High pressure - LOW velocity

R

rossneder

Guest
This is a mystery for me.

Fresh gun, just chambered. While doing a ladder test my velocites were really low so I blamed the chrono.

Unfortunately it developed pressure signs way before it should have for this ladder.

I readjusted the seating depth and tried again, still way to much pressure with a 300f/s loss from where the round should be.

Headspace is correct, new rounds fit perfectly, when I say high pressure I mean blown and or flattened primers.

In my book high pressure means high velocity so I'm stumped. This reamer was probably cut for a certain bullet and I don't know which but I'm using 140 AMAX in a 6.5x47 Lapua and getting 2600f/s with blown primers.

Quick Load cannot duplicate the results I'm getting even with a Start Pressure of 40,000lbs.

So, is there a chamber problem? The bore is a 6.5mm not a 6mm, dosen't look bent etc.

Thanks,
Ross
 
Pressure

Check your neck length and case length to see if the case is either being pushed into or extends into the throat. Also, all brass is not created equal; how heavy is your brass (diminished case capacity). Increasing seating depth or jamming the bullet into the rifling can rapidly increase pressure also.

If your pressure is going up and not your velocity your charge may be too high, but not always. Ball or extruded?

Giving some more specifics may help... case, powder, barrel length and twist, cleaning interval, etc. Does it foul excessively?

nhk
 
Chamber

Another thought; what's the neck diameter of your chamber? How much expansion is your fired brass showing over your loaded neck diameter? Can you pass a bullet through the neck of a fired case?

nhk
 
Barrel

What about the groove diameter of the barrel?

A number of years ago a fellow shooter aquired a 6mm barrel from a now defunct manufacturer, and it acted like yours. Both the land and groove diameter was really tight............jackie
 
I cut the chamber and don't remember anything wrong about the bushing size for the range rod so I suspect that the bore is 6.5.

I just checked the fired brass and I can push a new bullet into the case, it's tight but can be done with hand pressure.

It is a Brux barrel, Lapua brass - never fired and easily fits into the chamber.

I did notice that as the ladder test progressed that chambering the round became more stiff, that should not have happened since they were loaded precisely.

The chamber spec calls for .2925 and a loaded round measures .290; a fired case measures .2915. Please correct me but these numbers seem OK.

The case length is spec to be 1.86 in the reamer print and the new brass is 1.838 so lots of room there.

Maybe there is a small block or bulge in the barrel, I'll look tomorrow and post.

Thanks for the support.

Ross
 
The powder is RL17, 8 twist, 25" about 40 rounds since chambered. It displayed low velocity since the first shot so I doubt it could be overly fouled.

A really long shot would be a mis-cut reamer. I'll check that tomorrow also.

Best,
Ross
 
FWIW, I have been allowing .003" neck clearance on my 6.5 and a bullet slides into the brass after its fired very easily, maybe 1 lb force..I just went and checked.

Will be interested in what the bore/groove dia that was stamped on this bbl.
 
Al, I did not ream the holes and they are the small ones that Lapua uses.

The bullet was an 140 Amax that has a very long shank. For this reamer the cartridge had a max length of 2.6. This max was imposed by the shank/ogive junction jaming in the throat, still no rifling marks but that is due to the shape of the bullet since it is a secant design.

I wonder if this reamer was designed around a smaller (120-123) VLD with a long slender nose and a short shank?

Dave did not remember and it is not listed on his print.

I bought the barrel with 160 rounds through it, cut it off and re-chambered it so no numbers.

Ross
 
Pressure (from neck?)

Quoted: "I just checked the fired brass and I can push a new bullet into the case, it's tight but can be done with hand pressure.
The chamber spec calls for .2925 and a loaded round measures .290; a fired case measures .2915. Please correct me but these numbers seem OK.
The case length is spec to be 1.86 in the reamer print and the new brass is 1.838 so lots of room there."

