Bolt Head Recesses

[Ovenpaa]

The subject of using a .308 bolt face with smaller case heads was recently raised and Robert Chombart (RGC) emailed me his thoughts on the subject which I would like to share.

Robert tells me:

BOLTHEADS RECESS.

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 = 9,66mm.

--.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-centre ignition……

-5°) RANGE PRACTISE: 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.

RGC 14/05/2015

[Gaz]

Surely, the chamber being a very lightly tapered cone and the cartridge case also being tapered, the case will always sit a tiny bit low in the chamber anyway? Combined with a spring-loaded extractor, effectively the case is constantly sat at the 7 o'clock position?

[Laurie]

I've got to say I agree with Robert, but there are examples around where it seems to work out in practice even if sub-optimum in engineering terms.

The main one I have in mind is Canadian use of the Number 4 action with 5.56mm maybe 20 or so years ago. The Canadian government continued to supply the DCRA with heavily subisidised (maybe even free) milspec ammo from the then state owned IVI ammunition plant and this was the norm in 'Target Rifle' at most levels. After the Canadian forces switched from 7.62 to 5.56mm, the quality of the latter being produced on brand new equipment was good, very good indeed according to reports and I've seen it in print more than once that IVI SS109 type 5.56mm was the best available in accuracy terms. 7.62 still being produced for GP Machineguns on the other hand went rapidly downhill so that local 7.62 TR shooters joked that IVI stood for 'Impact Varies Indeterminately'.

Many Canadians swiched to 223/5.56, a local historical 'blip' that sees 223 still very popular today in Canadian prone shooting although IVI has long since been privatised and the government sponsored cheap ammo gone. Many prone shooters retained their No.4 7.62 TR rifles long after they became junk or cheap starter rifles here and there was a wave of rebarreling them to 5.56 during this period. The only other change apart from the barrel was a longer extractor claw. Rounds were single fed into the chamber and fired cases manually removed from the action. The No.4 bolt face is of course smooth with no rim to enclose the case-head of any cartridge design and had managed for 30 odd years around the Commonwealth with 7.62 / 308 and continues to do so amongst more than a few FB-UK members' rifles too even now I'd wager. In any event, the little 5.56 managed just fine at full NATO pressures of getting on for 62,000 psi sitting with acres of unspupported flat metal around it. Multiple reloads would likely be unwise, but for cheap club issued ammo, shoot and chuck, it worked fine.

I'm sure many Canadians carry on with this practice and I've seen at least one such in a club shooter's rifle in the UK.

[R.G.C]

I've got to say I agree with Robert, but there are examples around where it seems to work out in practice even if sub-optimum in engineering terms.

The main one I have in mind is Canadian use of the Number 4 action with 5.56mm maybe 20 or so years ago. The Canadian government continued to supply the DCRA with heavily subisidised (maybe even free) milspec ammo from the then state owned IVI ammunition plant and this was the norm in 'Target Rifle' at most levels. After the Canadian forces switched from 7.62 to 5.56mm, the quality of the latter being produced on brand new equipment was good, very good indeed according to reports and I've seen it in print more than once that IVI SS109 type 5.56mm was the best available in accuracy terms. 7.62 still being produced for GP Machineguns on the other hand went rapidly downhill so that local 7.62 TR shooters joked that IVI stood for 'Impact Varies Indeterminately'.

Many Canadians swiched to 223/5.56, a local historical 'blip' that sees 223 still very popular today in Canadian prone shooting although IVI has long since been privatised and the government sponsored cheap ammo gone. Many prone shooters retained their No.4 7.62 TR rifles long after they became junk or cheap starter rifles here and there was a wave of rebarreling them to 5.56 during this period. The only other change apart from the barrel was a longer extractor claw. Rounds were single fed into the chamber and fired cases manually removed from the action. The No.4 bolt face is of course smooth with no rim to enclose the case-head of any cartridge design and had managed for 30 odd years around the Commonwealth with 7.62 / 308 and continues to do so amongst more than a few FB-UK members' rifles too even now I'd wager. In any event, the little 5.56 managed just fine at full NATO pressures of getting on for 62,000 psi sitting with acres of unspupported flat metal around it. Multiple reloads would likely be unwise, but for cheap club issued ammo, shoot and chuck, it worked fine.

I'm sure many Canadians carry on with this practice and I've seen at least one such in a club shooter's rifle in the UK.

Laurie,
Every rule suffer exceptions. I know the example of converted Nr4s.
They were coceived around a rimmed case with a quite thick and strong case bottom . Moreover , the relatively small diameter of the bolt would not have allowed much room for a casehead recess !!!

Also, being a cock-on closing, I wonder how much the firing pin does not help in maintaining the case under the extractor during the final of the bolt pull ? I may be wrong here, eventually, correct me ?
Finally, my mention was on modern single shot actions including all the developments of the last some 50 years . For those actions, I think offering this ‘possibility’ is senseless.

Sincerely yours
R.G .C

[Laurie]

Finally, my mention was on modern single shot actions including all the developments of the last some 50 years . For those actions, I think offering this ‘possibility’ is senseless. [R.G.C.]

I quite agree Robert. One practice I've never been sure of is the use of intermediate diameter 220 Russian / PPC / Grendel cartridges in bolts configured for the 0.473" Mauser / 308 etc case-head to avoid buying an expensive new bolt (if available) or a complete action.