Inconsistent neck sizes on fired cases

[otters500]

Hi, just hoping someone can put some light on this one for me. I have a Sako 270, model L61R Finebear. As part of my quest to get this fire arm to shoot half MOA I bought new Norma Brass, neck turned them down to create an even sized neck, trimmed to length (neck and took a small amount of brass of the base to make them square) using a wilson trimmer. Neck sized with a Wilson neck die (.002 of tension) Hand loaded them with Berger projectiles using Wilsion dies about 2.5 Grains under what I normally use (did not want to fire form using too much pressure). Using a balance beam scales to measure powder and double checked each load, all 100% perfect.

What I have found is that about 30% of these cases after being fired will note allow a 270 projectile to fit back into the neck. I also noted that these cases have a lot of black carbon around the base of the brass. They fired cases that do fit a projectile measure .310 and the cases that do not measure .303, so a fair bit of difference.

I have attached a picture of the black carbon on the case if this helps.



Can anybody give me any ideas what may be happening here.

Regards Andrew

 

[Smithcollector]

Sounds like there's not enough powder to generate enough pressure to consistently expand the case necks, and you're not getting a good seal between the case neck and the chamber, thus (1) causing inconsistent expansion of neck and (2) allowing gas to flow back into the chamber, which is why your case is sooty.

While I don't know what your actual powder charge is, I think you could safely increase it and your problem will go away.

 

[damoncali]

Use full power loads from the start. There is no benefit to starting off light, and what you're describing is consistent with too light a load.