RCBS Precision Mic/Setting Sizing Die:

[Dinky]

I have now (1) read, (I think) most of what is available on the Web about this subject, (2) devoured the user guide, and (3) called RCBS and talked with a Tech about use of the instrument for determining head space. So I now understand the process (I think) pretty well--in theory. However, I would like to keep my rifle, not to mention my skull, intact, so I am going to post my findings--as well as what I think I have learned from those findings--for a little hand-holding on yall's part:

(1) I was concerned with the statement in the instructions to the effect that the Mic should not be used with cases that are to be either full-length or small-based resized. The RCBS Tech confirmed my assumption that this only applied to the use of the tool for bullet seating--and not the function of headspacing. So, I'm OK in using in my bolt guns.

(2) Next, I measured five rounds each of all my various batches of brass and loaded rounds, and have to admit I was somewhat blown away by the results:

(a) .300 Ultra Mag: I do not yet have any rounds fired in my rifle. I do have 100 virgin cases which have been processed only to the extent of primer pockets and flash holes as well as running an arbor through the necks: Results of measuring these cases revealed readings of -.001 (from zero) through -.003, mostly averaging around -.003.

Also, in order to get some factory ammo, I went online, mortgaged my house, and purchased 20 rounds of Nosler Custom ammo--wondering all the while what my Grandfather, born one month before Custer bought the farm at the Little Big Horn, would think of my paying what he shelled out for one acre of Texas ranch land--for a box of ammo! These beautiful rounds (and they ought to be!) measured consistently at -.005.

None of these measurements stood out particularly wildly.

(b) .300 Winchester Mag: Here, the results were mind-boggling, and I would like your impressions:

(Cases fired in my rifle): 10 cases, five of them Remington and five, Federal Premium Nickel) all came out anywhere from dead zero to +.002. This tells me that my chamber, as far as head space is concerned, is pretty much dead on with SAAMI Minimum specs.

Virgin Remington Brass: Five cases averaged -.013.

Factory Ammo:
First, Federal Premium Nickel: Five cases averaged -.018. Spread was -.018
through -.020.
Second, Remington: Five cases averaged -.015.
Third: Hornady: Five cases averaged -.010.

Question: Are these results amazing, or am I way off base?

Question: Finally, would I be way off base, given the approximation of my chamber headspace in the .300 Winchester of about + .001, to bump my shoulders back to approximately - .002?

Thanks,

Dinky

 

[woods]

Not surprising at all

First you need to understand that what you are measuring in the 300 win mag is not headspace since SAAMI defines it as

"HEADSPACE The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats."

SAAMI

which means that headspace on the belted case is measured from the bolt face to the front of the belt groove. IOW the case can move forward only until the front of the belt on the case hits the belt cutout in the chamber.

You are measuring from the bolt face to the shoulder which is different from headspace on a belted case.

Knowing this, the case manufacturers are not concerned with how tight the chamber fit in regards to the shoulder, the belt will keep any problems from occurring from the case moving too far forward in the chamber upon firing.

.013" or .018" is about average as I have seen them from .012" to .040", example of a Winchester Model 70 post 64 in 264 win mag that I bought for a donor action, factory new shoulder measurement to once fired shoulder measurement

 

[Dinky]

Wood,

Thanks. I was actually aware of what you said about the definition of head space on belted rounds--in this case, the 300 Winchester--and that my measurements were in fact relative. But I failed to wrap my mind around the IMPLICATIONS of that for factory ammo design. So I see what you are saying about their not having to worry about head space. And I guess, since the Ultra Mag is not belted, that would explain why the manufacturer stayed as close to the SAMMI minimum at a minus .005.

But check me out on this: taking the Remington cases as an example, the unfired factory rounds "measure", (relatively speaking), an average of minus .015, whereas the fired cases average right at a plus .001. Doesn't that work out to a forward movement of the shoulder of some .016? And what is the implication of that fact on brass length?

 

[woods]

Wood,

But check me out on this: taking the Remington cases as an example, the unfired factory rounds "measure", (relatively speaking), an average of minus .015, whereas the fired cases average right at a plus .001. Doesn't that work out to a forward movement of the shoulder of some .016? And what is the implication of that fact on brass length?

Yes and this is why the belted cases are more prone to a case head separation. The primer fires, the case moves forward to contact at the belt then all the rest of the case has to stretch to reach the shoulder. The brass is moving forward right when there is a lot of heat at the pressure ring right in front of the belt and it thins the most

But if you only set the shoulder back a minimal amount once the case has fully expanded forward and you get a crush fit, then you will not exacerbate the thinning at the pressure ring

Keep the "shoulder gap" (for lack of a better term) minimized and "headspace on the shoulder" and a belted case is just like all the others