Dasher blow length

H

harv50

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:confused:
Seems like everything I read about the correct blow length for Dasher cases is around the 1.553 length.
I can only get between 1.544 / 1.546...
I started with to low of a load and got 1.532 and worked my way up to where I getting the 1.544 /1.546 area with anywhere
between 32 gr and 33 gr of varget (32 gr made the same blow length as 33 gr)
I'm using the expanded neck (.257) necked down to a CRUNCH fit and heavy neck pressure (.267 loaded round, .263 bushing).
I'm sure the case isn't moving with the crunch fit, bullet in the lands 20-25 thousands.
I've tried Rem 7 1/2, CCI 450's, and 205 Fed's with the only diff being gratered Fed primers @ 32.5+ load);
Any idea why I'm getting what seems to be a "short" blow length.
Maybe it is just the way my chamber is but thought I'd see if anyone else has had this "problem".
It might not even be a problem as I haven't tried to work up a load yet, Just fire forming now but don't want to have a bunch of wasted cases from being to short of a blow.......
 
"Sounds" like you're using enough pressure now to fireform. Most times, it takes more than one firing to achieve full fireformed dimensions. Just make sure that on you're second firing that your bullet is into the lands and let er rip......then take another measurement.
Might also talk to the one that chambered it and get some accurate measurements of your chamber or do a chamber cast.
 
harv50 -

1.544/1.546 from "blue box" brass is respectable blow lengths for 6Dasher's - IME.
Since the old "brown box" days, it has been a challenge for me to get consistent blow lengths and lengths past 1.545". Where on the old "brown box" I could easily adjust my FF loads and get blow lengths to 1.555 to 1.560 with great consistency. Those same loads in the "blue box" brass would only produce lengths in the 1.530 to 1.540" range and with poor consistency.

Where I myself am at with the new "blue box" brass is I have adjusted my techniques to get blow lengths of +1.545" from the most of them, with a straggler here and there that will blow short (like say 1.536") that I cue out.
Also I find a lot a variation in the primer pockets to the new blue verses the old browns, and will cue out a couple that are loose after the blow.

I prefer the "false shoulder" method with the "blue box" exclusively and like to use a moderate charge from slow burning powders, to fill the case well. Soft loads and max loads give me to many inconsistencies to blow lengths and primer pocket dimensions.
I like a clean dry chamber and de-greased brass, with at least 20-thousands crush on the false shoulder to headspace off of.
Depending on which loads I'm using, I may jam the bullet along with the false shoulder, or may jump the bullet. The fuller the case, the less I need to jam the bullet, is my experience.

Down falls to mixing short blown Dasher's with longer blown Dasher's is capacity variance, from which I see an increase to velocity ES, hence vertical dispersion - IME
- Also I find the short blown Dasher brass do not grow much, and will typically remain short.
- I find the stiffer the FF-load, the more primer pockets are at risk.
- I find to light or to stiff a FF-load, typically blows them short.

The biggest downfall to using "short blown" Dasher cases, is an increase of carbon of the chamber-end/throat, that I see as as a result of using short brass.

Good Luck,
Donovan Moran
 
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Good post. Dumb question: Is it easier to blow short consistently, than longer? If it is, could you adjust the neck length of the of the reamer that you use to cut the chamber in which you shoot formed brass so that the space between the end of the blown case and the end of the neck portion of the chamber is what you prefer. I realize that this would entail using different chambers for fire forming than for formed cases, but if that was not a problem would it work?
 
If you can't make them long go for a 1.555 chamber length. You don't have to do anything but fire form because the brass is only 1.555 long so it can't move a head when fired. A fire form barrel made up like this would save a lot of work........ jim
 
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