Seating Depth and Pressure

[uncle-buck]

Was fire forming some .243 cases in a hunting rifle a few days ago with a mid-range load of 38.0 gr. of IMR-4350 and Sierra 100 gr. Pro-Hunter bullets seated right on the lands.

They actually shot pretty accurately. Now, I'm thinking I want to bump up the powder charges incrementally to get some more velocity and perhaps find another accuracy node.

But I want to be careful about pressure.

The Sierra manual lists 41.7 gr. of IMR-4350 as the max. load for that particular bullet. Is that likely to be a safe load for bullets seated on the lands? or should I seat them .005" or .010" off the lands when increasing the powder charge?

Thanks for any advice on this.

 

[RonAKA]

I don't think it is a good idea to try to seat right at the lands. You can't control every round to that kind of precision, so you will end up with a mix of bullets seated just off the lands, right at the lands, and slightly jammed into the lands. That does not improve accuracy. I would recommend going 5 or even 10 thou one way or the other. Into the lands is probably going to increase pressure some, so you should start lower and work up.

 

[MrBottleneck]

I would not recommend jumping up too max and on the lands. Seating on the lands with show a dramatic rise in pressure.
I generally work a load up determining what powder charge is getting me best accuracy. Once that is found I will then play with that particular load on seating depth. I generally start at .020 off and work towards the lands. Basically doing all my charges at .020 off the lands then once my most accurate charge is found I then will seat that said bullet (group of) so many varying thousands off lands seeing if one depth shoots best for that current powder and bullet combination. Thats what can be so fun and at times mildly frustrating about hand loading. Trying to get that perfectly accurate cartridge.
I am sure many do it different but, each hand loader does things his own way.
Here is a article to read Uncle Buck:
http://www.bergerbullets.com/wp-content/uploads/2013/03/COAL.pdf

 

[uncle-buck]

Thank you, gentlemen!

 

[Zero333]

I always work up loads jumping 10thou. I want to be as close to the lands as possible because when I find a good powder charge I don't want unexpected pressure signs when I play with the seating depth.

If you're using Winchester 243w brass you will not reach max until 43gr+ of IMR-4350

F.C. and Lapua brass reaches max pressure a lil sooner than Win. brass. Like a grain less.

 

[uncle-buck]

I always work up loads jumping 10thou. I want to be as close to the lands as possible because when I find a good powder charge I don't want unexpected pressure signs when I play with the seating depth.

If you're using Winchester 243w brass you will not reach max until 43gr+ of IMR-4350

F.C. and Lapua brass reaches max pressure a lil sooner than Win. brass. Like a grain less.

Am using Lapua brass and appreciate your advice,

 

[/VH]

uncle,

Regardless of where you are seating your bullets, you should watch for signs of high pressure as you work up your charges.
If your current seating depth is shooting accurately and doesn't cause other issues (like magazine length/feeding problems, sticking the bullet upon extracting an unfired round, etc.), I see no reason why you can't carefully work up your charge at that depth. As long as you're not trying to end up on the ragged edge of excessive pressure with your powder charge, you should then be able to tweak your seating depth while carefully watching for signs of high pressure, which I would not expect to see until you made a fairly substantial change in seating depth.

FWIW, I usually work my powder charges up with the bullet seated well into the lands.

 

[jepp2]

FWIW, the guidance I always use when working up a load where I will seat the bullet into the lands is a 7200 PSI increase in pressure. This value is provided in the bubble help for QuickLoad. It works for me, but I have no way of knowing how close that is to what the actual pressure increase is.

[URL=http://s860.photobucket.com/user/jepp2/media/Quickload%20Bubble%20Help_zpsb4klbii1.jpg.html][/URL]

 

[dmoran]

FWIW, the guidance I always use when working up a load where I will seat the bullet into the lands is a 7200 PSI increase in pressure. This value is provided in the bubble help for QuickLoad. It works for me, but I have no way of knowing how close that is to what the actual pressure increase is.

No where even close to actual, real world pressure resultants from In to Off.
Donovan

 

[jepp2]

No where even close to actual, real world pressure resultants from In to Off.

Cmon, don't keep me in suspense, let me know what the real world pressure results are from in to off, and how you came by that information. That is how I learn and improve.

 

[dmoran]

Cmon.... That is how I learn and improve.

jepp2 -

First off, and hoping you agree, that a 7200-psi variation in pressure is a very substantial amount !.!.!
In many of the commonly discussed high-power cartridges here, that would be a 10% to 15% change in peak pressure by just moving the seating depth a few thousandths. Also we need to look at basic's, and remember that velocity goes hand in hand with pressure, so a 7200-psi variance could also attribute to upwards +100-fps of velocity swing (and possibly more in some cartridges). Even though very few conduct actual pressure trace tests, most everyone can access what velocity variances of that magnitude would attribute to in POI variance on a target and on a chronograph, which can be looked at as forms of "pressure testing" themselves. Example below of a 1000yd seating test from .015-OFF to .010-IN from one of my comp guns:




Notice how there is a slight rise in POI to each increment from OFF to IN, which suggest a rise in velocity/pressure, but how little of significance it would actually be in amount of velocity/pressure.
Even though I did not pressure trace the above seating test, my estimation based on experience with an RSI Pressure Trace system would place the total pressure variation to likely fall under 1500-psi of pressure variation and probably more realistically around 800 to 1200-psi of pressure variation for the 6 seating increments, and some where less then 25-fps in total velocity spread.
Hope that helps....
Donovan

 

[hogpatrol]

To the OP, jump or jam has no set rule for pressure. It depends on the cartridge and the powder being used. Here's a good thread on this subject, which by the way refutes the 7200PSI "jam" paradigm.

http://forum.accurateshooter.com/threads/why-will-jump-have-higher-pressure-than-jam.3756844/

 

[jepp2]

Hope that helps....

That certainly does and I appreciate the thorough response and agree with the logic.

If I look at the RSI Chamber Pressure System website: https://www.shootingsoftware.com/pressure.htm and go just over 1/2 the way down to "Other Sample Traces" the first example where the actual pressure curve was measured and plotted on the rifle, The peak pressure with the bullet seated into the lands were 41056, 42734, and 41056 PSI, and the same rifle, loads and bullet with 0.030" jump recorded 35181, 33922, 34761, and 32633 PSI.

Data I have seen shows the lowest peak pressure occurs with the bullet having a jump of around 0.050" off the lands. Closer to the lands has greater case volume, thus lower propellant peak pressure but greater projectile start pressure. Further from the lands has reduced case volume, thus greater propellant peak pressure but lower projectile start pressure. Clearly a trade off.

But the entire scenario of the bullet path down the barrel, the point where peak pressure occurs (about 1 inch of travel), and how the gas generation occurs as the bullet continues down the barrel - continuing to accelerate while the gas pressure continues to decrease (from 60KPSI to 10KPSI) on an example I just ran, it clearly is the change in total energy under the curve that determines exit or muzzle velocity. And that is what your bullet impact clearly shows.