OK, finaly got to my copies of Otteson vol 1 & 2 lastnight, and lost the first copy of this post this morning.
The following are Otteson’s figures for how much a case head protrudes out of the breech end of the barrel with the bolt locked.
Ottesson reckoned on typical mass production clearances between the top of a bolt rim and the breech face of a barrel being around 10 thou, and diametrical clearence between the bolt nose and barrel counterbore of a Rem 700 being around 10 thou. And a simillar 10 thou diametrical clearence between the walls of the bolt head counterbore and the case rim and head. Because of difficulty achieving concentricity in the investment cast Ruger 77, it has a 30 thou diametrical clearance between case head and bolt face rim.
He cautions that a re barrelling a rem 700 to a closer fit makes the rifle much more sensitive to dirt, like unburned powder grains or stray cleaning brush bristles.
Case Head Protrusions after Otteson
Action______________Protrusion (inches)
M98_______________0.105”
03 Springfld_______ __0.147 to 0.148
700 Rem____________0.160 plus any chamber mouth radius
Sako 461 (Vixen) _____0.107 annular ring around breech face of bbl for extr nose clearence
Win mod 70 (post 64)__0.125
Weatherby Mk V_______0.13”
Savage 110__________Pre ’66 (rem type extr) 0.144”--- Post 66 win mod 70 type extr 0.128”
Ruger 77_____________0.132” (push feed – controlled feed may be less – I really don’t know)
Carl Gustav
(Dovetail shaped lugs)___0.120
Browning BBR_________0.160 (0.13 counterbore, +0.015 clearence + chamfer)
US Patent numbers:
Sako type extr 431,670 (Paul Mauser)
Win mod 70 / savage 110 type extr 431,669 (Paul mauser)
Rem 700 breech 2,585,195 M H Walker Apparently with v good description of tests.
The Mauser 98 sets the standard, as the thin extractor allows very deep seating, however this is achieved at the cost of cutting down the bolt rim for most of its circumference, and leaving a great gaping hole in the receiver inner collar for the extractor to go through.
The Sako which Otteson measured had very small case head protrusion, however at that time, the protrusion was not matched by the bolt face rim, so there was considerable potential for gas leakage into the action if a case head failed, and no measures at the rear of the action to deflect it away from the firer’s face. this may have changed since then.
As I posted previously, Remmington’s engineers achieved excellent shrouding with the unbroken bolt rim and the barrel counterbore, however the unbroken rim required a deeper bolt head counterbore to provide a lip to hold the little extractor in there. They also neglected gas deflecting measures at the rear of the action, to keep gas and debris leaking down the race ways out of the firer’s eyes, as they had such good gas sealing up front.
Using figures for the Sako Vixen head protrusion, if you can achieve that with a “Sako type” extractor on a Rem 700, then there is another 57 thou of the case that could be inside the chamber if the bolt face rim is reduced to allow the barrel to be set further back.
What importance is minimal case head protrusion?
So long as the cases are good and no one has been stupid enough to barrel the rifle to expose anything other than the solid case head webb, then all the actions listed have proved to be ok
For high pressure loadings, or with cases which have thin heads or dodgy metallurgy, or long history of re-loading or dodgy storage (eg corrosive or damp conditions, heat etc) things might be more dubious...
Bear in mind that the commercial actions covered were all developed in the 25 to 30 years after WWII, when western manufacture had the margins to support v good quality production.
Mauser’s military turnbolt design work was done when metallurgy and quality control were poorer, and case head failures were not so uncommon, and it was a military advantage not to loose to many eyes and faces from your troops due to case head failures.
With western manufacture having to cut costs to compete with Chinese workers paid less than $1 /day or manufacture being outsourced to such places, I think we are probably due a few more dodgy case heads.
In the Custom bench rest actions that Otteson covered, almost all used the win mod 70 type sliding extractor and had lugs continuing up to the bolt nose to support this, but in bolts that were otherwise copied from the Remmington 700. This is due to thin heads in some batches of .220 Russian brass, and also due to the high pressure loadings, tight throats and bullets seated to touch or just clear of the lands. Having bench rest loading and ejection ports on the left side also gives incentive to avoid case head blow outs.
The advantages of the mod 70 type extractor for allowing deep seating can be seen with the Savage 110, which started with a rem type extractor (mounted outside the bolt counterbore to get around Remmington’s patent) and went to a Win Mod 70 type extractor. Bolt face counterbore reduced from 0.134” to 0.118”.