OAL & Pressure

[ES13Raven]

How much of a difference does OAL make on pressure in 9mm?

For example, a 147gr loaded to 1.100 OAL vs 1.135 OAL?

I know it is powder specific, but lets say TiteGroup for example.

[Hi-Power Jack]

My guess is that except for extreme examples of OAL difference, or

for powders that fill the case, there is a small difference .

Probably so little TG in the case, that there is a large air space no

matter how long or short the OAL is (except for extremes).

BUT, best way to proceed is to lower your powder charge if you drop

the OAL and work back up with a chrono.

Good luck with it.

[ES13Raven]

My guess is that except for extreme examples of OAL difference, or

for powders that fill the case, there is a small difference .

So pressure is only an issue with powders that fill most of the case?

[Hi-Power Jack]

Depends on the powder, but I think any powder that we normally

use for 9mm minor probably is not going to be much of a problem.

If you use e.g. 5.0 gr TG at 1.35", I can't imagine you'll run into any

pressure problem by dropping the OAL to 1.10", because even at

1.10" you have a lot of air space ...

May be wrong, but I'd be safe by dropping that to 4.5 gr TG at 1.10"

and shooting thru a chrono.

[Beastly]

I think you will see both a velocity increase and a significant pressure increase. I definitely backed my load off when reducing from 1.135 to 1.085" and using a fast powder (comparable to TiteGroup) @ 134PF. Out of maybe 15-20K rounds I've seen 3 case blowouts but I'm not sure if this is the cases (e.g. Glocked range pick up brass with a weak spot) or whether I am that close to the edge pressure wise.

Find someone who has access to the ballistic software, QuickLoad I think, and run a quick check.

[IDescribe]

As the bullet is seated deeper and deeper in the case, the initial size of the combustion chamber decreases, which increases the pressure in the early stages of the burn, which increases peak pressure. You can assume that if you are getting higher velocity for the same powder charge with the same bullet in the same pistol in the same brass, that peak pressure has increased.


You are unlikely to find a user here who has access to and has run an OAL pressure test on a piezoelectric device, so no one knows what the pressure changes are.

[9x45]

You can't measure pressure directly without a test barrel setup, either piezo or copper crush. The difference between powder volume is a function of density, however, when the combustion chamber volume is reduced, the pressure increases. Problem is pressure is not a linear function and you don't want to be on the knee of the curve where a small change causes a significant increase. Since the chrono only measures velocity in a linear function, it won't give you a clue to higher pressures, only visual exam of the primer and case, and that is just an indicator.

[Chutist]

You can't measure pressure directly without a test barrel setup, either piezo or copper crush. The difference between powder volume is a function of density, however, when the combustion chamber volume is reduced, the pressure increases. Problem is pressure is not a linear function and you don't want to be on the knee of the curve where a small change causes a significant increase. Since the chrono only measures velocity in a linear function, it won't give you a clue to higher pressures, only visual exam of the primer and case, and that is just an indicator.

THIS...

Not evidence but... there are widespread antidotal reports of OAL being a critical factor with some fast powders. Clays in .40 for example.

I wish I had access to a test rig to verify some of the reports.

Edited March 1, 2016 by Chutist

[Miranda]

hi Forumers...

from all I have read, the powder you pick is the answer to OAL changes

and pressure changes that result.

from here on I am sure we will hear a lot of 'yeah but' and the like.

to explain why that answer (always a BIIIIG mistake)

lets look at some magical powders.

The ideal powder burns up to some useful pressure and then stops burning

and when the pressure drops a bit, it finishes burning.

it will never over pressure.

the opposite of that powder...

burns faster as the pressure rises and accelerates the burn rate in some geometric progression.

this is a case of a little extra pressure or a little extra powder is way over pressure.

this second magic powder is often called 'an explosive.'

Keep in mind, "a little extra pressure" is created by the case volume (or lack of it) that comes from a shorter C.O.L.

I do not know of a gun powder that does not burn faster as the pressure rises.

it may exist, I just do not know that detail.

end of magic powders....

I have loaded with Clays. It makes me nervous because it does burn fast and a little extra powder

is something the manufacturer warns against.

I lack the confidence in my process and equipment to hold the tolerances I feel are required.

once the bullet is moving, the pressure peak is far less of a concern.

the amount of time from ignition to bullet moving is where the speed of the powder

and peak pressure are in play.

miranda

edited to enhance clarity...

Edited March 1, 2016 by Miranda

[ES13Raven]

Not evidence but... there are widespread antidotal reports of OAL being a critical factor with some fast powders. Clays in .40 for example.

Is the first sign of too much pressure, flattened primers?

[Miranda]

the flattened primers is one way.

the soft primers will flow and flatten sooner.

my clays loads were variable. Some were flat and others looked .... normal.

That was when I decided to hold off on the clays for my 9mm ammo.

