Measuring Consistent BTO using comparator - am I missing something?

[aahunt03]

When I measure BTO using a Hornady Comparator, (or Sinclair,) I measure the same loaded round 10 times and get 10 different measurements. Just the slightest variation in position leads to an inaccurate measurement.

If I want to seat my bullet to X thousandths off the lands, how can I accomplish this when I cannot get an accurate and consistent BTO measurement?

In a world where we spend tens of thousands of dollars on the most absolute, definite, precise, and repeatable reloading equipment and gear, how can the BTO measurement be so inaccurate and unrepeatable? Am I missing something here? Is there some magic tool or gauge you are using which gives consistent readings down to thousandths of an inch? Or is the community just winging it on BTO and being absolutely precise on everything else?

 

[Olen_4504]

tagging myself in to see what other people say. i've experienced similar. you definitely need to spin the case to find the flat spot, but if you spin it too hard and too fast, you actually dig into the bullet and it seats further into the gauge. my problem is my gauge and my calipers don't seem perfectly square. i can't figure out of my seating process/neck tension is inconsistent or if my measuring is inconsistent

i'm curious what other people say.

 

[Threadcutter308]

When I measure BTO using a Hornady Comparator, (or Sinclair,) I measure the same loaded round 10 times and get 10 different measurements. Just the slightest variation in position leads to an inaccurate measurement.

If I want to seat my bullet to X thousandths off the lands, how can I accomplish this when I cannot get an accurate and consistent BTO measurement?

In a world where we spend tens of thousands of dollars on the most absolute, definite, precise, and repeatable reloading equipment and gear, how can the BTO measurement be so inaccurate and unrepeatable? Am I missing something here? Is there some magic tool or gauge you are using which gives consistent readings down to thousandths of an inch? Or is the community just winging it on BTO and being absolutely precise on everything else?

I'm not sure what brand comparator(s) I have, but the (mounting) slot that is cut in the back of the body is offset from the centerline of the bore. I always mount the comparator on the right hand/moveable jaw. That is because I want the cartridge case head to sit of the left hand/fixed jaw. Take a look at how the comparator body orients to the moveable/right hand jaw of your calipers and install the comparator so that it is inline/concentric with the fixed/left hand jaw of the comparator.

 

[Olen_4504]

I'm not sure what brand comparator(s) I have, but the (mounting) slot that is cut in the back of the body is offset from the centerline of the bore. I always mount the comparator on the right hand/moveable jaw. That is because I want the cartridge case head to sit of the left hand/fixed jaw. Take a look at how the comparator body orients to the moveable/right hand jaw of your calipers and install the comparator so that it is inline/concentric with the fixed/left hand jaw of the comparator.

that's how I arrange mine. but the comparator doesn't look to be perpendicular. it's ever so slightly canted to the right. i think that's why I have a hard time getting a good reading. for my case head to be flat against the fixed blade, the cartridge isn't sitting square in the comparator

 

[Threadcutter308]

When installing my comparator onto the moveable jaw of my calipers, I put the comparator onto the jaw and close the jaws so that the face of the comparator contacts the fixed jaw. I then tighten the setscrew on the comparator, fixing it to the moveable jaw. That also gives me the benefit of being able to see whether I have the comparator installed in the correct/concentric orientation. The fixed jaw should exactly bisect the hole in the comparator.

 

[Olen_4504]

When installing my comparator onto the moveable jaw of my calipers, I put the comparator onto the jaw and close the jaws so that the face of the comparator contacts the fixed jaw. I then tighten the setscrew on the comparator, fixing it to the moveable jaw. That also gives me the benefit of being able to see whether I have the comparator installed in the correct/concentric orientation. The fixed jaw should exactly bisect the hole in the comparator.

wow why didn't i think of that.

 

[Threadcutter308]

wow why didn't i think of that.

Don't feel bad.......it took me 12 years to figure it out

 

[Kraze]

Also, if you don't have the anvil base kit, I recommend it. It's not too expensive and makes it a bit easier to spin the case while keeping the case flat against the anvil base.

https://www.amazon.com/Hornady-AB1-Lock-N-Load-Anvil-Base/dp/B005CMMIM4

 

[aahunt03]

This thread has not been answered fully yet.
I guess everyone just uses OAL...which is inconsistent.

