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-   -   Barrel Pressures (http://pistolsmith.com/showthread.php?t=28626)

cadvan 05-31-2017 05:09 PM

Barrel Pressures
I've been doing a lot of research on calculating barrel pressures in order to work up my own loads. I have a mild plinking load I shoot in 45 acp just going off of published load data, but I want to develop a SD load, which would be near max. Can barrel pressure be accurately calculated without accounting for powder burn rate? I found an excel sheet that I updated which says that:

Barrel Pressure =(Projectile Mass)*(Projectile Velocity)*(Correction Factor)/(2*(Bore Area)*(Barrel Length))

where the correction factor supposedly accounts for the ballistic coefficient and such and equals .250. The output is in psi after unit conversions. The formula makes sense to me based on the basic physics I know, but I keep reading that burn rates affect pressure, though no one says exactly how to use it in a formula.

I have load data for the bullet/ powder combo I want to use, but the test barrel is 5", where my carry gun is 3.77" (Glock 30s). I assume the length difference equals a pressure difference, or is it only a difference in how soon the pressure drops? Obviously my velocity will be lower than the published data. Basically I'd like to confirm what the published data claims and that I'm shooting a safe load.

BTW I am not looking to spend a ton of money on a reloading calculator, I'm cheap haha. Nor am I looking for an argument about carrying reloaded vs factory SD rounds, or powder preferences. I have a feeling all 3 would come up if not mentioned now.

On a side note, is there a place to find the burn rate for a powder? I tried looking it up for w231 (for plinking loads) but could not find anything besides powders listed in order of relative burn rates (not very helpful).

dfariswheel 06-01-2017 03:23 PM

I've been out of reloading for a long time but here's what I'd go with:
Trying to develop a safe but maximum load by using formulas really isn't the way.
There's too big a chance you'll go over the safe limit by accident.

The best way is to pick the powder and bullet weight you want to use.
Consult at lease a couple of loading manuals to compare maximum loads.
Back off at least 10% ?? then work up to the max in short steps, watching closely for over-pressure signs on the primer and bulged cases.

The reason this is better then using formulas is because the formulas can't take into affect things like case head support.
One gun, even in the same brand may have less case support and will show bulges in the feed ramp area before another gun will.
This is why you have to work up slowly toward manual max loads, remembering that their max may well not be your max.

Last, even in a defense load there's no really good reason to push max loads.
The "target" will not notice a few feet per second difference.
That's more a function of bullet design.
An absolute blow-down-the-barn door load with a poor bullet won't work as well as a lower load with a great bullet.

So, this is one case where Real World observation is better then formulas in developing a hot load.

cadvan 06-02-2017 07:51 AM

My understanding is that for hollowpoints, high velocity is needed for good expansion. I was going to keep my personal choices out of it, but I guess that is kind of hard haha. Right now at least, I'm reloading for a Glock30s, which has less case support than other guns, so I believe some case bulging is normal. It being a 45 acp and being low pressure I am under the impression it doesn't give much pressure signs. I have noticed that both with factory ammo and my light target loads, my firing pin leaves a drag mark on primers (maybe because I put in heavier springs?), so I can't really use that as a pressure sign.

I am wanting to push close to max loads for defense, because the barrel is only 3.75". So going back to the beginning, I think I need the little extra speed. I am however going to experiment with Lehigh Defense's Xtreme Defender bullet, which also transfers more energy with more speed. I am also thinking of loading +P, which I have load data for for that bullet.

That's where I'm at, though slightly off topic. I am just a very calculating person, and would like to be able to double check what the manufacture claims with their load data. Hence why I'm curious about burn rates with relation to barrel pressures.

dfariswheel 06-02-2017 03:36 PM

It's very common, even normal for modern striker fired pistols like the Glock to leave firing pin drag marks on the primer.

noylj 06-04-2017 05:43 PM

1) If you think you need max pressure/velocity, move up in the cartridge.
2) Since burn rate varies with lot number and you can't measure burn rate and several other variables are unknown and I would expect a few more variables to be needed, I would forget the whole thing.
3) Remember mysterious "fast" and "slow" barrels? You take two barrels off the line, measure all dimensions and they are all "the same" to be degree you can measure them, you attach pressure transducers, and fire rounds through them. Despite the SAME average max pressure, one barrel shoots consistently 25-200 fps faster than the other. So, pressure is NOT the sole determinant of velocity. Life is not simple.
4) Load per 150 years of instructions.
5) First, you have to HIT the target. THEN you can worry about terminal ballistics, which, for pistols, really is not much effected by 100 fps.

noylj 08-08-2019 09:09 PM

Just to show the problems, a formula like that ignores pressure spikes.
Some manuals list the pressure of their max load, meaning that 0.1gn more powder exceeded one or more of the SAAMI statistical means for pressure. Sometimes you'll see one powder that is at a much lower pressure. This is almost always because that next 0.1gn of powder produced pressure spikes well outside of SAAMI limits.
This was discovered years ago when all of a sudden all the new manuals showed must lower maximum powder charges. Everyone started ranting about lawyers, but it was really SAAMI and the industry being able to observe instantaneous pressure over real time and NOT just how much a copper plug was reduced in length.

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