I started reloading 10mm ammo (https://www.bulkcheapammo.com/handgun-ammo/10mm-auto) & the load I'm using is:
10.2 gr Blue Dot
180gr Hornady XTP JHP
Winchester LPP
But the question I want to know is how come on factory rounds the shoulder of the bullet seems to start a couple hundredths above the case mouth when seated, but when I do it seems to be pretty much level with the case mouth?
1) Is the cartridge you are comparing your reload to a factory Hornady XTP JHP?
2) Does the COAL of your handload equal the specified COAL in the Hornady manual?
If your answer to #1 is no, then that is your problem, different bullets have different profiles. Many JHP have a radiused Ogive while Hornady tends to be a tuncated cone, that'll look different.
If your answere to #2 is no, then set your seating die to give you the COAL specified by Hornady.
Seating a bullet too deep can raise pressures a significant amount, but I'm talking about hundreths not a few thoundsandths of inches ( I usually seat my 10mm rnds a little long, but I have a revolver). The important thing is not the COAL but the setting depth (how far the bullet is sticks into the case) it's just that COAL is easy to measure, seating depth required arithmatic.
I am also curious what reloading manual you are using as a guide to the reloading process?
They want the tapered crimp to be on the actual largest diameter part of the bullet and if seated to deep the crimp would be on the tapered part of the bullet.
However some of the bullets, you need to work within the Cartridge Overall Length of 1.2500" - 1.2600" max. The exposed straight section of bullet on a 40S&W is usually more than we see on the 10mm auto...