Help with Dasher over pressure

[dkhunt14]

Noted with follow question: I prefaced my comment quoted above by asking about excessive headspace. If the headspace was long and the shoulder set back as mentioned, would the firing pin bump the cartridge forward to shoulder contact and ignition then drive it back to the boltface and lead to ejector marks?

Thank you

Yes that is possible. When You bump too much the case gets driven forward, now it fires and pressure slams it back to bolt face. That is how the blanked primers come about. The firing pin bounces and touches the primer again, poking a hole in it. It could possibly also cause ejector marks.

The biggest thing is it's hard on brass and eventually they will stretch and separate. A Dasher shouldn't be bumped more then .002 at most. It should not drive up pressure that much though. A good gunsmith holds .001 on headspace. Every 300 WSM and 6 Dasher I had were all the same headspace. Matt

 

[Radar]

HI there-
Im having trouble with two Dashers. Both are gong over pressure with moderate loads.
Today I did a test on one and I was still getting ejectors marks at 30 grains of H4895!

The kicker is that I have a third Dasher made with the same reamer that is not showing any signs
of pressure.

Both the over pressure guns are vintage 40x repeaters.
The good one is a Kelbly Atlas repeater.
All three barrels are krieger 8 twist with the same reamer. I don't know the freebore.

HEEEEEEEEELP!
Wade

Are you using Isso or JB to clean it?

 

[Leeman]

Yes that is possible. When You bump too much the case gets driven forward, now it fires and pressure slams it back to bolt face. That is how the blanked primers come about. The firing pin bounces and touches the primer again, poking a hole in it. It could possibly also cause ejector marks.

Matt,

In my experience, blanked primers come from excessive clearance around the firing pin hole. Ejector marks come from excessive pressure.

 

[dkhunt14]

If you bump them too much it can happen also. The case slams against the bolt face and the firing pin bounces. When it touches the primer it pokes a hole. Seen guys bump them at .005 plus in a BAT and they blanked BR4 Primers. They are the toughest primer. I told them to bump them .002 at most and the blanked primers went away. This was more then one person that did it.

Ejector marks usually comes from excessive pressure, but other things can cause it. A stiff ejector, brass debris in ejector, or even a rough ejector are a few of the things that can also cause it. Matt

 

[FrankG]

Here are a few things that can drive up pressure quickly. I have had 2 / .236 bores in Dashers and 2 / .237 bores. The .236 bores could not take anywhere near the powder charges that the .237s could. However, they could take more than you're pushing. If you have a "carbon ring" located in the chamber at the end of the neck where the rifling starts, is a major contributor to pressure. Additionally, if your freebore is short, depending on the bullet, can and will contribute to rising pressure. Finally, if you have an excessive length of the case necks AND IF they are jamming up against the end of the neck of the chamber, you will get pressure. There is at least a reasonable chance that one or more of these "problems" are in existence in your rifle. If not, I am at a loss..

Not just the tight .236" bore either.

What is the groove size? This will have a bigger impact on the pressure curve then the bore will but anytime you start changing the total surface area of the bore it will have an effect on pressures. If the grooves are at size or under and in combination of possibly running a fat bullet you will drive up your pressures.

Do you know the groove dimensions? If your still having problems with the two barrels and not the 3rd barrel and it's the same loads and same reamer used...I'd call the barrel maker and send the barrels in and ask them to look over the barrels and regauge them to check bore and groove size.

I just recently inspected some ammunition test barrels in 6.5mm and 5.56mm that was on a M4 (both barrels) not made by us. The ammo maker was having problems with pressures and velocities. The 6.5mm barrels for example the pressure was 8k psi higher with the reference ammunition. Both the bore and groove where .0005" under min. spec. and that was the issue. On the M4 the bore was .2177" and the groove was .2237" and the M4 was blowing 50% of the primers on box ammo.

Later, Frank
Bartlein Barrels

 

[Mudder]

Are you using Isso or JB to clean it?

I use Boretech carbon remover. I have soaked both guns several times to make sure they are clean.

I think it's a bore size issue

 

[JFM33]

I had a similar issue with a 6br it turned out to be excessive gap between the bolt face and the barrel.Do you know what yours is set at or measures?

 

[dkhunt14]

I use Boretech carbon remover. I have soaked both guns several times to make sure they are clean.

I think it's a bore size issue

It still needs mechanical cleaning with Iosso or JB every so often. Carbon builds up and gets hard. Not just the neck area but also in the throat. I do mine every 40 rounds to keep it out. Matt

 

[Mudder]

It still needs mechanical cleaning with Iosso or JB every so often. Carbon builds up and gets hard. Not just the neck area but also in the throat. I do mine every 40 rounds to keep it out. Matt

Im sending both to the gunsmith to be checked for carbon ring, bore size, and add some more freebore.