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 maximum clearance of 0.060 mm (2.4 mil) between these two specifications and accuracy will not be good.  The closer to zero clearance one goes, the better the alignment will be and the better 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 when the cartridge casing expands. The casing 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:
1.  Under-size bullets,
2.  cartridge case necks that are too thin
3.  or an over-size chamber neck area.

The solution is:
1.  If the bullet is monometal, use the correct diameter bullet.
2.  Use cartridge cases with thicker necks.
3.  Have the rifle rechambered.

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.

THE TEST

The maximum allowed difference between the diameter of the case neck and the diameter of the chamber is 0.060 mm (2.4 mil).  Parcel tape is 0.040 mm (1.6 mil) in thickness. Use parcel tape to do this check.  Remember that the parcel tape dimension must be doubled.  The Scotch Tartan 305 or Scotch Tartan 369 ranges are close to 0.040 mm (1.6 mil) thick. Any other tape such as heavy duty parcel tape (Scotch 311, 313, 371, 372), sellotape or masking tape is too thick, of variable thicknesses and should not be used.

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.  Do not put powder in the case or a primer in the base of the case.

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.

Effectively, the case neck has now been increased in diameter by 0.080 mm (3.15 mil) and it should be too big for the chamber.

If the combination used does not chamber normally and the tape is pushed back, there is less than 0.040 mm (1.6 mil) clearance between the case and chamber neck. Acceptable hunting rifle accuracy should be possible. Ideally, the less the clearance is, the better, as long as there is room for the case to release the bullet.

If the cartridge chambers easily and does not push the tape back, add another layer of tape.  Check again until the tape prevents the cartridge from chambering.  Remember that one layer of tape should push back and more than one layer is not a good sign. If the test shows excessive space but accuracy is achieved, the chamber and rifle concentricity is good and easily deformable bullets with soft lead cores and thin jackets will work best.

If measurements need to be smaller, measuring instruments can be used for this, but requires a bit of practising.