6 Dasher: Chamber, 6 BR Brass, Reamer and Fireformed lengths quandary.

[Steve Wilson]

New blue box Lapua 6 BR brass measures 1.5545 with the occasional 1.555, based on the sampling of my two 100 round boxes. Dave Kiff told me that the No Turn Neck reamer I bought from him (.2704 neck, .104 freebore) would cut a 1.555 chamber, using his Go Gauge to headspace the chamber. That makes sense, the length of the parent brass has to chamber in order to fireform. Right?

In my "All the stuff for my new 6 Dasher is finally here" thread, I was advised, by a fellow member, to short chamber the barrel, or my brass would come out too short. In the order of 1.535 to 1.540 or so, if I remember correctly. But two things jump out at me with that line of reasoning. If you short chamber, how can the parent brass fit if you don't trim it back also? Do you just mash the bolt closed and pinch the heck out of the case mouth/bullet? Or will the end of the loaded round shove the shoulder of the brass back to a shorter length when it comes in contact with the neck step at the end of the chamber?

I have a Redding bushing style, full length sizing die. Going back to the short chamber idea. How does that compare to where the sizing die is going to locate the shoulder? I am only in the fireforming stage now but it seems to me that the die and reamer makers took all of this into consideration when the cartridge was designed. If short chambering is required, the headspace gauge should be sized accordingly, along with the caution that new brass needed to be trimmed back to a certain length.

When we cut my chamber, we put dykem on the shoulder, neck and mouth of a new piece of brass, after headspacing with the Go Gauge. No marks on the shoulder but the dykem was wiped off of the end of the mouth. A tight fit on the length of the brass when it hit the end of the chamber. We could set the barrel back a few thousandths to minimize any excessive gap between the end of the neck and the step for the neck, in the chamber. Which brings me back to what length is the die going to size the shoulder back to?

What am I missing here or is it much adieu about nothing?

 

[lpreddick]

please don't force a loaded round into a chamber too short. the crimping of the neck into the bullet will INCREASE pressure to very large numbers....bad outcome. i've done this and only blew primers. my 22 brx reamer from pt&g cut a chamber SHORTER than the lapua 6br brass previously fired that i turned and necked down gradually to 22 cal. i fireformed them using the cow method and noted slightly tight bolt closure. i was proud that i had set the false shoulder just right. extracted a nicely formed 22 brx case only to notice the mouth was rolled in and would not allow to seat a bullet as it should, being formed in a .251 neck. measured the chamber with the sinclair chamber plug and BAM! my chamber was .0009+ thous shorter than the case! trimmed the next few, fireformed and noted this case chambered easier! bang. extracted a perfect 22brx whose mouth was straight and accepted a bullet with very slight resistence, less so when powder residue was removed. measured this case and &%@#!! the length was the same as the chamber! i believe it is suggested to leave about.015 thous clearance between case mouth and end of chamber...that's where the "carbon ring" forms. i trimmed them again, shot a fully loaded round, measured length and, thank you, it did't grow. factory brass treated the same may have worked without all the fuss, but i have a lot of good 6br lapuas to use and all the trimming, etc is good brain/hand exercise.

 

[johara1]

If your chamber is 1.555, yes the unfired case will touch the end of the chamber. As long as the cases are trimmed to that length i don't think it will hurt to use that to keep the case from going forward instead of the false shoulder. Your fire forming load will be a lower pressure load and the case will get shorter when fired,so it's not a pressure issue......jim