Been having a few customers asking the question of being able to used a .223 case in a .308 bolt face so i asked our designer, Robert Chombart, to give a detailed explanation why this is not acceptable. I thought i would share this with everyone.
Being asked the question of having for instance a .308 bolt face recess used for a .223 case, my answer can only be negative for different reasons:
All rifle action manufacturers but one, offer a specific bolt head recess for each cartridge case dimension, such as .223 , .308 , PPC and Magnum for instance.
This recommendation corresponds to CIP chamber dimension R1, who are, for instance:
--223, R1 = 9,06mm.
--.308, R1 =12,06mm.
SAAMI specifications are similar, but are only recommendations, whereas CIP are the law in our CIP countries. I really doubt a rifle with, say, a .308 bolt face recess would be accepted for proof as a .223.
The reasons for a closely dimensioned bolt case head recess are multiple, and this list is not exhaustive:
-1°) SAFETY : The case head must be closely held in the bolt head recess to contain, in case of overpressure, the brass flowing freely, nothing preventing its rupture with important gas and debris escape as a consequence.
In relation with this, even with relatively mild ammunition, the non-externally maintained case head will expand under firing, allowing the primer pocket to open up, no longer maintaining the primer and considerably reducing case life.
2°) CYCLING. With a correctly dimensioned bolt case head, the left side of the recess counteracts the extractor spring pressure, to allow a good bearing to ensure the correct hold of the case, its good extraction and eventually ejection.
Also, the empty case will remain fixed in the bolt recess up to the end of the bolt pull.
The normal test consist to take the bolt, insert an empty case in the recess and gripped by the extractor and turn the bolt in every direction and upside down.
The case must remain in place during the whole operation. This is impossible with an oversized recess.
-3°)ACCURACY : It is important to have the chambered round perfectly cantered in the chamber. Many single shot match actions have for this reason no ejector or one not pushing the cartridge sideways in the chamber.
The pressure exerted radially by the spring-loaded extractor will push sideways the case head not supported by the opposite face of the bolt recess. This will have an effect in cartridge placement even worse than the one caused by a spring-loaded ejector.
4°) IGNITION : The side pressure then exerted will cause off-center ignition……
-5°) RANGE PRACTICE: A fired case, not being maintained, will automatically fell in the receiver. Not a problem for recovery in an action with a largely dimensioned loading port, but what about a misfire or an incident on the firing point requiring official intervention?
-6°) MAINTENANCE: Each case head dimension needing a different extractor, multi-calibre fitting would each time require a change of the extractor.. Moreover, the prudent who want to keep always a spare one will then need one per calibre…
Notwithstanding the risk inherent for each extractor change.
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