260 Ackley Improved COW Fire Forming inconsistency

[WWP2012]

Good afternoon all, with pretty much the whole state on quarantine I decided to do some fire forming. The cases turned out nice with 20.5g of LIlgun, Paper towel, and COW with the exception of COAL length. I bought a bulk bag of 100 new Nosler Brass from SPS and measured the cases prior to fire forming, they were either 2.016 or 2.017. After fire forming I ended up with a COAL from 1.990 to 2.028 and everywhere in-between. 44 were from 1.990 to 2.000 and the other 52 rest were at 2.010 or higher (4 cases were junked from the bag). I am wondering if theses were mixed lots in the bulk bag that caused the issue or if it is normal to have that happen using the COW method?

I loaded up some 140g ballistic tips to break in this new barrel and those #1 Nosler cases have a starting length of the same 2.016/7. I fired a few to test them and afterwards the case length was 2.013. I sorted thru the brass and ended up trimming the ones I could to 2.010 which is quite a bit shorter than what I would of liked as a 260 has another .025 per SAAMI. Not sure why Nosler only makes cases to 2.017 verses taking them out to 2.025.

Anyway, if you have some experience or insight you would like to help with, please let me know.

 

[JMW67]

Good afternoon all, with pretty much the whole state on quarantine I decided to do some fire forming. The cases turned out nice with 20.5g of LIlgun, Paper towel, and COW with the exception of COAL length. I bought a bulk bag of 100 new Nosler Brass from SPS and measured the cases prior to fire forming, they were either 2.016 or 2.017. After fire forming I ended up with a COAL from 1.990 to 2.028 and everywhere in-between. 44 were from 1.990 to 2.000 and the other 52 rest were at 2.010 or higher (4 cases were junked from the bag). I am wondering if theses were mixed lots in the bulk bag that caused the issue or if it is normal to have that happen using the COW method?

I loaded up some 140g ballistic tips to break in this new barrel and those #1 Nosler cases have a starting length of the same 2.016/7. I fired a few to test them and afterwards the case length was 2.013. I sorted thru the brass and ended up trimming the ones I could to 2.010 which is quite a bit shorter than what I would of liked as a 260 has another .025 per SAAMI. Not sure why Nosler only makes cases to 2.017 verses taking them out to 2.025.

Anyway, if you have some experience or insight you would like to help with, please let me know.

I have been using Peterson 260 srp brass in my 260ai brass measures 2.224 before forming I am not doing the cow method just a fairly stiff load of imr 4350 and a 130 seated into the lands it is working very well and the fire form loads are plenty good for hog hunting out to about 300

 

[WWP2012]

I have been using Peterson 260 srp brass in my 260ai brass measures 2.224 before forming I am not doing the cow method just a fairly stiff load of imr 4350 and a 130 seated into the lands it is working very well and the fire form loads are plenty good for hog hunting out to about 300

What do they measure after the first fire?

 

[codyadams]

Only brass I have used is Peterson and Lapua. after forming my brass didn't vary that much, maybe .005" or so in 100 pieces, but they did all come out short, I had 12ish firings on my Lapua brass, and only trimmed one time. My peterson brass is at 5 fireings, and I haven't trimmed yet, the average is right at the trim to length.

 

[WWP2012]

Only brass I have used is Peterson and Lapua. after forming my brass didn't vary that much, maybe .005" or so in 100 pieces, but they did all come out short, I had 12ish firings on my Lapua brass, and only trimmed one time. My peterson brass is at 5 fireings, and I haven't trimmed yet, the average is right at the trim to length.

What COAL did your Lapua brass end up after FF?

 

[codyadams]

What COAL did your Lapua brass end up after FF?

I dont recall exactly, but I think about .015-.020" under max case length

 

[drmarsh]

Not to hijack but what is max length on the 260 ai supposed to be

 

[codyadams]

2.035, same as a standard .260 rem