I expect I'll try clays for the 45 when I get to loading them.

miranda

[9x45]

primer flow, then flat primers, then primers that look almost 50% bigger, hammered head stamp, small bulges at the base. Lots of Hosers run TiteGroup and 147s at those OALs. What's your charge weight? We just chron'd one of the guys who went from 1.13" to 1.1" to get more juice out his loads without adding powder, and it recorded about 80 fps faster with the shorter OAL so you know the pressure had to go up. If your'e running book loads from Hodgdon, I would doubt you are anywhere near over pressure.

[9x45]

Here's a link to burn rates, Clays is #10, Bullseye #13 and TiteGroup #14. You have to be real careful with hot powders and heavy bullets. The oldest .45acp 230 grain load on the planet is 5 grains of Win 231, # 29 in hotness.

https://www.hodgdon.com/PDF/Burn%20Rates%20-%202015-2016.pdf

[ES13Raven]

primer flow, then flat primers, then primers that look almost 50% bigger, hammered head stamp, small bulges at the base. Lots of Hosers run TiteGroup and 147s at those OALs. What's your charge weight? We just chron'd one of the guys who went from 1.13" to 1.1" to get more juice out his loads without adding powder, and it recorded about 80 fps faster with the shorter OAL so you know the pressure had to go up. If your'e running book loads from Hodgdon, I would doubt you are anywhere near over pressure.

Running 2.9 grains of Clays with 147gr coated Ibejiheads. I was running them at 1.110 OAL, but I now have my barrel reamed so I can run them longer. No issues so far, but I know I have to be careful with Clays.

[Scrmblr]

I run a 124grn round nose coated bullet, with 3.8grn TiteGroup, at 1.11 OAL. It makes 1095-1100-fps with no feeding issues in my 1911 or Tanfo Stock 2.

[JoeDean]

This from the Speer reloading manual #10: "If the bullet is easily moved deeper into the case, malfunctions may occur. But more important, loads that produced 28,000 cup went to 62,000 cup when bullets were purposely seated .030"deeper!"

[Red Ryder]

This from the Speer reloading manual #10: "If the bullet is easily moved deeper into the case, malfunctions may occur. But more important, loads that produced 28,000 cup went to 62,000 cup when bullets were purposely seated .030"deeper!"

Definitely not a proportional equation. Shortening the oal can exponentially increase pressure. Causing an explosion, a broken Gun, and possibly a trip to the ER!

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Edited March 6, 2016 by Red Ryder

[bigbob21]

This from the Speer reloading manual #10: "If the bullet is easily moved deeper into the case, malfunctions may occur. But more important, loads that produced 28,000 cup went to 62,000 cup when bullets were purposely seated .030"deeper!"

Definitely not a proportional equation. Shortening the oal can exponentially increase pressure. Causing an explosion, a broken Gun, and possibly a trip to the ER!

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Daym guys... Just ordered a 650 from benos with all the books and DVDs...

So is the advice to load as long as possible for initial testing(to be safe), check if it feeds and chrono's well , and then go shorter later if saving powder is the goal?

That's what I'm getting from this thread....

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[Miranda]

this is hard to explain...

you really want to meet specifications.

the guys here say you can get improvements by changing

the specifications or recipes to match your pistol.

To make such changes, you should know why the change and what the change affects.

Then you need to know which changes are less safe.

You have listed the direction of safer as I know them.

as an example...

You want to be sure when you are near pressure limits that

you are sure you do not have bullets that set back into the case.

... or have extra powder.

it is a lot to absorb.

miranda

[Guy Neill]

The type of powder will not be too much of a factor. The amount of gas, combined with the internal volume is what counts.

The relationship is p1v1 = p2v2 (this stems from the gas law of pv = nRT, but nRT is constant if we are pushing the bullet in deeper)

So, if p1 = 30,000 psi in a given volume v1, then reducing that volume will raise the pressure assuming a given amount of gas. Since the bullet begins to move under pressure, the volume enlarges, reducing the pressure.

However, if you seat the bullet deeper, the initial internal volume is reduced.

I don;t know the internal volume of a 9mm Luger, but I know it is fairly small, making it a worst case scenario for pushing the bullet deeper.

If we take the initial volume, v1, as coming from a cylindrical area, then v = πd^2/4. d = 0.355 and, for the sake of argument, we'll assume l = 0.25".

This gives v1 = 3.14 * 0.355^2 * .25/4 = 0.0247 in^3.

If the bullet is pushed deeper, the new volume is as above, but with the new, shorter length. Let's assume the bullet is pushed back the 0.030" mentioned in an earlier message. The means the length of the volume reduces such that l2 = 0.25 - 0.030 = 0.22" This gives v2 = 3.14 * 0.22 * 0.355^2/4 = 0.0217.

Back to the original relationship, p1v1 = p2v2, solve for p2, giving p2 = p1v1/v2 = 30000 * 0.0247/0.0217

p2 comes out to a bit over 34,000 psi

If the initial internal volume of a 9mm is smaller, the effect will be greater than in this set of assumptions.

Bottom line - the OAL is important and you should not simply push the bullet in deeper without dropping the load and working it up again.

Guy

[IDescribe]

Okay, the type of powder will be a MAJOR factor. Specifically, the faster burning the powder is, the faster the pressure will climb, and the higher the peak pressure will be. It's peak pressure that's going to wreck your gun and your day.

