I use a little adpter on the calipers, from memory Stony Point make it. It's caliber specific and should help you get more consistant measurments.

I find differences of as much as .3mm(.1") which for some may be negligable, but to get readings varying from say 86.28 to 86.56 is frustrating.

Usually I can perfect this by running the bullets through the seater again, giving lifting it a little harder or with a rap.

So what gives? Why does my seater(s) not place the bullets to the same depth?

I do my best to use a consistant stroke, and a fairly light one at that.

Any suggestions?

Any chance you are using compressed loads? If so, that would expain the variation. This has happened to me before when I try to crush the powder; there is some spring back with the case.
Ron

Ah ha! I had thought about this, and as it happens an hour ago I loaded up some .375H&H with Partitions. One load was with VV N140 and not compressed, the other VV N160, and therefore compressed. The second batch gave me more trouble and are what gave me the results I stated above.

The first batch I managed to get to 86.28mm and VV N160 were not going to go deeper, so I put the to as close to 86.5 as I could get.

A Lee Factory Crimp Die would be a good investment if you are going to persist with your compressed loads.

I`ve found another problem that will affect your OAL, although it`s only happened with one seating die to me. The plastic tips on some bullets are narrower then their lead counterparts and will cause the die to seat the bullet off them instead of the ogive. The seating "cup" can be drilled out a touch to stop this if it`s happening with your dies.