Seating depth and pressure

L

Longboat

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Up until now I have just been seating my rounds to loading manual OAL.

On my 308 I have had a good load with 168 grain ELDM with 43.0 grains of IMR 4895. After measuring I see that these loads have been jumping .096" to the lands (CBTO).

My question is, if I want to set my bullets to jump .020" to the lands, should I be starting with reduced powder charges again and working up? Or will I still be safe with 43.0 grains of IMR 4895 going from a .096" jump to a .020" jump? Would that increase pressures significantly?

Thanks.
 
Conventional wisdom has it that if you set the bullet for a larger overall length and keep the charge the same, the pressure decreases. So, yes keep your starting charge the same as when you loaded to a 0.096" jump.
 
If you're loading 43.0grs powder, .096" jump to lands and making a predicted speed/pressure of 2609 fps / 58,738psi (20" brl), changing to .020" jump would give a predicted 2593 fps / 58,682 psi

Loading closer to lands will increase pressure. Lengthening out the OAL increases internal capacity which reduces pressure when using the old charge weight of 43.0

Seating closer to lands does not mean it will shoot more accurate, but could be worth a try. I like to set the bullet jump to function in the rifle and tune the powder charge to change accuracy. Numerous times I've found great loads that had bullet jump of up to 1/4" of freebore
 
todbartell said:
If you're loading 43.0grs powder, .096" jump to lands and making a predicted speed/pressure of 2609 fps / 58,738psi (20" brl), changing to .020" jump would give a predicted 2593 fps / 58,682 psi

Loading closer to lands will increase pressure. Lengthening out the OAL increases internal capacity which reduces pressure when using the old charge weight of 43.0

Seating closer to lands does not mean it will shoot more accurate, but could be worth a try. I like to set the bullet jump to function in the rifle and tune the powder charge to change accuracy. Numerous times I've found great loads that had bullet jump of up to 1/4" of freebore
Click to expand...

Did you mean to say loading closer to the lands will NOT increase pressure in your second paragraph?

And you are right about my current velocity. It's right about 2610 fps.
 
I meant that a shorter jump causes higher pressure, but is offset by the larger internal capacity of the cartridge when the bullet isn't occupying as much of the case by seating it out further. Jamming the lands and adding more powder is where trouble can happen
 
If accuracy is your objective, then changing the seating depth by that amount can mean the barrel harmonics are different.

I would not be concerned about over-pressure unless you stuck the bullet into the rifling.

Seat the bullet out where it belongs, about 20 thou off the rifling (why did you use the book OAL?- they weren't testing with your rifle).

Then make 10 each of the same powder charge and with plus and minus 0.3 gr and test for accuracy.
 
Yeah I will make a few different charges to test for sure. Just wanted to make sure I didn't have to start back at starting loads like 10% back and waste time and components.

I never had played with seating depth yet because I just started reloading last fall. And had 4 weeks to make 400 loads for a competition. So I kept it simple and was basically copying factory ammo that shot good. Never had the tools to measure chambers etc yet. I stilled nailed quite a few V bulls at 600. I'm just getting a little more advanced as I go.
 
todbartell said:
you can always fall back to your previous load that gave you good results. Experimenting is good
Click to expand...

For sure, that's half the fun for me, its very interesting to play with. Reloading filled a hole for me and replaced other hobbies I've moved passed.
 
As you increase the COL, the pressure will drop until you get near the lands, then the pressure will begin to increase. The people selling the RSI Pressure Trace System have an example on their page where they seated a 6mmppc bullet .030" off of the lands, and then just touching the lands. The pressure was 7000psi higher when seated on the lands than when seated with a .030" jump. Unfortunately, they didn't seat the bullet with a longer jump, or with a jump between .030" and the lands, so we don't know exactly where the pressure was lowest, or when it started to increase. The link is below, scroll down to the 6ppc graphs.

https://www.shootingsoftware.com/pressure.htm
 
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