Seating depth change causing over pressure

S

Stats Shooter

New member
i have a strange situation brewing regarding over pressure appearing where I would have thought it shouldn't have


200 gr partition
Norma brass
H-1000
Fed 215 primers
Once fired brass


I was working up the load and found best accuracy pushing the partition hard. Max velocity according to quickload is 3020, im pushing it 2997 at 61k psi. This load has no signs of pressure at all.

The jump at the initial base to ogive of 2.78 is 0.13" to the lands, so certainly a good jump. Max mag length is a COAL of 3.40" or a base to ogive of 2.79". That would make the jump 0.11.

I had a decent load at CBTO 2.78" and decided to try 2.785" and 2.79" CBTO to see if I could tighten the groups up just a little.(Thats the 1.1 moa load I described in the "partition accuracy thread)

I loaded up 5 each at 2.78 (for varification), 2.785, 2.79" CBTO. The 2.78" load shot a nice 0.8" 5 shot group. The 2.785" load had a loose primer, shot poorly, and had a difficult extraction (test ceased with one shot remaining). The 2.79" load test had to be stopped due to over pressure (after 2 shots). I pulled the remaining bullets and checked my records to make SURE i didn't make a mistake on the scale when hand weighing the charges and the powder weight was correct and the exact same as the cartridges assembled at a base to ogive of 2.78".

Can moving in just 0.01" with a jump as long as 0.12-0.11 cause such a pressure spike? I mean, these are quite a ways away from the lands. Im going to load 3 at the same charge at 2.78" CBTO to verify that they do indeed function as before....but after 15 rounds already within those specs Im sure that they will.

The strangest part is how BIG the difference was. The velocity of the 2.78" CBTO load is approximatly 3000 fps but the sticky bolt loads were 3040-3050 fps.

Anyway, any thoughts on this would help....my inital reaction is to drop a grain or so because now im afraid that a hunt could get ruined if a bullet gets seated a little long.
 
Tag to follow ,

I would not think while still having that long a jump the pressure would go up that much . On the other hand if you're at max pressure .120 off the lands , that's a pretty hot load to start with .

You say

The velocity of the 2.78" CBTO load is approximatly 3000 fps but the sticky bolt loads were 3040-3050 fps.
Click to expand...

Is this chrono verified or QL estimates ?
 
I have always been aware that (in bottle neck rifle cartridges) the closer you seat to the lands with all other things being equal, pressure will increase.

Here is a good thread on the subject from the Nosler Reloading Forum.

There are some good graphics along with the posts.

https://forum.nosler.com/viewtopic.php?f=4&t=27836

Seating deeper will decrease pressure and velocity.

That is why your velocity jumped when you seated longer.
 
Nice link Dufus.

It is still however surprising that 0.01 would make as much difference as it did in my case considering how far off the lands I am. But, I maybe crossing a threshold compounding the problem to make it much worse like more obturation of the bullet / case preventing more of the gas from escaping.
 
You identify the case, powder, primer, and bullet, and lots of information about the length of the loaded rounds.

I don't see any mention of what cartridge you are shooting, nor what the powder charge is. Nor what the gun used is. I assume a rife of at least .30 with that bullet weight, and a large case if you can use as slow a powder as H-1000, but it would be nice if you actually told us, so we don't have to guess.

It MIGHT be that your powder charge, in your gun, with your components is right on the edge of showing pressure signs, and a slight change in seating depth MIGHT be enough to push you over the edge so pressure signs show up.


MIGHT be.
 
I automatically assumed it was one of his 300 Win Mags.

I knew also that by his velocity figures, he is at or above the load spectrum with that load data (200 gr partition and H1000).

Any small change in seating longer would and did produce the results that he had.

I have had similar results in the past.
 
You identify the case, powder, primer, and bullet, and lots of information about the length of the loaded rounds.

