c-g-designs

THROATS.

       I have been recently asked questions about throats dimensions.

       It is a suhject of importance, too often under-estimated.

Two values are to be borne in mind:

       1°) The location of the base of the cone in relation the front chamfer of the chamber.

       2°) The angle of the cone.

       Those dimensions are close if not absolutely similar in both CIP and SAAMI norms.

       In the CIP norm,their conventional symbols are as follows:

               S = For the length. This dimension is of course the initial relative minimal position who can vary  with the bullet shape and its relative position in regard to its taking of the rifling. 

               I = The angle of the slope of the throat (1/2 of the apex angle).It is this angle,of which the importance in relation with the weight and ogive shape  of the bullet used condition the value of this angle.

       For the clarity,those two symbols who are used in the official CIP nomenclature are those who wil be used here.

       NOTE before we start; Too often, the precision chambers are cut together with the throat using a single reamer. Unless specified on order, those reamers are made to CIP/SAAMI specifications and no adaptation is then possible.

       Therefore, to realise a precision chamber,perfectly adapted to the chosen bullet (s),it is preferable to use two reamers, a chamber and a throater and proced to the two operations separately.

       The choosen angle is function of the weight and ogive shape. The CIP values of a few calibres serve as reference:

-- .308 Winchester, i = 1°45’. Tgis is valid for the NATO bullets, FMJ 144 to 155grains. This value is already down to 1°30’ for the 155 PALMA and the G.Kolbe’s recommendation for bullets 180-210 HPBT is 1°.

-- 7-08 Winchester: I = 3°. This caliber was initially intended for short ogive bullets. For heavier bullets,the recommended angle become also 1°30’. This caliber with the modified angle is know as the 7-08 Obermeyer.

-- .284 Winchester: I = 1°47’. Intended for heavy bullets. Would be best at 1°30’.

-- 6mmBR NORMA: I = 1°30’. This caliber, derived from 6BR Remington, have had its angle reduced to suit the 105grains bullet. The angle for this caliber could experimentally be reduced to 1°?.

       I think also the new bullets (hybrids..) could benefit from a reduction of I at 1° like for the .308?.

       Back to the separate reaming of the taper: This necessitate more care and the use of an adjustable reamer stop to precisely adjust the throat and frequent measuring and cleaning during the process. For the use of heavier bullets, it can no longer be question to set the throat and cartridge OAL for the bullet to be closer to the rifling shorter than the 1/16 to 1/8 gap recommended by many specialists.

       R.G.C