Actual Pressure Difference from OAL?? -9mm

S

Seb7701

Inactive
Hi All.

Forgive the newby post, but would very much appreciate a few thoughts on the pressure differences likely from different OAL's for a 9mm.

I am aware that a reduction in OAL causes an increase in pressure, but how much of a difference exactly is required to affect things?

My example -
Was just setting up to load for my new Kimber when it became apparent that it wasn't nearly as 'easy going' as my 92A1 when it comes to OAL!!
I've been using 124gr LRN which my data tells me needs to be a min. OAL of 1.15" (which I know is pretty common...), except the Kimber doesn't like them until they're more like 1.125".

Now we're only talking .025", but I am trying to work out whether it is just a case of reducing the load to compensate or just switch to something like 115gr which I can run at around 1.100" to 1.125" which will then suit both guns.

A bit long winded I know, but any wisdom would be appreciated!!

Cheers, Seb
 
Details, please. What is your load data? Where did you get the data? What is the max charge from that source? Is the data for a lead bullet like the one you're using?

We (usually) don't have a way to measure pressure, so we can't answer your question directly, but knowing your data might help us answer a question about safety.
 
Unless your loads are near the max level, that small diff is not an issue. Lead bullets normally aren't loaded that heavy in 9mm so I doubt you're in the "hot" load range and 0.025" may be within the variance of bullet nose form anyway.
 
Thanks for the replies guys - appreciated.

Load data is as per ADI powders here is Oz.
124gr RN, 4.2gn AP70 powder (equiv. to Hodgdon Universal) (max is quoted at 4.4gn)
OAL 1.15 stated, but the Kimber doesn't like them long at all, while the Beretta, god bless it's heart, will take anything!!

Here's my data -
http://www.adi-powders.com.au/handloaders/pistol.asp?Calibre=9mm+Luger

I received a PM with some great reading, but it included this post, which indicated that .020 diff in OAL can shoot pressures up to 50,000 psi???
http://forums.brianenos.com/topic/177401-seating-depth/
 
Here is how I do it. First load a dummie round extra long, then keep reducing the length until it passes the plunk test. Then I reduce that by .01". Then work your load up from the recommended minimum watching for over pressure signs. Personally I go up until I reach the point where the gun functions reliably with good accuracy, and you don't see signs of overpressure. Once I find this load I load up 50 rounds as further testing and if no problems are encountered load away. If I see problems at this point start all over again, possibly using a different powder depending on the issues encountered.
 
I asked a similar question a while back; "how much pressure rise in my 9mm if the bullet is .010", ,025", .050" deeper". Or "how much pressure rises with each .010" deeper than book OAL?". No one could answer.
 
Seb7701 wrote:
how much of a difference exactly is required to affect things?
Click to expand...

Seating a bullet deeper than specified in the manual reduces the capacity of the case. The reduced capacity means the burning propellant has less volume to expand into so pressures increase. As pressure increases, so may the temperature of the burning. The burning rate of the powder is not linear; a 5% increase in pressure may increase burning speed by, say, 20%, while a 10% increase speeds burning by 50%, so the maximum pressure reached by seating a bullet 0.005 inch or 0.010 inch deeper cannot be predicted.

But this note from the Speer Manual #11, page 385 should give you an idea of just how much of an effect small changes can make: "...loads that produced 28,000 c.u.p. went to 62,000 c.u.p. when bullets were purposefully set 0.030 inches deeper!"

So, particularly for a high pressure, low volume cartridge like 9mm Luger, it is best to stick with the OAL provided in the reloading manual.

If your gun doesn't like that OAL, get different bullets.
 
If you start at the start load, any COL difference is just part of load work-up.
The whole COL issue started from an article a decade or two ago about LEOs who keep re-chambering the SAME cartridge every time they have to unload and load their gun. Reports came in about KBs on the firing line and investigation showed that rounds so treated tended to have severe bullet set-back.
So, they show a normal 9x19 cartridge that produced 33ksi. They seated the bullet 0.250" (1/4") deeper and pressure was 60+ksi.
However, pressure goes up exponentially. Because of this, if I have two data points and connect with a straight line, my "interpolation" between the data points would OVER state the change.
So, let's take the historic report, actually reported in the literature, where COL of X give 33ksi and X+0.25" gives about 63ksi. This would imply, and be an over-statement given the exponential increase in pressure, that a 0.025" decrease in COL will increase pressure by about 3ksi, so a 0.025" decrease in COL will increase pressure by 300 psi.
So, first find the COL that functions in YOUR gun, and then work-up the load—always beginning with the start load—and I always check several sources and start with the lowest start load.
Per my e-mail to Speer:

James: I do not hold the data from the test, the guy who did it is dead now. I would believe that it is in my First Printing in 1987 as well as the 4th in 1991, if it had been in error, it would have been change in the later editions. It could have been related to a particular bullet in 9mm, but I did not see any qualifiers to indicate such.
Coy

-----Original Message-----
From: noylj [mailto:noylj@live.com]
Sent: Thursday, March 10, 2016 11:52 AM
To: speerexpert
Subject: Re: Speer Bullets - Ask Speer Form

Thank you so much for response.
NO, my question was if the 0.030” set back REALLY increased the pressure as the article said (from 35 ksi to 65 ksi), since I quite clearly remember another article (and it was, I thought, Speer also) that showed a 0.25” (25 hundredths of an inch) set-back in 9x19 for the 65ksi pressure.
I just would like to see the report where a 0.030” set back increased pressure so much.I mean, that is 3/100 of an inch and most folks get 1/100 of an inch variation in COL any way (whether from bullet length differences at meplate or flex in press or whatever) and never notice any thing.
Just thinking that someone may have moved a decimal place or something and would just like to see the original report that such a “small” variation could really lead to so a disastrous an outcome.
On 10 2016, at 11:20, speerexpert <speerexpert@VistaOutdoor.com> wrote:

