the bullet to turn away from the centerline of the barrel.

Photo of parcel tape and dummy cartridge.

CHECK THE DIMENSIONS

The maximum allowed difference between case neck and chamber is 0.060mm.  Parcel tape is 0.040mm in thickness.  Use parcel tape to do this check.  Any other tape such as sellotape or masking tape is too thick and should not be used.

Photo of an untaped cartridge

Make a dummy cartridge if reloading or use a factory cartridge to do this check. The case must be loaded with the bullet type being used and the case must be prepared as it normally would.

Photo of taped cartridge

Cut a section of tape as wide as what the case neck is high. Tape around the case neck and ensure that the ends of the tape do not overlap. A small gap between the ends of the tape does not matter.

Chamber the cartridge

Effectively, the case neck has now been increased in diameter by 0.080mm and it should be too big for the chamber.

Photo of tape pushed back.

If the cartridge chambers easily and does not push the tape back, the combination of this cartridge and chamber has too much space to produce consistent accuracy.  If the test shows excessive space but accuracy is achieved, the chamber and rifle concentricity is good and easily deformable bullets will work.

Photo of measuring instruments

If the combination used does not chamber normally and the tape is pushed back, there is less than 0.040mm clearance between the case and chamber neck.  Acceptable hunting rifle accuracy should be possible.  If measurements need to be smaller than this 0.040mm tolerance, measuring instruments can be used for this.