Inconsistent BTO Measurements

[JFrank]

With so many variables introduced each and every time you pull the trigger , can you tell the difference between bullets seated .002 longer than the last round ?

At 1000 yards the answer is ‘yes’

 

[dellet]

I think the problem is run out. You need to incorporate a concentricity gizmo into the process. If the bullet is not straight you’ll only get the same measurement if you index cartridge so you can measure it in the same place.

But first I’d ditch the anvil.

 

[Rwfish]

You need to know what your tools can give you for accuracy. Remove the cartridge and run your anvil up against the comparator and zero the caliper. Now repeat this action ~10 times using your “normal” finger pressure and log your difference from zero for each check. When complete, these numbers will be the best accuracy you can expect from the tools and method you are using. If your baseline is .0015” at best you can’t expect to achieve better results when measuring your cartridge. Another thing to look at is your neck/bullet concentricity. If this is poor you will find it difficult to get good alignment in the comparator gauge as you rotate to check in the caliper as you are doing.

 

[quest450]

At 1000 yards the answer is ‘yes’

Absolutely YES !
A recent seating depth test, paper doesn’t lie

 

[MikeRu]

more than a couple thoughts FWIW:

get rid of the round base or anvil, mine is also sensitive to how tight the screw is set so it creates errors just as described above.

2 thou seating depth is about the size the optimal window quite often; you should be able to estimate base to ogive or BTO to the nearest 1/2 thou or 0.0005 with pretty good reproducibility; easy to sort and then retest one group to see how reproducible it is. Make sure the little backside screws are correctly adjusted on your mitutoyo, that seems to matter -- touching but not overtight.

if problems persist then one option is to mount your calipers on the Hoover Accuracy One meplat trimmer body and stand, it has an ogive bulit in and creates a very nice hard stop that seems very reproducible to me for getting BTO; no more using your thumb on the roller you just slide back and forth.

CTK precision (Curt Knit) has a nice substitute for the Hornaday BTO adaptor, it measures a little further out on the bullet nose compared to the Hornady ogive adaptor; Curt's idea is that you get a more reproducible read out on the nose where there's more slope. So it's a good surrogate for BTO and still reports your seating depth just fine (although your absolute number will be different).

I use the Accuracy One Comparator ref'd above to double check datum to ogive AFTER seating bullets to the same base to ogive/BTO. This checks for cases with a different shoulder bump that you might use for a folwer etc.

 

[ELR LVR]

Absolutely YES !
A recent seating depth test, paper doesn’t lie


View attachment 1701501
View attachment 1701502

I like the repeating pattern shown here, Nice Job!!!
Exit timing is critical, consistent exit timing even moreso
(For long range accuracy anyway)

 

[Linko]

Of all the methods/tools that I have seen. The accuracy one tool measuring shoulder to ogive impresses me the most. I like the concept of shoulder to ogive better than BTO.

that's just my take away

 

[DShortt]

are you using the correct seating stem? you may be seating off the point of the bullet

I've made a few seating stems so that.the contact point is closer to the ogive.
If you remove your seating stem, take a bullet and insert it into the stem and it wobbles, that's never going to be consistent.
To be fair, I know why they are made that way. In order to accommodate all bullet nose profiles.

 

[D-4297]

From the FWIW Dept. As a retired Aerospace Master Die-maker , I have never worked in a Toolroom that allowed "ANY" Electronic Calipers in the room . There is a reason for it !