 

[Olen_4504]

This thread has not been answered fully yet.
I guess everyone just uses OAL...which is inconsistent.

you are right, it went on a little tangent

but maybe if the OP tried what was talked about, he might get more consistent results?

 

[Threadcutter308]

This thread has not been answered fully yet.
I guess everyone just uses OAL...which is inconsistent.

I may be missing something........the term "BTO" means "Base to Ogive", which implies measuring from Cartridge Base to Bullet Ogive and has no direct correlation to "COAL" (Cartridge Over All Length), or "OAL" (Over All Length).

I have been measuring and working with BTO for so many years, I pay very little attention to COAL anymore. I only handload for bolt guns and as long as my handloads fit in and feed from the box magazine, I don't care what the OAL is. You are not wrong about OAL/COAL being inconsistent.

I think many others here are expressing the same/similar thoughts. Or, did I miss something ?

edit; FWIW, I do get BTO variations of as much as .002" on my handloads. After chasing the variations around via constant adjustment of the seating die (impractical) for years, I've concluded the variations are due to slight differences in dimensions of the bullet(s) during the manufacturing process. I'd like to see the variation be under a thou, but it doesn't look like that is realistic. And yes, my seater stems are all "VLD type" and do not touch the bullet tip, ever.

 

[Kraze]

I don't use OAL, I use BTO measured with a Hornady Comparator and the anvil base kit mentioned, and I use the same method that Threadcutter described when attaching the comparator to the caliper jaws.

I just went and measured a couple different loaded rounds multiple times and got the same measurement every time. So in my mind, as long as you have a decent set of calipers, you already have the right equipment to accurately measure BTO, and I think plenty of others are getting accurate measurements with the Hornady comparator as well.

So to recap, use the anvil base kit if you don't have it already, set up the comparator as Threadcutter described, and make sure to spin the case until your reading stabilizes. If this doesn't get you the same number every time, I don't know what to tell you, other than maybe it's your calipers.

Edit: I suppose you could be getting different numbers on your measurements on the same case if you basically have no neck tension, and you're seating the bullet in a bit farther every time you measure. This should be easy to diagnose though. For one, each measurement would be getting shorter. If that's the case, take a loaded round and lightly press the tip of the bullet into your reloading bench and see if you can seat it deeper. If this is what's causing your varying measurements, you have bigger problems than just getting an accurate BTO measurement though.

 

[towerofpower93]

As stated above, the anvil gives a larger piece of real estate to rest the case on while slowly turning it in the comparator to ensure the round is standing upright and not off kilter.

The anvil also has a small set screw which allows you to move it perpendicular to the caliper jaw so it mates up squarely under the comparator body. The Giraud 338LM bullet/headspace comparator uses the same method to get proper alignmment between itself and the anvil, but with larger, finger adjustable screws.

This helped me with getting consistent results and I can now measure the same loaded round with the appropriate caliber comparator and get the same reading multiple times.

 

[MarinePMI]

I use the anvil base and after cleaning up the rod, get consistent results. In my case, the rod had a lot of flashing from the moulding process which was causing the rod to bind up and give erratic readings. Once I took a knife, and trimmed all the rough areas of the rod smooth, it functioned much better.

 

[aahunt03]

I bought the anvil and since creating this thread I have measured about 100 rounds with consistent results. Thanks everyone for your suggestions.

 

[Deleted member 10043]

I didn't pay tens of thousands of dollars for my Hornady measuring shit. Are you sure you bought the right stuff?

 

[whatsupdoc]

I have no issue getting repeatable measurements using the sinclair tool.
I clamp the cartridge between the tool and the caliper jaw with minimal pressure.
Then I slide the base of the cartridge parallel to the caliper jaw until it is flat on the caliper jaw.
Next I slide the cartridge base perpendicular to the caliper jaw until I get the lowest reading.

Seems complicated but in practice it is fast and accurate, not sure if it make sense but thats it.

 

[Lynn Jr]

If your bullets have been properly sorted and you get large variations make sure your primers are seated correctly.