Shoulder length and bump questions.

[Colin78]

I'm scratching my head and hoping some of you can help me understand what is going on what how to mitigate it.

I'm using digital calipers and Whitten shoulder bump gauge and am getting different shoulder lengths depending on how hot my load it. When doing a ladder test the shoulder length increases by around .003". That is the first thing I don't understand. The ladder test is not from low "under" pressure to over pressure, but more of a top end ladder. From say mid pressure up to very slight ejector marks. So I would have thought all those would have the case expanded fully against the chamber.

The next issue is when resizing those cases. If I take the longest case and set it to bump the shoulder back .003" when sizing the short cases it is still bumping them back as well. I'll give you an example.....

Fired case shoulder length range from 1.617 to 1.620

So I set my die to bump the shoulder back to 1.617 using that longest fired case of 1.620 resize several of the long cases and they all end up at 1.617. Perfect just what I wanted. But without touching the die when I get down the the shorter fired cases that are already 1.617 and run them thru the resizing die they go down to 1.615. How is that possible? If I have the die set to have a shoulder length of 1.617 I would think the fired cases that are already 1.617 would stay 1.617 not shorten down to 1.615. Really scratching my head.

I am seeing this on more than one caliber. This is Lapua brass for one caliber and ADG with another. Using Hornady one shot case lube and allowing approximately 5 minutes of drying time from spraying the cases to resize it them. This is RCBS dies on one caliber and Whidden on the other. I have tried removing the bushing and expander rod so all I am doing is sizing the body and bumping the shoulder to see if that made a difference but nope. Same result.

 

[dflyfish]

Different brass will re-shape differently as far as the adg vs the lapua. This is just do to the meteorology between the two.

The case length is much more controlled by the contact point of the die vs the chamber.

Seeing different lengths would leave me to believe something is off in the chamber or the die.

Do you have a shoulder gage?

 

[jimbires]

this all makes sense to me , I've been here . new brass is not fully expanded on the first fire . it may take 3 or 4 cycles to get it fully expanded . you'll want to take a few pieces of brass , fire them , and neck size only . fire again and neck size only . do this until you get 4 or 5 pieces of brass that is getting tight to chamber . now this fully expanded brass can be used to set up your die .

next , I don't like spray lubes . I've used them with poor results . I'm back to using a lube pad with RCBS case lube .

if you're setting up the sizing die with a gap between the shellholder and die , you're going to have inconsistent shoulder bump . I use the redding competition shellholder sets . by using these shellholders the die will make solid contact with die . the problem with a gap here is ; harder brass will not size as easy causing the clearances in the press to add up to less shoulder bump . friction by using a poor lube will only add to this problem .
softer brass and good lube , will allow the brass to size easier . by not stressing the press clearances this brass will size more .

poor lube won't allow the brass to slide into the die easy , so it won't go in as far as a better lube would allow .

 

[lancetkenyon]

One other possibility.
With the hotter loads, you are also expanding the body a bit more. Meaning slightly more resistance when FL sizing. Maybe they are sticking to the wall of the die just a touch more, causing slightly longer headspace. Lube the body up and try running them through a second time.

I also use RCBS case lube on a pad, no spray lube.