Alright, probably a silly question. But here it goes. I've always measured COAL when developing loads. I switched to a Hornady comparator to measure BTO and wrote down my measurements for loads that have already been established. Well, since switching to measuring BTO, I've gotten a little practice in measuring it. I typically will spin the case a little bit to get a good reading and apply pressure until I get a measurement that doesn't change (shortest measurement). I decided to go back and measure the previous ammo I already measured originally and found a shorter measurement and wrote those down. I'm assuming that when I first started measuring BTO, I wasn't applying very consistent pressure to get the shortest reading. Just to clarify, I am not forcing the calipers, I'm just applying consistent pressure to get a consistent reading. Is this what you guys do when measuring CBTO?

I use a Hornady comparator to determine where the lands are (for a given bullet) and to help make up a dummy round for a specific load.  Once I have the dummy made, I measure it with an RCBS Precision mic and use the RCBS instrument during production (bullet seating operation) to check QC along the way...

For both instruments, even/consistent pressure is important to help get reliable/reproducible readings.  I also measure several times and rotate the bullet between measurements - there should be good agreement between all the measurements.  If there isn't I figure out why and fix the "problem".

Measuring at the ogive should give much more consistent readings than measuring COAL.  For CBTO measurements, I expect the ES for a batch run to be no more than 3 thousandths and nothing should differ from the target CBTO by more than 2 thousandths up or down. Obviously, less is better.  Bullet quality also plays a part here so cheap blasting bullets might not have the consistency of precision match bullets...

Huh, I pretty much do what you do with the exact same tools even...
I will add when I measure with a caliper I spend lots of time and effort trying to be as consistent as possible...oddly enough the RCBS PM will let you know if you are consistent on the calipers...

RCBS PM..I also use it to verify shoulder setback...

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Interestingly enough, after reloading for several calibers, I did just recently decide to make a dummy round for my 6.5 CM load (boy, I'm stingy with my brass and bullets ). So the other reason this question was brought up is everytime I check the BTO, I am closing the calipers with consistent enough pressure that the comparator leaves a slight ring around the bullet where the ogive is. So again, I'm not sure if this is too much pressure, but the measurements are all within .001 of each other.

Ok, that's valid. It doesn't feel like I'm pushing that hard, but I'll try to dial it back and see what my measurements are.

I have never left a ring or mark while checking the CBTO with either instrument.  I have only ever measured jacketed bullets - typically from Hornady (Interlock), Sierra (SMK), Berger (Hunters), and Speer (HotCor).

GMTA!  

So this is what mine looks like. And this article mentions scratches on the bullet from measuring. You guys think this could have an adverse effect on accuracy?

No that is normal...but with that saiid, I strive to use light but consistent pressure on my calipers...leaving marks maybe a bit more pressure then needed...I can normally get the pressure right to the point I can take my thumb off the wheel without either the reading changing or the piece being measured moving...

None of the bullets I measure end up looking even remotely like that.  I think that "scratch" was "enhanced" for the purposes of photography to make a point...  but I could be wrong...

That's solid advice, I'll give that a try

EDIT: Just gave this a try and instead of it reading 2.1625, I got 2.1630.

There is an uncertainty in the readings from the instruments regardless of the technique or which instrument we are discussing.

What gets frustrating is learning to accept a non-zero uncertainty, and learning when uncertainty is too high due to other imperfections in the ammo.

Part of the uncertainty is caused by the samples we are looking at and the fact that some have low runout on the base of the case and also on the runout of the bullet, and some don't.

The nature of trying to settle the gage pressure on a long cartridge using a narrow base against the point where the intersection of an ogive and a circle are minimized, depends on cartridge runouts and the gage pressure.

That straight line may or may not be the stiffest if the base doesn't represent the centerline of the cartridge. If it is tilted at all, the tip of the rim is loaded and it isn't as stiff as one where the base area is fully supported.

If a tool is very square to a perfect bullet, then the intersection is a circular ring. But, what happens when a bullet is not straight?

That intersection starts to be more like an ellipse, but the line of action of the tool is only getting supported on the long axis of the ellipse.

So for a given gage pressure, the measurement uncertainty will grow when there is misalignment in the base or bullet.

If the ammo is sorted by runout, including the base of the cartridge and the bullet centerline, you will notice a more repeatable reading with lower uncertainty.

The opposite is also true, try the same readings on samples with bad runouts and the CBTO readings get more inconsistent too.

If you are getting an uncertainty of +/-0.001, you are doing pretty good.