Case neck condition testing

[jrock]

Its been on my to do list to do some accuracy testing based on the type and quantity of lube left on the inside of the case neck after sizing. I see a wide range of ideas and methods so I figure I need to test it for myself.

I'd like to test several options such as, clean necks after ultrasonic cleaning, wet SST media, tumbled with a few different dry media, and wet and dry lube right after sizing. I also want to test freshly loaded rounds vs. rounds left to sit on a shelf for a few weeks.

Since muzzle velocity will probably be the biggest tell in this test, what sample size do folks on the forum use for gathering this type of data? 5 shots, 7 shots, 10 shots +?

 

[Mikecr]

I doubt dry lubes and other frictional levels will make a difference to MV.
Wet lubes could cause a hydraulic suspension, non-gripped condition, and should never be done IMO.
Setting up for a few weeks could matter (regardless of dry lube).

The thing is bullets are not normally pushed out of necks, but released from necks with neck expansion(which is always 1st expansion).
Normally tension grip is lower than purely frictional grip. So the neck expands any amount, and the bullet is swinging in the wind from there.
Given this, it doesn't matter what your seating friction is, as that doesn't change tension (a bullet gripping force).

But with a wet lube your friction could actually drop to near zero, and then no amount of tension, high or low, would matter anymore. There would be nearly nothing holding the bullet from either leaving, or pushing further into, necks. So a bullet could then move before neck expansion. That could play hell with a load..

 

[MagnumManiac]

My take on this, and I have tested extensively, is that a dry lube is best.
Have tested finely powdered graphite, mica and HBN.
Regardless that I use HBN coated bullets, I always found tighter groups, with set sizes of 10 shots minimum over several days in differing weather for a 50-100 round total count, that powdered graphite applied to the inside of the necks after/during sizing produced the best results.
In a given sample, the amount of graphite applied is very, very small. Hardly visible to the naked eye…removing it opened groups substantially and I rarely use neck tension/interference greater than .0015" as measured AFTER spring back.
HBN alone did not tighten groups, but made tuning easier and with the addition of graphite in my process, the tuning was such that I could open and tighten groups as I changed the HBN tumbling/coating time.
As Mikecr said, bullets are not released by friction, but they are held by friction for a very short time and this influences peak barrel time, as does seating depth.

Cheers.

 

[jrock]

My interest in testing came from a rifle I got a number of years ago with a tight neck chamber. I turned the brass and used just a bushing die to size. I'd used RCBS lube on the body of the case and dipped the necks in Imperial dry lube prior to sizing. Since the bushing die only touches the outside, and I don't use expanders (yet), it left the lube on the inside of the necks for bullet seating. For some reason, with that gun, if the ammo sat on the shelf for two weeks, the MV would noticeably go up. Figured it had something to do with the neck lube change, neck tension, or possibly cold welding if the dry lube was removed during seating. I hadn't noticed any issues on other guns using expander balls and RCBS lube. That gun has sat idle for awhile due to the issue and I'd like to get it sorted out.

Adding an expander mandrel to the bushing die process sounds like a good place to start and possibly playing with neck tension. In the past few years looking into this issue, I've read that one should never use clean brass when seating a bullet as it doesn't provide consistent seating pressures. Some sort of lube seems to help the consistency. I've wondered if wet lube helps prevent cold welding since I was seeing a spike in MV with dry lube over time.

 

[Mikecr]

It wouldn't be cold welding with dry lube, unless the ammo was kept in a very moist environment.
RCBS lube inside necks with seating could cause MV to either go up or way down, or go all over the place from shot to shot.
A seating force variance leads to inconsistent seating depths. No change in MV though, as seating force is not tension.