I don't see any mention of what cartridge you are shooting, nor what the powder charge is. Nor what the gun used is. I assume a rife of at least .30 with that bullet weight, and a large case if you can use as slow a powder as H-1000, but it would be nice if you actually told us, so we don't have to guess.

It MIGHT be that your powder charge, in your gun, with your components is right on the edge of showing pressure signs, and a slight change in seating depth MIGHT be enough to push you over the edge so pressure signs show up.


MIGHT be.
Click to expand...


I automatically assumed it was one of his 300 Win Mags.
Click to expand...

Good Guess!
The cartridge is a .300 win mag. Im reluctant to show powder charges when im up near or over max for the simple fact that what is MAX is different depending on chamber, powder lot, seating depth, and in the case of a .300wm, if it is fire formed vs. new brass.

In the .300wm, the case water overflow is 7 gr. higher in Norma vs. Remington brass. I am using Norma. What would be max in a New Remington case isn't even the middle charge for a Norma case. Then, the .300 win mag shoulder datum to case head can grow by as much as 0.02"!!! This is common, but it means that a formed case, head spacing off the shoulder is remarkably larger (in volume) to a new piece of brass.

The most extreme example would be a piece of new Remington brass vs. a fire formed piece of Norma Brass. Remington case water overflow (new) of 88 gr and the fire formed Norma (tested by me) was 96.5 gr water weight.

And yes, the velocity of around 2990-3000 FPS for a 200 gr partition is maxed out. The only reason I have done testing at this max (actually just over max) charge is the groups being produced.

Uncle nick has suggested I try and bring it down a couple grains and fudge the seating depth some, I am going to try that.

I did re-test the load at the shorter cartridge seating depth on saturday and it extracted fine. There are some slight extractor marks and the extractor groove grew 0.002". So This would be a MAX load, and moving into the lands apparently pushes it way over the top.
 
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I have always been aware that (in bottle neck rifle cartridges) the closer you seat to the lands with all other things being equal, pressure will increase.
Click to expand...

Same thing is true going the other way.

F. Guffey
 
My last seating depth experiment my Enfeild. Before this I was sure that the pressure would rise as the bullet was seated deeper. I was told what would happen but I had to see for myself. Note that the velocity did increase at first, then at .006" jump it began to drop due to gasses escaping around the bullet. This round was IMR4064 and Speer 150 HCSP bullets.
5 round groups all the same starting powder charge:
lands - 2.5 MOA 2576 fps
.002 off 1.25 MOA 2598 fps
.004 off 1.5 MOA 2603 fps
.006 off 1.25 MOA 2601 fps
.008 off 1.0 MOA 2591 fps
.010 off .75 MOA 2585 fps
.015 off 1.75 MOA 2582 fps
.020 off .5 MOA 2579 fps
.030 off .5 MOA 2569 fps
.040 off .5 MOA 2558 fps
.050 off 4 MOA 2547 fps
.060 off 2.5 MOA 2530 fps
.070 off .5 MOA 2535 fps
.080 off 1.5 MOA 2531 fps
.090 off 1.5 MOA 2516 fps
.110 off 3 MOA 2519 fps
.130 off 2 MOA 2528 fps
.150 off 2 MOA 2529 fps
.170 off 2 MOA 2519 fps
.190 off .5 MOA 2508 fps
 
Well that appears to be what I experienced. Going into the lands increased my pressure even though my jump was over 0.1" at the time. My belief was that, at that distance, pressure would decrease going into the lands. But, it increased.

My velocity jump was more dramatic, but same phenomenon.
 
Be careful with that. Some powders are more sensitive than others. Also make sure you are finding your lands and not jamming. I have made that mistake in the past. A worn throat is a bit trickier to measure precisely. If you are jamming and think you are at .005 off you might still be at the lands. I had a big lesson with 4350 recently. It is not as forgiving with pressire spikes. I got lucky. If you seat on the lands with one powder you would be safe to ease into it with another powder.
 
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