Max load vs Max pressure

[Woodswalker]

Hi everyone, I've been reloading for a while, but I've never wanted to exceed max load. But now I'm looking to get the most out of my 10mm for bear defense and noticed that often the max load listed for a powder/projectile combination will not get close to the max pressure.

For example, Hodgdon's reloading data (http://www.hodgdonreloading.com/data/pistol) for 10mm, 200 gr shows a max load of 8.2 gr of Longshot, giving a pressure of 35,000 PSI. This is 2,500 PSI below max pressure.
They also show a max load of 5.9 gr for Universal, giving a pressure of 36,900 PSI. This is much higher pressure, though is still 600 PSI below max.

I'm wondering what factors go into deciding the max pressure. I noticed for 800-X, the pressure for the max load is 32,500 PSI - I could imagine that is because it meters poorly, meaning more of a safety factor must be used to ensured that large deviations in 800-X loads still won't go above 37,500 PSI.
Is this the only factor? Is it even a factor? I've also wondered about whether it takes into account if powder would be crushed after a certain number of grains, or perhaps if accuracy would suffer terribly. I'm just struggling to work out why some max loads for some powders are so close to max pressure, while others are far below it.

Does anyone have any insight on this? Is there any reason why one powder at max pressure (even if it's way over max load) would be any more dangerous than another powder at max pressure? Does anyone know what charge weight of Longshot gives close to max pressure?

Thanks!

[Benchrst]

Possibilities:

Charge for max SAAMI yielded no gain in velocity over published max.
Charge for max SAAMI yielded erratic results.
Published max has a built in buffer for varying component / chamber combinations.
Published max has a built in buffer for component variances.
Published max has a built in buffer for powder variances.

The list could go on an on...

[sqlbullet]

Benchrst hit on the big ones.  It would be nice if we had access to the ballisticians (sp?) notes.

[Geeman]

This is an interesting subject.

As I see it, Max Load is something determined for a particular gun, not a universal standard for every gun.  My Witness can take more pressure than my Glock, and my Encore can take more than either.  For me its what I feel is safe in any and all conditions.  My Max load is found by testing, carefully stepping up until I'm either satisfied with a load, or it begins to show signs of too much pressure, then is backed off to within safe limits.

The closer to the edge, the smaller the margin of safety is.  What happens when you take the ammo out of a hot car?  What about when you run out of powder and the new is from a different batch?  Same thing when you run out of a particular primer.  It can be a pain to re-test to ensure the new load is still safe with different components.  Then there is the bigger issue.  How many of us run brass mixed head stamps and how many keep track of how many times they have been fired? 

Internal ballistics is a science where everything is a variable.  Everything can and does change the result.  Cartridge length, first shot vs subsequent shots, gun clean or dirty, temperature and how long the round is left in a hot barrel before its fired.....

I find a load that I feel is safe in any and all of my guns and stick with it.  If I want to push it, I use new brass and look for signs to verify it is safe. 

Greg

[The_Shadow]

Woodswalker, Welcome to the forum!

Benchrest & Geeman provided a few good instances for published data being what it is...However take into account 800X which doesn't meter as well, sort of like paper plates.  Progressive press operators wanting speed over precise hand weighed measures as a factor and even corporate lawyers as you can get a feel for the published data.  An then there is a double charge possibility with some powders!

Then there are some who go beyond the published data to yield even more pressure and velocity...you are on your own, as you work blindly without benefit of pressure testing barrels and lab quality equipment to verify the results.

No two guns may be the same even though they are made by the same manufacture, sometimes differences do exist!
Brass cases, as new or used, can have different properties and working at the upper extremes can have disastrous results!

The best advice is to get to know, understand the abilities of yourself, your equipment, your firearms as you approach any handloading processes. 

When I am testing I check my loads in several different firearms to see if the results look favorable for all of them.  You don't have to have the absolute fastest velocity to have reliable and accurate rounds to shoot!

[my_old_glock]

Possibilities:

Charge for max SAAMI yielded no gain in velocity over published max.
Charge for max SAAMI yielded erratic results.
Published max has a built in buffer for varying component / chamber combinations.
Published max has a built in buffer for component variances.
Published max has a built in buffer for powder variances.

The list could go on an on...

Published max has a built in buffer for temperature/weather variances.



.

[Woodswalker]

Thanks everyone for the great responses! Sorry for the slow reply - I've been busy lately. I managed to get to the range and try out loads from 7.0 gr up to 8.4 gr (Longshot powder) with 200 gr hard cast projectiles.

I don't have a chronograph (yet), so I'm not sure how they went velocity-wise. But the casings all seem quite good after firing. The 8.2 gr loads had fired casings roughly the same diameter (top and bottom) as lower loads (around 0.425" to 0.427", with some 0.430" right in the middle), but no "Glock smile" despite using a Glock. My 8.4 gr casings do seem quite a bit larger - about 0.430" to 0.432" around the base, though still no apparent Glock smile nor any permanent mark.

So now I'm wondering about reloading this brass. I assume I should be all good to go reloading the 7.0 gr to 8.2 gr load brass with loads up to 8.2 gr (is that true?), since it seems ok for diameter, length and has no permanent markings. But what about the 8.4 gr load brass? Is it at all safe to reload? Should I reload it only with reduced loads? Would it even be fine reloading at 8.4 gr again?

Thanks so much again everyone - I'm new to (slightly) pushing max load boundaries, so any advice is appreciated (and obviously all risk and responsibility is on me ).

[The_Shadow]

If you use the Pass-Through sizing method prior to your regular sizing you should be fine for reloads up to the same 8.4 load.
That method helps to get the area where normal sizing can't reach because the shell holder or plate and the radius in the mouth of the sizing dies.  Using the LEE FCD (guts removed) and a bullet sizing push pin, I have found the benefit of pass through sizing to be effective with 10mm, 40S&W, 357Sig and 9x25Dillon casings.   
I also pass through size 380ACP and 45ACP for the same reasons and benefits.

[Taterhead]

Ive spoken with ballisticians from some of powder companies a few different times. 800-X came up more than once. They reason why it is  published with maxes far from SAAMI, was due to volatility at higher pressures. It had nothing to due with metering (besides, some extruded powders meter like crap too, but aren't similarly derated). At least that's what two different Hodgdon ballistics guys told me. This is also why certain loads aren't published at all (like when I called about 9mm data for WST).