How much does bullet seating depth influence chamber pressures?

The reloading manuals I have state their tested powder charges and give a C.O.L. and pressure for a given bullet weight, but not a specific bullet. The arse end of different bullets of the same weight will determine how deep (or shallow) they can be seated. i.e: BT's verse flat base.

I find I am seating my bullets significantly deeper than what the manuals state as I don't feel comfortable with only 1/2-1/3 of the neck gripping the bullet.

No pressure dramas so far as this is my first attempt at reloading and I am still about 7% below their stated maximums, but am I likely to see pressure signs well before I would if I used their tested C.O.L's.

This brings me to another question. If seating the bullet deeper increases pressure, is it relative to an increase in velocity.

I don't have access to a chrony or pressure measuring equipment so your help on this would be appreciated.

Cheers...

I want my bullets seated at least one diameter deep. Example: a .308 bullet should be seated .308 Minimum deep.

I doubt that there's any difference in pressure by changing the seating depth....at least that's never been a concern to me and I've reloaded for forty years now.

Further it's not necessarily true that an increase in pressure will yield an increase in velocity unless that increase in pressure is maintained all the time the bullet is in the barrel. Most increases of pressure caused by reloading errors (case too long) only spike the pressure momentarily and then the pressure comes down quickly. However that spike can be very dangerous.

Seating depth does influence pressure. The longer you make the cartridge, the lower your pressure will be, until you make the cartridge long enough to touch the rifling. Then pressure will go up.

seating deeper simply reduces case volume and thus an increase in pressure and velocity. Using quickload "what if" shows for 375HH, 300g Hyd RN:

code:

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seating  
depth     velocity  pressue
0.2"      2411fps   40,600psi
0.3       2445      43,100
0.4       2452      45,900
0.5       2520      49,200
0.6       2561      53,100

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-steve-

PS Brent, thanks.

[ 08-23-2003, 04:07: Message edited by: steve505 ]

I'm not knocking 40 years of reloading experience, but I know for a fact that pressure does come up when seating them deaper. How much? It doesn't come up enough most times to get you into trouble, and I hesitate to say this, even near max load it's probably not ever going to blow the gun up but, you could get into pressure signs pretty quickly depending on the load combination and how it reacts near maximum loads. I've had rounds drop off in pressure and velocity like a rock near max load, but mostly seen them climb, some climb so damn fast you'd shit yourself. I'd recommend backing off a bit and working back up if the seating change was more than about .050" to .100", depending on the cartridge size. Pressure can run up about the same degree if getting on the lands, not always, but it does happen frequently enough on the long side of OAL that I'd watch for pressure signs as I inched closer to the lands looking for a sweet spot, velocity will be a good indicator also if pressure's on the move upward. There isn't no free lunch, when velocity moves, pressure is too... Likewise, if pressure moves so will velocity. It isn't linier, but it's related in about as close fashion as it gets. Others will disagree, but I know what I see from the Oehler 43 and it very closely related.

It don't take full caliber in length on the bearing surface to hold bullets on recoil if neck tension is .003" or better, besides you stay away from any "doghnut" that's forming at the neck/shoulder junction by seating to just above it anyway, better accuracy with consistant tension and the dougnut is growing all the time so it ain't the same down there. 1/2 to 2/3rds in the neck is what I shoot for. Just my 2 cents...

Steve, I think one of your MV's came out haywire...

Just looking at that reminded me, some shots in groups will vary 2 and 3000 psi themselves, and in the .100" seating depth change that's about what I'd expect, maybe a bit more if your real deap already or real close to the lands. Bottom line is it will change pressure, so be carefull to watch for signs of it getting high in any change you make, OAL included.

If the book specifies an OAL, I take precaution if I've worked up to max and change anything. If I've got a .100" or .200" jump to the lands yet, I know pressure might come down a bit before it comes up, if it does when I get closer to them tuning the load, and that's if mag length will even alow that great feat in most new rifles put out.

Here's one for ya; I had loaded up some 200gr Accubonds for the Ultra at 3.835" OAL to hit lands hard, single fed in the Rem 700 of course, did some pressure testing and at 90gr Retumbo they were about 62kpsi and 3080 fps. I had to drop back to 3.660" in a hurry to go hunting with it, as I was out of the RL22 I'd been using. I dropped the OAL back and fired it over the Oehler 35P just before I left to see where velocity went to with the change in OAL, it fell to 3045 fps so I know presure was a couple thousand psi below where it had been seating them .015" into the lands like thee first ones tested were. I was good to go and the load turned out to group under an inch, even while making minute scope adjustments to zero it up perfectly. I'll see what pressure is reading with the load at the range with the Oehler 43 next time but, I suspect it's near 60kpsi now. I could get another 100 fps out of it easily but will accuracy be there still, we'll see... Killed a little fork horn bull Moose with it yesterday, still lokin for the bullet, it didn't exit so I may find something in the off side shoulder that it passed through and shattered.

Thanks guys,

Your imput has been great,
Being out of the sport for 25 years I have been lacking in knowledge as technology and methods have progressed. You are certainly building my confidence and understanding of what's actually going on when I pull that trigger.

Cheers..

FYI
You can see the difference in pressure between seated on the lands as compared to off the lands if you take a look at one of the sample traces on this page
http://www.shootingsoftware.com/pressure.htm

The traces are a comparison done with a 6PPC. You can find it about halfway down the page.