Pressure changes with seating depth

[Mram10us]

Good ol' Chrony. All things equal (powder, primer, bullet, case, rifle) then V = P for all intents and purposes.

I understand your theory, but I'm not 100% sure that the speed is a good way to judge peak chamber pressure. Maybe ignorance on my part. I'll study up

 

[Quicksilver338]

Little background I guess on the setup. Ryan pierce built 300 norma mag improved. The Lapua brass is essentially indestructible and primer flattening and bolt lift are the easiest ways or only ways you're going to see pressure. You do see some marks in the bolt face once you start running up there but you see slight marks at lower pressures too so not a for sure way to detect pressure.

I'm running n570 powder and starting load development with the 199gr hammer hunter. Barrel is a 1:9 proof. I am currently loaded .010" off the lands since I believe being up closer shows pressure signs sooner than a large jump. I have yet to do testing beyond the initial ladder test but using a magnetospeed I found a wide node but just beyond that seemed to be getting pressure signs like bolt click, heavier lift, etc. It seems like most people running this bullet are jumping them quite a ways, which doesn't say mine won't shoot at .010" off but if I did have to seat them deeper to say .050" to get them to tighten up the groups the would it spike my pressure out of that node. Being a norma improved and along with the hammer bullet there is really no load data out there and am flying blind.

Checking here to see if I can get any insight before I further test things.

 

[Rhovee]

I do seating depth from 5 in the lands to 40 off on all my rifles. I have never had any issues. At 5 in, the velocity is about 15-20 FPS faster. From 3-40 off it's the same. Now if you were going 10-40-90-120 like Berger suggest then I could see it becoming an issue. Larger the case less influence. Check and see if you'd web is getting sized at all. And I own a 300 NMI, 338 NMI that I just did this on last week. Yes less case volume with more jump. But less pressure because of more jump.

 

[entoptics]

I understand your theory, but I'm not 100% sure that the speed is a good way to judge peak chamber pressure. Maybe ignorance on my part. I'll study up

My statements were a bit of an oversimplification of course. I'm not judging "peak chamber pressure". I'm judging relative pressure in a load that is not exhibiting signs of high pressure already. In that case, velocity is a direct indicator of "integrated pressure", all other things being equal (e.g. neck tension, charge, primer, etc).

I was suggesting that if the OP is pushing the Peak pressure already (e.g. bolt lift, ejector wipe, primer flattening, etc, etc.), then care must be taken with small changes. IMO though, seating a bit deeper (e.g. seating depth tuning) will not radically affect the Peak pressure, it will affect the integrated pressure, due to small changes in the expansion volume.

Regardless, to the OP, if you are at max pressure already, proceed with caution when making pretty much ANY change to your load. IMO, getting close to the lands will likely have a more rapid and profound effect on pressure (peak or integrated) than a few 10 thousands of seating further from the lands.

 

[Canhunter35]

What have you found in your reloading experience? Does seating a bullet deeper in the case raise or lower pressure?

let's say charge weight stayed the same, but you went from being up on the lands to a .050" jump. Have you found this to change pressure much?

If you come off the lands to say .050" jump you will decrease pressure but if you keep going deeper past .080" jump say it'll start decreasing ur internal volume pushing pressure back up

 

[shinbone]

Not a lot of data, but I have seen an apparent slight velocity drop with increasing bullet jump. I would think velocity and pressure are closely correlated.

 

[supersubes]

I've done a fair bit of testing/observation on this. Pretty easy for us to rationalize that deeper seating will increase pressure. The reality though is that we're working fairly close to the lands most times. I can't ever remember a load combination where pressures went down as I got closer to the lands. Seems to me that force required to engrave a bullet is far greater than the force required to overcome neck tension.

 

[BrentM]

I too noticed a wave curve. Seating to the lands, which I never do anymore, produces pressure spikes, seating further reduces pressure, until you get to a point you compress the charge and reduce case volume and then pressure goes back up a little. It seems to me I had to get past 150 thou jump to see much of increase depending on the powder charge etc.

Recently I have been playing with the 156 in a prc case and found max load was well below published and velocity was low. There was no room for the powder and it spiked. Switched to a faster burn and less volume powder with the same seating depth and it produced better velocity at a lower pressure sign, or lack there of

 

[5.56×250]

It seems to me that bullet diameter would have an effect on how much pressure increased too, at deeper seating depths. A 338 cal bullet would decrease or increase volume in the case more than a 264 for a given depth change, correct ?

 

[JASmith]

I did the exercise using Pressure Trace on a TC Encore chambered for the 6.5 Grendel.

The results were largely consistent with the QuickLoad manual admonition to boost the starting pressure by about 5,000 psi when the bullet is in the lands.

So, there was a gradual decrease in pressure as the COL was increased followed by a jump in pressure when the bullet was touching or jammed in the lands.

The rate of decrease and added pressure were not, however, as significant as I would have expected from the QL calculations. That may have been a result of the compund 1.5 - 0.5 degree throat in a SAAMI-spec Grendel chamber.

 

[Philward]

Does powder burn rate figure into all this pressure equation. Do the slower burning powders (4831) need more unfilled case vs. 4064 at max case capacity with seating depth determined by space for powder to burn.

Yes but rate figured into the equation, but slower powders can be filled and compressed whereas faster powders, not. In general.

 

[freddiej]

I reckon since I am all out of popcorn I should do something with my brain and my fingers...
I have found that most everything I have read here is correct except for a few things.
jamming the ogive in the lead will bring up pressures exponentially.
the problem with the opposite, dropping the projo as far into the case, is that I have yet to find how far in you have to go to bring the pressures back up. I have two friends whom are engineers and they told me there has to be one point.. we have tried. I sunk my 45-70 slug almost all the way to inside the case and have not yet seen pressure signs with a 405 cast lead slug and H-4895. I chose a straightwall case for a few reasons.. faster burning powders, measurable reduction in combustion chamber, and to **** off the two engineers. I ran the projo all the way down to the powder charge and packed it like black powder, without any pressure signs. nearly did not get it out my 28" barrel as a result of this. the powder can not get a head start on building up pressure. the projo starts moving, increasing the volume of the combustion chamber, immediately after the primer is struck.
Just my two cents worth and my R/L experience trying to see how far is too far inside the case to bring back pressure.

 

[5.56×250]

Speaking of 45 70s, a customer that use to come in the store I worked for had just started reloading and ,( im going from memory on powder nominclatures) exchanged accurate 17 for reloder 17 (?) In a mutual friends limited edition browning falling block 32" barrelled buffalo gun . He exchanged a slow burning rifle powered for 10mm auto powder ! A full charge too. Unknown to our gun owner they went to the local range to test loads. When he ( the gun owner) touched off the first round he blacked out and woke up laying on the ground behind the bench .The charge erased the numbers and letters from the case head and the primer flowed to the point the case head looked like a smooth flat piece of brass ! Other than a black and blue shoulder, the shooter was ok, and believe it or not, the browning falling block was none the worse for wear either. The scariest part was the marlin lever gun in 45 70 that was laying on the bench to be tested next with the 10mm powdered loads ! I doubt there would have been any hope for the marlin staying together ..
Sorry guys, subject shift, but had to share that pressure story.