How to find headspace and CBTO in blaser r8 chamber

[Datchew]

Hi gents,
I've mapped bolt gun and falling block (ruger no 1) chambers to find where i want to set the shoulder for fire formed brass. Several different techniques.
The one i like best is to simply drop the shoulder when re-sizing with the exchangeable redding shellholders and then gently feel where the bolt or block closes without resistance.
I've also resorted to measuring shoulders with the RCBS vernier mic tool.

I'm commencing some 300WM and 375H&H handloads with a blaser r8. It seems the bolt carrier "shroud" closes too much to do mapping in a similar fashion to prior experience.
Do you have any advice on how to determine headspace of the chamber or am i stuck resorting to measuring fired brass and bumping back a few thou?

 

[AimSmall]

Good question.

Best solution I'd offer - don't fret about it. The R8 will shoot .5moa in 300 and 375 without much trouble. Actually, 7 calibers I load for it will.

 

[Datchew]

That's a surprisingly interesting note.

So just load for the median and if it shoots moa anyway (likely) then leave it alone

Good call. I'll leave the genie in the bottle if I get the desired result
Thx much

 

[fourfive8]

Precisely measuring the headspace length and comparing to the resizing headspace length produced by any one die is very difficult if not impossible. Judging correct headspace it based on "just enough" resizing or set back for resistance free chambering is the product and desired effect. Overall, for hunting cartridges, I adjust my resizing die in small, 1/4 turn increments, testing for chamber fit after each adjustment- of course starting long and working toward correct minimal headspace. After each adjustment and sizing with bullet only and no powder, I open the chamber and hold the rifle vertically muzzle down and drop the test cartridge directly down into the chamber. If it slides to a stop it needs a tad more set back or resizing. If it goes "plunk" it is good to go. Some call that the "plunk test". The difference between each 1/4 turn of the sizing die is a very few thousandths, so is used as a NOGO-GO gauge of sorts. Hunting ammo chambering should be completely resistance free. Target ammo can have a slight compression against body taper and/or shoulder datum ring contact when chambering.

On rare occasions, a rifle's chamber will be so dimensionally different from the sizing die that it becomes difficult to determine exactly what is causing chambering resistance or headspace control. Both body taper and shoulder shape can affect this resistance. Even seating a bullet can affect the location of the headspace control datum ring section on a shoulder. That is why I always test a sized case in a chamber with a bullet seated. Of course this whole discussion is about cartridges that headspace on the shoulder. Those that headspace on a rim or belt- different discussion.

 

[fourfive8]

Feeling for bolt closing or breech block closing resistance or even the plunk test as a way to determine headspace can give false readings. Several things can cause resistance to free chambering. Bullet engaging lands, mouth rim contacting end of chamber, shoulder on case contacting shoulder in chamber and body taper (contact). Many methods will work to get to minimal and functional headspace control. Just make sure to eliminate the false headspace controls that may mimic the correct controls, some of which can cause dangerously high pressure spikes. The bullet contacting or engaging the lands can increase pressure. The case mouth contacting end of chamber has a real potential to dangerously increase pressure because the end of chamber constriction can crimp the bullet in place preventing proper release upon firing. That potential obligates paying attention to trimming to correct case length!

An interesting test to determine how positive any cartridge-chamber headspace control really is is to size and prime a cartridge but not seat a bullet nor charge with any powder. Fire the cartridge. Inspect and measure primer protrusion after the firing. Sometimes an eye opening exercise.