UPDATED: Meet... the 6mm Belted Dasher.

[spclark]

Re: Meet... the 6mm Belted Dasher.

That 1:7 may be having a negative effect on safe pressures in your 6mm, particularly if you're seating them on or into the lands.

A true 1:8 is generally regarded sufficient to stabilize anything over 100 grains (save for Berger's 115's in many barrels) where 1:7.5 if you want to have a margin of safety out to 1000 yards in colder temps. 1:7.75 is often a happy medium. Anything faster & not only do you risk bullets coming apart on their way down range but chamber pressures peak rapidly while those bullets are overcoming inertia getting up to the fast rotations imposed by the rifling.

Here's one example (by no means the only & please note the mention of moly-coated bullets) of common practice with the 6mm Dasher:

/www.6mmhot.com/6mm_Dasher.html

 

[7stubby]

Re: Meet... the 6mm Belted Dasher.

The reason for the 7 twist is Krieger had it in stock at the time, and I had money so I bought it. Good or bad here I am. Increase in pressure with the 7 twist I figured it would be, but not this much.

 

[Kenny474]

Re: Meet... the 6mm Belted Dasher.

I still feel that you either have an issue with the chamber or bolt to barrel clearance. It takes 6-8 seconds for brass to anneal at 700-750 degrees, so unless you are annealing with an acetylene torch you are far from over-annealing.

Check the bolt to barrel clearance and possibly the chamber opening for an excessive chamfer. Even being a little over pressure, you should see normal pressure signs LONG before you get a belted case if the chamber is sound. In a proper chamber the portion of the case that is expanding on your rounds is fully supported.

Do a little research on "Glocked brass". As Glocks do not have fully supported chambers they often produce brass nearly identical to what you are getting.

Do the right thing and check for possible issues. Otherwise, at least let others at the range know you are firing a potentially dangerous rifle so they can get the hell out of the way. If one of those cases ruptures you can injure yourself and anyone around you, not my idea of a fun day at the range.

After having a rifle come apart next to me last year, I have zero tolerance for unsafe practice. It's not worth my life, or another innocent person's, to look away while someone is careless.

I am not trying to offend you, and I am sorry if I have. But safety is number one. If it means you need to rework your rifle for it to be safe, than so be it. It's better than getting your face reworked.

 

[jb1000br]

Re: Meet... the 6mm Belted Dasher.

Please let us know what the clearance is between bot head and case...the gap has to be a big one...saw this before, only worse, on my buddy's rifle.

JB

 

[Bulseyetom]

Re: Meet... the 6mm Belted Dasher.

My first BR rifle in 6PPC did that and everyone at the San Gabriel Range pitched in and offered their explanation. Finally, Gene DeLoney took a look at my belted cases and said "son, you have too much gap between the bolt and the barrel!" He told me how to check it using pieces of scotch tape and told me how many peices of tape was excessive. When I got home, the gap was 3 times more than allowable and I never shot that barrel again as it was not worth setting back. Seems like the coned bolt did not match the coned chamber but I don't really remember, but instead of the .003 to .005 gap Gene recommended I had .018 or more when I quit adding tape to the end of the bolt and gave up as a lost cause. This was a poor man's non-machinist way of checking the clearance but it showed me I had a dangerous weapon at the node I wanted to shoot at. Tom

 

[JMC]

Re: Meet... the 6mm Belted Dasher.

7Stubby,
I looked at your picture which is a little dark and hard to tell exactly what you are looking at but I have to agree with Lynn. One thing that stands out though and does seem a little odd is it looks like you have a blackened case about 2/3 of the way down, it might be just the picture but that is too much of the case blackened to be high pressure unless your case is split..

I recently changed lots of Varget and blew a primer out and the small primer pocket looks like a large primer pocket now. I just backed the load off and problem solved, I shoot a 6 Grendel. My case looks the same as yours except I can not feel any type of ridge where the case head had expanded but visibly it looks like one is there.