FWIW: If it was me (and it's not) I'd turn 0.002" off the case neck diameter. What I was getting at on case length was actually the headspace length of the case before and after firing. If the shoulder of the case is set back too far the case can be driven forward by the firing pin and the case mouth will act the same as a if the neck is too long even if the total case length is 0.022" under spec. I'm not sure if I understood if you are jamming your bullets or not, which would help fire-form your case. nhk
 
High pressure and low velocity with all that has checked out is really weird.

Is the chronograph accurate?
 
Chambering became more stiff

rossneder said:
I did notice that as the ladder test progressed that chambering the round became more stiff, that should not have happened since they were loaded precisely. Ross
Click to expand...

FWIW: How much neck tension are you using and are your loads compressed as they go up the ladder? I've had compressed loads 'grow' after sitting overnight and that could account for stiff chambering because you are then jamming the bullet. Did you trying extracting any that chambered stiff to see if you had rifling marks? I've even had the jam pull the bullet when I attempted to extract the round. nhk
 
Here is a shot of the fired cases.
Left to right
unfired - light load - max load - way overpressure load

nq5q3k.jpg



I also ran a known load over the chrono and it was perfect.
 
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RE: fired cases

It's hard to tell perspective from the photo, but it appears the cases have grown(?) in body length from left to right, is that the case?

I do see bright bands on the case neck which indicates to me the necks are dragging on extraction and could use some clearance.

The line around case head probably corresponds to the end of the chamber and there is definitely pressure to print that and you may be looking at a head seperation fairly soon. How much expansion are you getting in the diameter of the case head?

It will be interesting to see what the rest of you think. nhk
 
Using a 30 cal comparator that stops at the neck-shoulder junction I have between .001 to .005 increase in length depending on how much powder was used.

There are no extraction marks on the neck, it may look that way but it's just reflections.

Best,
Ross
 
rossneder said:
Here is a shot of the fired cases.
Left to right
unfired - light load - max load - way overpressure load

nq5q3k.jpg



I also ran a known load over the chrono and it was perfect.
Click to expand...

hmmm

We gotta' source that ring...... it ain't from headspace caused by repeated sizing because this looks like once-fired brass.......in fact to me it looks to be too far down the case to be an "incipient casehead separation" ring anyway.

It looks specifically like a pressure ring to me.

So where's the pressure from?????

It ain't from the shank hanging down into the powder chamber. I've got loads with a quarter inch of shank down in the powder.....

It ain't radial clearance of the necks. I shoot MAX loads with clearance at zero and no pressure.

It ain't from touching the lands.

I don't believe it could even be a tight bore, not that radical a problem. A tight bore can push hot loads over the edge but even then you can load for a tight bore just by backing down the powder charge.

The neck pinching onto the bullets still seems most logical BUT... you say the numbers on the reamer check out. Now, have you measured to find the end of the neck? I'm using a WSM right now where the reamer is spec'd to be 2.000 oal and the chamber measures 2.022. Twenty two thou is a big difference. If these numbers were reversed...... and if I didn't actually measure it....

And judging from your description to this point (and from those pictures!) this isn't false signs of high pressure, it IS high pressure.......

Neck crimping on the bullet and the wrong powder, like too fast for bullet weight, are the only two things I've seen act this way. And it ain't too fast powder.... RL-17 is the ballistic twin of H4350, either of which fill the bill perfectly.

Manually measure the neck??? Or easier, try shortening some cases and see of the problem resolves......

al
 
Is the powder a given?

I once had a new can of powder that was completely off. It gave pressures greater than powders from the same manufacturer several burn rates faster.

What about your primers? Haven't got pistol primers by any chance?
 
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I just scoped the chamber and the muzzle (only a 6" borescope) and everything is perfect.

I ran a patch feeling for tight spots - nope.

Not the best primers for this but they are Win small rifle. I usually mix my 1 lb cans of RL17, e.g. run down to 4oz and add to the next lb to keep it fresh.

I'm going to load same 107 VLD short and see if that solves it.

Best,
Ross
 
Be sure your pin protrusion is correct and the spring is new if the action is not. You may not have high pressure but an ignition problem which can make the primers look that way.
 
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