[realbuffdriver]

I am considering purchasing an RCBS Precision Mic to determine precise headspace measurements for loaded rounds. It occurred to me that .308 Winchester and .243 Winchester have identical headspace. What is the difference, if any, between the precision mic gages? Is it only the plastic bullet device, used to determine where the bullet touches the lands?

If any of you have both gages, I would like to know what you've found.

Thanks!

RealBuffDriver

[Fatelvis]

Realbuff, I've used both the RCBS mic and Stoney Point's guages, and like Stoney Point better. The RCBS is accurate, and well made, but the SP lets you check measurements alot quicker, and after the initial investment of the headspace and Comparator setup, you'll be able to check headspace and optimum seating depth on a whole bunch of cartridges, relatively cheap.(Compared to buying a specific Precision Mic for every cartridge you want to work with). I hope I helped you out!-

[Poper]

Real:
I am sure there are others on this forum more knowledgeable than I. However, I offer my humble opinion for your use as you see fit.

I have the RCBS Precision Mics for .222 Rem., .243 WSSM, .308 Win., and .300 WSM. These are fine tools and will tell you a great deal about the chamber dimensions of your firearm. I do not have any of the Stony Point devices, so I cannot speak for their precision or usability, but the RCBS units do perform as advertized. Unless you have a large number of guns, the cost would appear to be nominal.
I doubt seriously the .243 and .308 are interchangeable eventhough they are, technically speaking, the same case. The headspace measurement is taken from the datum line which is near the mid point of the shoulder (between the body and the neck). Simple geometry indicates the shoulder for the .243 will be wider because of the smaller neck diameter and therefore the datum line will not occur in the same place. This would give an incorrect headspace dimension reading if you used the .308 in lieu of the .243 instrument.
Because firearms are prcision instruments, in my honest amateur opinion, it is rarely safe to substitute such things unless you are really sure of what you are doing. It is entirely possible you may get false readings that might disguise a potential problem or create a hazzard. I believe the $35 or so would be a wise investment. This really isn't the place to skimp on $ IMO.

I hope this is helpful.

BTW, I am trying to locate one in .260 Remington. Midway: nope. Cabella's: nope. Local shops: nope.
Any assistance would be greatly appreciated.

Poper

[realbuffdriver]

I can certainly see your point as to safety.

I would respectfully disagree with your point about the datum line. The reason being that I bought a rifle headspace gage, and it is good for .308, .243, .260 Remington, 7mm-08 Remington, and .358 Winchester. If the same headspace gage functions in all of these chambers, then headspace should be measured the same for all of these rounds.

It appears, after looking at the product on RCBS website, that there are two "caps" for the micrometer and one "base". One of the caps most likely measures headspace and the other measures bullet seating depth.

Again, it would be nice if someone who owns the product could confirm, but I'll bet that there is no functional difference in the headspace "cap" for all of the above calibers. Obviously, the bullet seating would vary among these calibers.

Thanks for your reply. I agree that substituting would be potentially dangerous and won't do so unless I can determine that the parts are in fact identical.

RealBuffDriver

[Poper]

Realbuffdriver:
This subject is an interesting one. It is good business practice for a mfr to reuse as many common elements of a design and spread the costs across as much product line as possible.
The cap substitution may be possible if:
1) The SAAMI headspace dimension is identical for the two cartridges,
2) The dimensional calibration of the two RCBS units are identical (likely)

Obviously the chamber/throating measurement tool would not substitute as it is strictly bore dimension dependent for its measurements.

A consultation with a qualified gunsmith may be worthwhile.

I hope this helps.

Poper