The more pressure smokeless powder is under, the faster it burns, and the faster it burns, the faster it builds pressure. This is why graphs representing pressure are curved instead of linear. It is a positive feedback loop, and anything you do at the beginning of the burn to increase pressure, such as reducing OAL and shrinking the initial size of the combustion chamber, will result in a geometric increase in peak pressure.

So as the powder starts burning, that functions to increase pressure. Then, as Guy described above, as the bullet moves, it expands the volume of the combustion chamber, which functions to reduce pressure. While the effects of the rapidly expanding gases on pressure outweigh the effects of the expanding combustion chamber, pressures climb, then when the effects of the expanding chamber outweigh the effects of the expanding gases, pressure starts to fall. Then the bullet leaves the barrel and all pressure is relieved.

The type of powder definitely has an effect. Smokeless powders are essentially a lot of nitrogen bound into a solid compound with a number of other chemicals included for a variety of effects. Slower burning powders are just that -- they burn slower and release their nitrogen at a slower rate. This allows you to put more powder into the case, which allows you to get more expanding gases into the process without reaching dangerous peak pressures. The slower powders will not reach the same peak pressures, but they lengthen the time that the effect of the expanding gases outweigh the effect of the expanding combustion chamber, which lengthens the time that the rate of acceleration is significant, which results in higher velocities at lower peak pressures.

Insofar as the Speer book goes, we have some bullet in some caliber of some weight at some OAL. Peak pressure is 28,000 CUP. When OAL is reduced .030, pressure climbs to 62,000 CUP. Fine. So in some perfect storm of components and dimensions, this happened. That does NOT mean that 34,000 CUP pressure changes is what tends to happen with .030 OAL changes. To realize the insanity of that, all you have to do is ask yourself what would have happened if they had lengthened by .030. Would we have seen pressure drop 34k CUP? Of course, not that would result in a vaccum. Would it have dropped even significantly? It would certainly drop a little, but not anything dramatic. The example in that book is certainly a perfect storm. My assumption is that It was a super fast burning powder, it was in a high pressure cartiridge, the bullet weight was heavy for that caliber,it was lead sized large for that caliber, and the depth to which the bullet was inserted into the case was already at or past the "danger zone" for that case. THEN, the .030 reduction in OAL sent things wildly out of control.

Remember that the example in Speer book is a known tested load, and the OAL was then reduced. When you're developing a load, you start with trusted load data, start at the BOTTOM of the load window, and work upward. Using the "starting load" is what protects you. And load data is NOT specifications. It's a report on what they tested. You should determine the optimum starting OAL for your bullet with your pistol. You might later play with OAL to find the accuracy sweet spot for that bullet in your pistol with that powder. You should NOT assume that the OAL in the published data is going to work for your bullet and pistol, nor do the publishers expect you to. Just be aware that as bullets go deeper into the case, pressures go up, and if you have reason to think that is what's happening in your load vs the published data, you want to either reduce starting load or not go all the way to max load. You can also get a chrono and monitor your progress that way, not exceeding the max velocity in the published data. Those are the simple ways. If you work the load up from the bottom, "short" OAL is not something you have to sweat over.

Edited March 6, 2016 by IDescribe

[Red Ryder]

The powder does matter. That's why I was upset that IMR SR 7625 was discontinued. You could seriously over load and over pressure the round witb this powder...... Without blowing up your hand.

[motosapiens]

when working up a defensive load, i found that 5.8gr of n340 at 1.14oal gave a slightly lower velocity than 5.2gr of n340 at 1.08 oal. 6 tenths of a grain less powder, but higher velocity from an oal change of barely a 20th of an inch. That convinced me to tread slowly and chrono frequently when reducing OAL.

[SW39]

Good afternoon all,

I posted this in another thread before finding this closer thread

Was happy to see this thread is pretty close to what I am looking for?

Loading 5.1 gr of CFE-P on a dillion 550 and I set the C.O.L. to 1.151 (+/- 0.003) with 124gr RN Plated from CMJ,

I use range brass I pickup besides my own, but clean, check and sort the brass by head stamp.

So I was loading to the above referenced load with RP brass, when I ran short (only about a dozen left) of RP brass and so I grabbed the box with FC brass off the shelf.

Well being the geek that I am I ran five rounds and checked them, what the heck, they are running 1.144 - 1.148 oal, now, I went back and double check the loads from previous with the RP's and they were all right on at 1.150 - 1.152.

So this being a new load for me that I am working on, brought up a red flag and I am worried about the change in col for over pressure, besides wondering why the case is making such a change in the COL, the only thing I can figure is the weight of the brass is making a looser fit with the head and sliding in more.

I kind a confirmed this when I went back and grab 5 RP brass cases from the box (only about 15 in the box) they ran at 1.151 & 1.152, thus its a case thing on the OAL.

But I am wondering what effect this will have on my loads for over pressure.

Input is welcomed this has got me confused......

[Red Ryder]

None, or close to none at that OAL. That is a normal variation of OAL with mixed brass.

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