Brass is shrinking...

[BZR]

I have been prepping my standard 260 brass to fire-form to a 40 deg shoulder. I have been following steps suggested in articles on this site but this is my first go of it...

http://www.6mmbr.com/260AIforming.html

I have expanded my 260 brass to 7mm using the mandrel from a 7mm die then sizing the neck back down to 6.5 mm (.288 bushing) leaving a false shoulder to make that brass fit snugly into the chamber.

However after this process my brass is .010 shorter. Will the neck push back out after fire forming or am I destroying my brass?

What is your experience?

 

[canderson]

I have never done a 260 improved but, your method is very similar to the way I did my 6 dasher. I would go ahead a fireform one piece and remeasure. I don't think that 0.010 would hurt anything as long as there is plenty of neck to grab the bullet. It might keep you from having to trim as much.

 

[Cheechako]

BZR

Get ready, it's going to get even shorter. You may find that a fully fire-formed Ackley will be as much as .025" (or more) shorter than the parent brass. Nothing to worry about. It's normal. The brass to do the changes has to come from somewhere and that somewhere is the length. If they don't get shorter it means that you are stretching the brass and stretching usually means the area in front of the solid head is getting thinner and that's not a good thing. That's why case heads seperate. Trim all the fire-formed cases to the same length and don't worry.

Ray

 

[BZR]

Thanks for the info I was a bit worried.

If the brass is shorter than the chamber does this cause more ware on the end of the chamber?

 

[fdshuster]

BZR: Cheechako is correct: anytime you "work" the brass, like changing neck dia., you will lose some case length. Going from Lapua 220 Russian to 6ppc, usually shortens the cases by about .008" to .010", when I reform. Same going from 6BR to 22BR. And, yes it will create a longer gap in front of the case mouth, but does that cause more wear in that area? I've never seen any indication of that with my Hawkeye borescope, and I keep a very close check on that area with all my rifles. That gap will fill-up with a ring of carbon though, kind of like firing 22 shorts in a long rifle chamber. You may want to consider measuring the actual chamber length of your rifle ( they all vary, does not seem to be a standard, it's whatever is on the chambering reamer. Gauges are available from Sinclair at approx. $6 per caliber, and are re-usable. record that length, and if you're getting some case stretching, you may be able to let them "grow" a bit. When I see the carbon ring build-up, I am easily able to remove it with JB on a bronze brush.

 

[BZR]

How short of brass would be considered too short?

 

[fdshuster]

BZR: Guess that's a personal preference/decision. When I spec out and buy a new chambering reamer (Pacific) for a new barrel re-placement, one of the dimensions I request is that the chamber length be .010" longer than max case length. Example: for a Lapua 6BR at 1.560" case length, the chamber length was 1.570". For a recent tight, no-turn necked 223, chamber length 1.770" for the max case length of 1.760". So, .010" is my "standard": what I'm comfortable with. Don't get too hung up on the case length issue, rather, get the chamber oal gauge from Sinclair, ( Catalog 2009-B, page 32, part# G-243, $6.95), measure the length of your chamber, subtract whatever "clearance" you are comfortable with ( I like .010"), and trim them back when they approach your "limit". The main concern is to not let the cases(s) stretch longer than chamber length: nasty things will happen. Actually, with your 6BR, if you are properly sizing the case/not setting back the shoulder, the brass will stretch very little, if at all, so you will always have at least a small "gap" in front of the case mouth. Then it's a matter of keeping it relatively free of the carbon ring build-up. Sorry for the rambling post. When I get started, it's sometimes difficult to stop.