Seating depth measurement

[datguy]

When I measure seating depth to ogive with a Mitutoyo digital caliper and Hornady style comparator inserts my lengths vary about .004".

When measured with a Redding Instant Indicator in my T-7 they vary less than .001".

I've checked my method with the caliper and it's consistent. I sorted bullets with a Sinclair sorting stand and a good dial indicator. All were within .0005" of one another base to ogive.

Wanting to sort out why the two methods are inconsistent. Any ideas?

 

[datguy]

The ammo in question is 308 Win. The instant indicator has a .300 bushing in it.

Maybe the comparator insert is wonky? Not sure how to evaluate that.. It is the Sinclair piece..

 

[ZiaHunter]

Most likely an issue with Hornady guage. Check to see if the rounds measuring long have a protruding primer.

 

[mdmp5]

Ok, I had a major issue with this myself in the past with 175 smks, but was unable to figure it out until recently. I was doing the same thing as you, except I was using the Sinclair comparator and the instant indicator. What I did not realize back then was that while the redding bushing is .300, the opening of the Sinclair comparator for .308 is .308", not .300 like the redding. They make the openings of their comparators with a taper, so that the bullet inserts into it further before it makes contact. This is what is most likely causing the disparities in readings between the 2 tools. I trust my instant indicator, and only use the Sinclair to take a reference measurement so I can label a dummy round.

 

[datguy]

mijp5, you nailed it! That is exactly what is going on. I have the Sinclair comparator.

I'm thinking about ordering a .308" bushing to put in the instant indicator. Seems like it would be useful in this application.. I do understand that what matters is that they are seated to the same depth. My thought is it makes transferring a measurement between the instant indicator and the bullet comparator work better.

 

[datguy]

Using a Starrett small hole gage and micrometer the large part of the hole in the Sinclair comparator measures .304".