Seating depth variations in compressed loads?

[dboyles]

I was loading up some .223 Rem rounds to test some varying charge weights of Varget under a 69gr SMK. I wanted these rounds jammed .010", which my Hornady tool tells me is a base-to-ogive length of 1.903" (measured with Hornady comparator and digital caliper). With a dummy round loaded to this length, I get roughly "square" marks on the bullet.

Using a Forster Co-Ax and Redding Competition seating die with a VLD stem, I seated 3 rounds a bit long and worked my way down until I had my die dialed in to give me 1.903" plus or minus a couple of thousandths, at least based on those three rounds.

I loaded the other 18 rounds and then checked several with my caliper and comparator. The trend was clear: cartridges with more powder had the bullets seated farther out. I verified my die settings (they hadn't changed), ran all 21 rounds back through, and measured each individual round:

I took one of the longer rounds, measured it at 1.9135", and ran it through the seating die three more times, rotating it slightly each time it came down. I remeasured and again got 1.9135". (Edit: I checked COL in addition to base-to-ogive and it shows the same trend, ranging from 2.285" - 2.303" as I move up in charge weight)

Is this a known phenomenon among folks with more reloading experience than me? I can't come up with a better way to explain such a trend. If so, do I need to dial my micrometer die down as I go up in charge weight?

 

[jonbearman]

More neck tension would probably solve it.As you seat a bullet on a 100% load approx ,will seat just fine with the press at the bottom of the stroke.Then when retracting the seater the bullet will follow it till it doesnt have enough pressure of the powder pushing against it to push it up slightly.

 

[Erik Cortina]

Try a drop tube.

Compressed loads will push a bullet out if the case doesn't have enough neck tension to hold it in place.

 

[dboyles]

Neck tension is logical, and I'm surprised I didn't think of it. I cleaned these dry, in a vibratory cleaner, and hadn't done anything additional to clean the inside of the necks. My nominal neck tension is .003" which should be enough (I'd think), but I figured the carbon residue could be acting as a lubricant and letting the bullet slide back up as Jon describes. So I tested one case with a clean neck interior and one without, both with a 26.6gr charge that practically fills the case. Both of them came back with the same basic result: about .010" longer than intended, as with the rounds loaded earlier.

I used the .248 bushing on those, so I pulled out a .244 bushing and tested again. Same thing. I just don't think there's room for the bullet in there. These Lapua cases will hold 30.6gr of tap water, which is actually a tenth more than I get from my Lake City 11 brass, and is in line with the numbers on the 223 Rem page. The Sierra load manual calls for a max of 26.4gr seated to 2.260", which is at least .020" deeper than I'm seating.

So does this still sound like a neck tension issue, or a case capacity issue? Or is there something else that I'm missing? I'm stumped.

edited to add: It occurred to me that I needed to try seating the same bullet/case with and without powder. The case without powder seated properly to the ~1.903" depth whereas with powder it's pushed out to ~1.914". I also tried loading a Lake City case and got the same ~1.914" result. These rounds were to be on the high end of my scale for pressure testing, so it may be that they're just too full of powder. Still, rounds that are well within published values are having this issue even when COL is pushed out .020" from the published load. Seems odd.