Dasher Case Length On Fire Forming

[rcw3]

Just trying to get an idea what 6mm Dasher cases measure in OAL when you are done fire forming them.

The 6BR Lapua brass I have measures right around 1.555" and I know when you fire form and blow the case out to a Dasher, it does pull back the length some. What I am trying to figure out is about what you wind up with in the length of the fire formed Dasher cases.

Robert Whitley

 

[benjamin199]

Robert,
An average of 5 cases I have that have been modified with a false shoulder read about 1.550" before firing, after firing another five average about 1.541" these have been fired once, then had the shoulder bumped back a about .001 using a custom Neil Jones die, but have not been trimmed yet.
Hope this helps you out.
Ben

 

[johara1]

Robert,
I think the info. you have on the BRX is the same. The blow out length is from 1.555 to 1.550,they will grow for two more firings then they almost stop. If you form short they will not be the best for a 1.570 chamber......jim

 

[rcw3]

Ben

Good info - thanks!

Hopefully others might share their info too.

Robert

 

[rcw3]

Robert,
I think the info. you have on the BRX is the same. The blow out length is from 1.555 to 1.550,they will grow for two more firings then they almost stop. If you form short they will not be the best for a 1.570 chamber......jim

Jim

A am not sure what you are saying here. A Dasher case typically also fire forms out shorter than a BRX case, so I am not going to "mix and match" data. Seems like responses thus far suggest a Dasher fired brass typically comes out after fire forming in the range of 1.535" to 1.545". Naturally there are exceptions to every rule but I am looking for hard data people are getting. I do not understand the numbers in your post? Three different batches of 6BR brass I have measure out to start around 1.555", so I do not know what you mean when you say it blows out right around that length - the case length does not blow out at all, it shortens on fire forming in response to the case body blowing out.

Robert

 

[johara1]

Robert,
What i said was the same things apply to the dasher. I will not use cases shorter than 1.550 when fire formed. The ways to make the case stay the same length is simple, as stated when you ask how to do a the BRX. The load will vary the length and remove all lube and the temp. of the chamber will reduce the length also. My load is a 115 DTAC and 30.5 gr. RL-15 with a standard primer and i use the false shoulder. move the load around till you get the length you want. Make sure your chamber is dry and lube free! and the cases also...you should see 1.550-5 right away.............jim It has to grip the chamber walls or you get 1.545 or less

 

[Cheechako]

Jim

I can't help but reply. I have made quite a few wildcat cases over the years, including the Dasher (and my own Prancer), that involved moving shoulders, increasing shoulder angles, and blowing out case bodies. I always expect the case to shorten somewhat, the exact amount depending on the case design.

So, I have to ask you - where does the brass come from that is used to produce the improved shape, using your length-growing method?

If your cases are growing, and continue to grow for 2 or more firings, you are creating thinner webs and/or case walls. A dry chamber to ensure the case "grips" only adds to the problem. What you want is for the case head to be solidly against the bolt face. Some will go so far as to lightly lube the case to make sure it does slide back.

JMHO

Ray

 

[johara1]

Ray,
It will get short if any kind of lube is present. The load will vary the length and it is not hard to retain the same length if you do things right.
I was of the same belief as you till i learned from somebody that made over 2000 dasher cases. Now i can make them grow if i have too. If your chamber is over polished will also affect the length. The case has to hold on to the walls. My brass lot right now is running 1.555-6 blow out length is 1.550-5,after 2 more firings they will 1.560+.......jim

 

[Cheechako]

Jim

I understood what you were saying. But, you didn't answer my question - where is the brass coming from that causes your cases to stay the same length or to grow???

Ray

 

[TheBlueEyedBear]

Good question, Ray!
I measured 10 cases after their second firing and they are right at the 1.541 to 1.542 length. I use just a slight touch of lube prior to the first fire-forming shot.

 

[johara1]

Ray,
I would say stretching of the brass, the same thing you do in making a false shoulder. i really don't see where your coming from you shrink and stretch brass all the time. If you want to shoot with .030 short cases it's fine with me.
Bear, what did you gain by lubing the case? it still stretches the shoulder and it ends up .030 short,the case doesn't grip the walls of the chamber. Your case only expands at the shoulder instead of over the length of the case.
You are loosing .020 of your neck, that is a very high percentage......jim

 

[TheBlueEyedBear]

The question still remains: When you form the DASHER case you are adding 11% additional capacity to the standard BR capacity; where does that brass come from to form the extra case capacity. My belief is that it is a simple physics 'thing'. If you add more brass somewhere it has to come from someplace else. Otherwise it would have to magically reform the case thickness to get the brass. Don't imagine how that would happen.
Just my opinion! And, I am not a 'rocket scientist'. Heck, I didn't even play one on TV.
Sure would like to hear a professional opinion on this matter.
Barry

 

[johara1]

Bear,
Ask yourself an other question,where did the brass go to make .030 shorter......jim

 

[TheBlueEyedBear]

It went 'south' to make the body of the original 6BR case 111% larger.

 

[Cheechako]

Bear,
Ask yourself an other question,where did the brass go to make .030 shorter......jim

The brass that results in a .030" shorter case goes to make the larger diameter shoulder and longer body length.

Think about the opposite. If the case doesn't get any shorter, where did the brass come from to make the larger shoulder and longer body length. It came from the rest of the case, most likely the web if you fire-formed with a dry chamber. That's what causes case head seperations.

A .020" shorter neck doesn't hurt a thing. When I am having a reamer ground for an inmproved cartridge I will figure that into the design and make the chamber to fit the shorter case.

Or, in the case of the Dasher, simply go to my cartridge, the Prancer, which is a Dasher with a longer neck.

But, short necks are not a killer of accuracy. Look at the .30 Wolf Pup.


JMHO, as usual.

ray

 

[johara1]

Cheechako,
If you have dry lube free chamber and use the right load it will not get short. The case will hold on the chamber and retain it's length.
I had short one's to start then i learned how to do it right,but if that doesn't matter, then short is fine.You can also vary the length by the load and the temp. of the chamber. the false shoulder keeps the body back to stop case head
To shorten the chamber less than 1.570 is treating a symptom rather than curing an illness.....jim

 

[Cheechako]

Jim

Have you ever known a shooter who resizes his brass too much, pushes the shoulder back each time, creating excess headspace at each shot? Not a lot of excess, maybe just a few thou each time. What usually happens is that he willl get a head seperation sooner or later. But what happens before the seperation that should warn him? His case grows in length and he has to trim all the time. The case is growing longer by taking brass from the web area and flowing it forward. By fire-forming with a dry case and chamber you are doing the same thing. The case is gripping the chamber walls and when the 50K pressure tries to expand it, the case head is moved to the rear until the bolt face stops it, stretching the case walls and web. A shooter who neck sizes can shoot the same case dozens of times and never have to trim once. Shooters using grand-daddy's 30-30 experience the same thing because the lock up of the lever action is so weak that it creates it's own headspace problems at each shot. 30-30 cases have to be constantly trimmed until the eventually wear out.

I don't thinlk either of us will convince the other so we'll have to agree to disagree.

Ray

 

[johara1]

Cheechako,
Number 1, i use a very tight fit to form with the false shoulder,2 i don't touch the shoulder on the next firing due to spring back. The third firing the die is set .0015 from then on the case stays very close to that length, growth has almost stopped.
To compare a dasher to a 30-30 it a joke..........jim