NRA Rule 150 , Barrel and Chamber Dimensions
For the reasons given below, and since it is possible that different standards may have been used in the
last few years to test the compliance of Target Rifle chambers with Rule 150, the NRA Council have
decided that all TR chambers are to be re-checked this year. All target rifles whose chambers comply will
be marked with a green coloured ‘sticker’. Please note that the previous ‘stickers’ (blue or orange) will no
longer be valid.
All competitors should therefore have their chambers tested as soon as possible. Chambers may be
checked in the NRA Armoury or by other accredited gunsmiths. The names of accredited gunsmiths are
available from the NRA Armourer. Testing in the NRA Armoury will be free of charge unless the test round
is marked or pushed back, indicating a failure, in which case a charge of £1 will be applicable to replace the
test round and the chamber must then be rectified and re-submitted for test.
The rules on barrel and chamber dimensions for NRA Target Rifle are set out in Rule 150 of the NRA
Bible but reference must also be made to the current agreement between the NRA and
UK Proof Authorities
in respect of the minimum permitted bore, groove and throat diameters on 7.62mm/.308 Target Rifles
submitted for proof.
Rule 150 Barrel states that the barrels of all Target Rifles must be suitable for firing either:
(a) the standard 7.62mm x 51 NATO military cartridge, or
(b) the .308 Winchester commercial cartridge.
The references to the .303” Mk 7, 5.56 x 45mm and .223 Remington cartridges in this rule are not relevant
as these calibres/cartridges are not permitted for use in TR during the Imperial Meeting.
The implication is that the chamber dimensions must comply with the CIP minimum chamber drawing
other than the exceptions outlined below. For all practical purposes the CIP and SAAMI drawings for
minimum chamber dimensions may be considered identical. Effectively, therefore tight chamber body
dimensions and or tight necks are not permitted.
The exceptions to this, as agreed with the Proof Authorities, are:
Minimum Bore Diameter : 0.298 inches
Minimum Groove Diameter : 0.3065 inches
Minimum Throat Diameter : 0.3085 inches
Maximum Cartridge Loaded Length : 2.800 inches
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The restriction on bore, groove and throat diameters are not directly specified in Rule 150 but are
governed by the minimum diameters that are acceptable to the UK Proof Authorities on submission of a
target rifle for proof. In addition there is a maximum pressure restriction on the ammunition which is, of
course, outside the competitor’s control with issued ammunition.
The restriction on cartridge loaded length is specified by Rule 150 with reference to the 155 grain RG
projectile, i.e. if a cartridge loaded with this bullet to an overall length 2.800 inches is loaded into the rifle
chamber, the bullet must not be in contact with the rifling.
In practice compliance with the throat diameter / length restriction is checked using a dummy test
cartridge. If the bullet of this test cartridge is “ring marked” or pushed back into the case either the throat
diameter is too small, or the leed is too short, or both, and the rifle may not be used until the chamber has
been rectified.