So I decided to mic the case head and it mic'd out at 0.4529" and a brand mew case mic'd out at 0.4387" a difference of 0.0142" definitely an extremely hot load of which I backed way down on. I also looked at my other cases and they look similar and it is not from running hot loads but where the resizing die stops on the case. Maybe your sizing die is off in size my sizing die was made with a sizing die reamer I had ground when I had my chamber reamer made. So my die is not the problem

Mic the case head of the one in question and a new case and let us know what the difference is. If they mic the same then I doubt it is strictly a pressure problem. Let us know what the case head mic's out at. This information would be very helpful in know if you have excess pressure or not

John

 

[rcw3]

Re: Meet... the 6mm Belted Dasher.

7 twist on the barrel and naked 107 SMKS.

I can almost guarantee the pressure issues are related to the 1:7" twist. I have a couple 1:7" twist 6mm barrels (one Hart and one Krieger) that I consider junk, only because they are a 1:7" twist and deliver nothing but pressure, pressure, pressure, and no decent velocity without a lot of pressure.

I also had two other shooters I know purchase 1:7" twist 6mm barrels and have nothing but pressure issues with them too.

1:7" twist is at or right near the point of no return with a 6mm bore. A 1:7.5" twist is fine, but go any tighter and you're "at the red line".

There's a point where the twist becomes more like a wall to a bullet and less like a ramp and it's right there for a 1:7" twist 6mm barrel.

Been there, done that - never going there again!

Robert Whitley

 

[7stubby]

Re: Meet... the 6mm Belted Dasher.

I am using a Redding Type S Neck bushing die in 6 BR, that is adjusted to size only half the neck. It doesn't touch any other part of the case. I took a paper clip and ran it down inside the case and can't feel anything that would indicate case head separation. Using the Hornady Comparator, loaded rounds come out to 2.865 with the 107 SMKs. To touch the lands is 2.885. I will try and measure the case head and get that posted. Thanks

 

[BoydAllen]

Re: Meet... the 6mm Belted Dasher.

The depth of a Remington bolt face specs at .150, clearance should be somewhere around .005, or a very little more. if you open your dial caliper to say .155 and use the bottom end like a depth mic. you will see that .155 comes just a very little bit above the extractor groove on a BR case, if that. Looking at your pictures, I would say that the belt that you have comes more like .2 above the head. There is no doubt that the back of your chamber is too far from your bolt face... no doubt at all. No amount of monkeying with anything will make this safe for high pressure loads, or make the problem go away. Fixing this requires a knowledgeable gunsmith and a lathe. Like,trust or a list of previous good work will not either.

 

[mikecockcroft]

Re: Meet... the 6mm Belted Dasher.

Thats what my first couple of cases looked like fired from a just built 6br. The fourth case about took my head off.
Shooting a right bolt gun left handed. After I got control of the blood pouring out of my face, i was able to call a friend to come get me using a cell phone and dialing by feel because i couldn't see. Took eye doctor a while to pull all the varget out of my eyes.
The rifle was shipped back to builder and the bolt face to back of chamber was way off. Shouldn't have left his shop that way but it did.

 

[Kenny474]

Re: Meet... the 6mm Belted Dasher.

Mike, that is definitely a horrible story and I hope it in no way left you scarred for life or had any negative effect on your vision. That had to be a scary situation!

Having the ability to detect an issue before a catastrophic failure of this kind can often save your life or at least save you from serious injury. It always pays to err on the side of caution.

Mike, I am glad you are still here writing with us and the outcome wasn't any worse. Thanks for sharing your incident, maybe others will learn from it.

Just goes to show it can be the fourth round or round 500, if there is an issue it can change your life, or even end it, in a second if left unattended.

 

[7stubby]

Re: Meet... the 6mm Belted Dasher.

Well, here is a update for you guys. I took the barrel off and sent it back to Krieger. They took a look at it and determined that it was tighter then .236. Turns out it was .2356 at the breech and .2357 at the muzzle. This is what was causing the pressure problems. They are in the process of replacing the barrel with another one. It wasnt the work done by the smith, it was the barrel that was causing problems.