I was using my LNL AP for some 10mm
Here is the breakdown
10mm Hornady XTP 180gr
10.2gr blue dot (pretty conservative, I already did my build up from 9.6)
CCI 300
Aiming for 1.255 OAL
Station 1 - RCBS size and decap
(prime)
Station 2 - powder through/bell
Station 3 - empty
Station 4 - Redding competition seater die
Station 5 - RCBS seat/TC die with seater plug removed. So its just taking out bell
Charges are perfect, but the Hornady PTX is rough and inconsistently belling, so I remove it, set it up new, and bell with an RCBS belling die
While running cartridges, I am getting lengths all over the place. 1.255 as a nominal, and the ups and downs going from 1.265-1.250
After running a bunch, picking out the good ones, I figured I would tune them with the redding competition seating die in my single stage to fix them. Simple right? NO
I unscrew the redding die from the LNL bushing and set it up fresh on the LNL classic. After dialing in to 1.255 on a round....my next rounds are going nuts.
I am getting no movement before and after running them on some, and on some getting movement all the way to 1.240 and up to 1.260.
Another note, at the end of the AP run, I put cases in the sizing die to keep the plate even. So that isn't a factor in this case
I have heard of others having COAL issues, Is your seater plug removeable? Don't know if this is your problem? ???
If yes, take it out and observe the bullet's fit. Does the bullet contact the tip of the nose? or does it contact the olgive section?
The reason is if bullets are pushed via the sides/olgives they may show differences in COAL if the nose is distorted any. I have seen a bright ring on the slope of the olgive section where they make contact. If the bullet meets extra resistance the nose plug may be pinching the HP closed, causing it to grow in length slightly.
Finding a seater plug that fits better and still pushes your bullets straight will help! I use my RCBS seater with the die lifted (stacked spacer rings) so the taper crimp doesn't close the casing any while seating to depth. After all are seated I remove the spacer rings, adjust the seater stem to just touch the already seated bullets as I taper crimp to finish the process.
All this is done on a single stage press...yes it takes time but results are unifority!