James: the quote is from Speer Manual #11 (page 385, in mine). Max Average Pressure (MAP) for 9mm Luger is 35,000 psi, for 9mm luger +P, 38,500 psi and +P+ a MAP of 40,000 psi. It would be my contention that as the pressure rises in the approved loads so would the pressure rise when the bullet is set-back. Some of that issue would be difficult to measure as the bullet would mask the transducer which measures the pressure of the round. Good taper crimp will help, poor will allow set-back during feeding, too much will not allow the round to headspace properly.

Shoot Straight!
Coy Getman
2299 Snake River Ave.
Lewiston, ID 83501
Sr. Technical Coordinator
Alliant/Blazer/CCI/Speer Bullets & Ammunition
(800) 379-1732
 
I plugged a speer 124g bullet and 4.2g Universal into Quickload. col was 1.169 22,793psi
1.119 was 27,583
1.069 was 35, 286 well over the 9mm max of 33359
 
Again, thanks guys - some very good food for thought there. Thanks in particular JohnMoses for the Quickload data!

I had a further play with the barrel out of the Kimber last night and found it functioned/hand extracted just fine (no hint of binding...) at around 1.110.
A bit finicky to be honest...

I ended up playing with a conical nose pill which would have helped but am thinking I'll just grab some 115gn LRN pills to make load development straight forward, given there are OAL's listed at 1.1 for that bullet. Berry's 115gn RN was just fine at that length, however we are unfortunately not allowed plated/jacketed rounds at our range....dammit!!!
 
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The main problem with the QuickLOAD data in this situation is it assumes the powder ignites then starts building all the pressure until the start pressure is reached all by itself. But in cartridges with a small powder space, the primer can unseat the bullet ahead of the powder burn getting fully underway, which throws the calculated estimates off. You can't count on that happening; especially not with a very fast powder. However, it does tend to happen in many loads. Even the .22 Hornet is famous for erratic velocity problems because of this, requiring very mild primers be used in it. In any event, this explains why a lot of people who've had bullets set deeper by feeding in semi-autos don't have a case burst. On the other hand, just because we know about this doesn't mean you should tempt fate by counting on it to happen every time.
 
Well there you go eh? Berry's plated RN are spot on at 1.125", meanwhile a LRN at 1.125" won't even drop in the chamber properly!! Back to the projectile-buying drawing board...
 
Seb7701, not sure what your goal is, but I wonder if you're overthinking this.

A bullet can be loaded to any overall length you desire as long as it fits in the magazine and chamber and feeds reliably. If it is seated to less that what it was in the load guide, then reduce the charge to account for the shorter OAL.

Bullets with short noses might end up being loaded short. For example, Sierra lists the OAL of their 115 JHP in the 9mm as 1.050".

You might find this article of interest: http://38super.net/Pages/Bullet Design and Feeding Reliability.html

Check out figure 13.
 
75A95 wrote:
If it is seated to less that what it was in the load guide, then reduce the charge to account for the shorter OAL.
Click to expand...

So, how much should the OP reduce a powder charge of a powder claimed to be equivalent to Universal for a 0.025 inch reduction in OAL?

And are you going to cover his medical bills if the result turns out to be closer to the Speer experience than what Ramshot reported with Zip?
 
noylj wrote:
However, pressure goes up exponentially.
Click to expand...

The rate at which smokeless propellants generate gas is governed by a differential equation with pressure and temperature components, not an exponential function, so the rate of change of pressure generation is changing as the conversion process proceeds. The equations are unique to each powder and if the ending data point is selected after the rate of change has dropped, then linear interpolation will result in values that are LOWER than actual.
 
hdwhit . So, how much should the OP reduce a powder charge of a powder claimed to be equivalent to Universal for a 0.025 inch reduction in OAL?
Click to expand...

hdwhit, you over interpreted what I posted.

The Ramshot data was not posted to suggest that it was the same as the OP's powder, and I never said that it was. It was posted to show actual measured changes in pressure as OAL changes.

People should contact the powder manufacturer and ask for their advice.

See my earlier post,
We (usually) don't have a way to measure pressure, so we can't answer your question directly, . . .
Click to expand...
 
Thanks for the ongoing thought AND charts gents, seriously.
Yep, I am probably overthinking, but the issue is this - due to our range, I am somewhat limited to a local manufacturer's non-jacketed pills, which seem to work fine for most others around here, although there are more STI's than Kimber I suspect...

So far, they have to be seated a LOT less than say, Berry's plated RN which will function nicely at OAL 1.15, whereby the local pill needs to be just UNDER 1.125".

Now my definition of 'function nicely' revolves around the case seating in the chamber in the same way that an empty re-sized case would, i.e. the 'plunk' test.
Just got some 125gr conicals and even they are just o.k. at 1.125" which is the shortest documented OAL I have for that kind of pill. Slide seems to close and more importantly (hand) extract without issue, despite chambering just a fraction further out than an empty case would.

I know, it's obviously a finicky gun and possibly less than perfect projectiles... Argghhh!!!!
Everything's better with photos right? Here we go. The 1.084" is the seating depth required for the rear of the case to sit nice and flush in the chamber. Meanwhile, the Berry's does that just fine at 1.125!!!
 

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