carbon and seating pressure

[Benjerman08]

I have about 8 firings through some lapua .243ai cases. They shoot good, the runout is great. The only thing that has ever bothered me is that around 10% of the cases have hard bullet seat compared to the butter smooth seating of the rest. Yesterday I decided to start marking these instead of just seperating them into their own pile to be shot seperately. After shooting, I took 5 of the cases marked "hard" (yes they do make p.o.i. shoot slightly high) and 5 that seat easy and ran them by hand through my 6mm empansion die. The 5 regular cases move up and down easy on the expansion shaft. (normal). The 5 marked "hard" have a slight sticking point about 3/4 into neck. It is not much as slight pressure pushed them on down the shaft. I clean necks with a nylon brush on a drill but don't get much out. Always figured a little carbon helped with seating. I chucked up a brass brush in the drill and cleaned the necks on the 5 hard seat cases. Upon re-checking with the expansion die, they slide fine like the others now. Ya, I know problem solved, but what was causing this? I thought about donuts at first, but the brush shouldn't remove them. Does carbon build any type of ring in cases?

 

[fdshuster]

I also think there may be the beginnings of donuts forming. A simple test is to take an as fired case and drop it into the case. If it falls all the way in, no donut. If it hangs up with the base of the bullet at the neck/shoulder junction, donut. Not saying this is the cause, but could be, & a quick, simple test.

 

[mikecr]

The mandrel of your expansion die is not copper like a bullet. So you might not have solved anything.
I think the brass brush removed enough material to reduce seating force of a mandrel, possibly increasing seating force of bullet, and not affecting actual neck tension at all.

What made 'hard necks' increase velocity is an increase in springback force gripping the bullets. This is not the same thing as increased seating force due to friction, but greater springback does leave neck IDs smaller after sizing(increasing seating force), and increases expansion forces to a bullet being seated(again, increasing seating force).
A donut can increase neck springback even while you're not seating into it. Especially if you FL size the necks.

Otherwise, it's time to anneal all necks in your batch.

 

[Martin52]

At a recent 800m match I inadvertently introduced 2 reloaded cases that should have been separated from the match set due to noticeable increase in pressure when seating the projectiles.
The result was the POI was 2.5 MoA high.
This reconfirmed to me the importance (amongst other reloading procedures) of ensuring consistent neck tension to the point that I am considering an arbor press with gauge.
I also anneal after the 4-5th firing from new, then after every firing.
Martin

 

[mikecr]

Nothing wrong with an arbor press type measure, you can set departing seating forces aside.
Keep in mind though that seating forces do not directly correlate to neck tension.

In the past I used an electronic loadcell under Wilson seating dies.
Now, I go a step further to match seating forces. I happen to use Sinclair mandrel neck expansion no matter what, so I added a loadcell to the backside of the mandrel, and built a preamp/indicator for relative use.
This is how I've come to learn that seating friction can alter seating forces, regardless of actual tension.
Mandrel expansion of necks is more consistent than bullet expansion of necks(due to material), and so provides less presumption of seating force -to- tension.
I have a design around here for a tool to measure tension directly, but haven't got around to it (guess I need to put the computer down).

 

[Benjerman08]

Brass was newly annealed. When I get a few more of the hard seat cases shot up I can reload them and see if they shoot good after the aggressive neck cleaning with bronze brush or if I need to just keep sitting them aside or culling.

 

[markm87]

Brass was newly annealed.

But you've fired them 8 times. I can feel seating pressure variance in my press after 2-4 firings.

 

[Benjerman08]

mark, do you mean 2-4 firings without annealing or feel difference after 2-4 total?

 

[Chuckhunter]

I think the whole batch of cases would benefit from a good sonic cleaning and re-annealing. Re-size them after annealing and I think you'll find everything will be beautiful. I anneal my 243AI cases after two firings.