So, I've been thinking about something and have tried looking up some info online but haven't found much... My question is, if a reloading recipe calls for let say an AOL of 1.075" on a 9mm round, and I go ahead and make my OAL 1.125", am I effectively reducing the ammount of pressure that is put on the bullet by a major factor when the round is shot? Is there a way to find out by how much, without using a chrony? I was thinking that if a recipe shows you pressure info with their OAL, what kind of formula can I use to figure out the change in pressue with longer OAL I will use?

Not sure that pressure and velocity are linearly equivalent, so short of having a test barrel with a pressure transducer it would be next to impossible to arrive at a formula to do it statically. Plus, different powders would have different pressure characteristics, so, if you could arrive at a formula, it would only be valid for that specific powder/bullet/OAL combo.

 

You must be an engineer or an analytical personality type (often one in the same).

 

Adios,

 

Pizza Bob

You cant tell as far as I know, but you will indeed be significantly reducing pressure without losing significant velocity. Even with a chrono, you wont be able to know the pressure because they arent %100 related.

 

What are you trying to do?

Not sure that pressure and velocity are linearly equivalent, so short of having a test barrel with a pressure transducer it would be next to impossible to arrive at a formula to do it statically. Plus, different powders would have different pressure characteristics, so, if you could arrive at a formula, it would only be valid for that specific powder/bullet/OAL combo.

 

You must be an engineer or an analytical personality type (often one in the same).

 

Adios,

 

Pizza Bob

 

That's what I figured... but thought I'd check with you more experience reloaders anyway. This was just more curiosity, as when I look at some of the recipes that I'd like to use in my reloading books, they list a shorter OAL like 1.075" or 1.095". For my 9mm loads I try to stick around 1.125" which seems to work well in my Berettan 92. By using the longer OAL, I'm trying to figure out if I'm significantly reducing pressure and/or velocity.

 

Another question I think of is, if I use somewhere near max charge for any given powder in a recipe... and you're using a longer OAL than the recipe asks for... are you still really at max charge? I'm thinking that the longer OAL reduces the pressure being built up behind the bullet... is that right?

 

My background is technical and electronics engineering, but I do have a very analytical personality. It's a curse sometimes, because I can be to analytical and anal with details.

You cant tell as far as I know, but you will indeed be significantly reducing pressure without losing significant velocity. Even with a chrono, you wont be able to know the pressure because they arent %100 related.

 

What are you trying to do?

 

Just trying to figure out the effect of increasing OAL, from that which is asked for in a reloading recipe. I need to get a chrono eventualy to try some of my reloads and see what kind of velocities I'm getting.

The assumption would be that with increased interior space, you would have a pressure drop. But again, it may not be that simple. It would be dependent on when the pressure peaks. It may be possible to actually see a velocity increase with a longer OAL (which isn't necessarily indicative of a pressure increase (too many variables).

 

Pardon me while my head explodes.

 

Adios,

 

Pizza Bob

According to my Hornaday Reloading guide, using a longer OAL (seating the bullet not as deep into the shell) will INCREASE pressures and velocity. The reasoning behind this is that the bullet does not get a running head start into the bore, and expanding gases do not have a buffer, as a longer OAL could potentially force the bullet right into the bore/rifling upon the cartridge being chambered. A bullet seated with a smaller OAL will leave some space between the bullet bearing surface and the bore/rifling, allowing a head start and room for expanding gases to move while the bullet gets seated into the bore/rifling.

 

Generally bullets seated with a longer OAL in which the bullet seats into the bore/rifling will be more accurate, but you should compensate for increased pressure by backing down on your powder charge some.

Dan there's a difference between seating too long and creating an over pressure situation and seating longer than the stated COAL and creating less pressure. Creating a longer COAL doesn't automatically correlate to higher pressure. It's dependent on the chamber of the gun and where your rifling starts etc.

a long OAL will only spike preassures if you are jamming the bullet into the lands/grooves

a long OAL will only spike preassures if you are jamming the bullet into the lands/grooves

 

I'm definitely not touching the lands/grooves with the bullet... So, it shouldn't be building up additional pressure.

There is an empirical formula for figuring out pressure due the propellant decomposition (given initial volume, free space, grain geometry, bullet weight, blah blah blah) but you'd have to know what the exact propellant mix was. Mfr's aren't super forthcoming about that unless they have to, with competition and all.

 

There's a lot of good textbooks on interior ballistics out there if you ever wanted to take the plunge.

Yeah this is a valid technique. You will also see some competition guns cut with a longer chamber to allow for longer loading without having the lands problem. This is quite common among certain USPSA guns because some folks like to load very fast powders in .40, which is generally a bit nutty, so they go with over OAL as long as it will fit the mags to lower the pressure, and cut the chamber longer to not run into the other side of the pressure problem.

That's what I figured... but thought I'd check with you more experience reloaders anyway. This was just more curiosity, as when I look at some of the recipes that I'd like to use in my reloading books, they list a shorter OAL like 1.075" or 1.095". For my 9mm loads I try to stick around 1.125" which seems to work well in my Berettan 92. By using the longer OAL, I'm trying to figure out if I'm significantly reducing pressure and/or velocity.

 

Another question I think of is, if I use somewhere near max charge for any given powder in a recipe... and you're using a longer OAL than the recipe asks for... are you still really at max charge? I'm thinking that the longer OAL reduces the pressure being built up behind the bullet... is that right?

 

My background is technical and electronics engineering, but I do have a very analytical personality. It's a curse sometimes, because I can be to analytical and anal with details.

 

I load out to 1.160" for my 92FS and I am not hitting the rifling. SAAMI max OAL is 1.169" so it should not be a surprise. Generally speaking, the longer OAL should increase accuracy as the bullet has a shorter jump to the rifling. Only testing different OAL's within safety limits, can you find the most accurate OAL for your pistol. I tried different lengths and did not find a difference in accuracy. The other consideration with OAL is reliability of feeding. Generally, the longer, the more reliable. These are generalities and may not hold true for your pistol. Make lots of test batches and shoot 'em.

Be careful when using a recipe for different bullet lengths with the same weight. Some bullets are longer than others so OAL needs to be adjusted accordingly to keep the pressure within spec. For example, a 9mm hollow point recipe will most likely have a shorter OAL than a FMJ using the same powder and bullet weight. If the recipe called for 1.07 OAL on a hollow point, you wouldn't want to seat the FMJ to the same OAL if the FMJ is longer. The same OAL would cause the bullet to seat deeper in the case causing higher pressure.

Be careful when using a recipe for different bullet lengths with the same weight. Some bullets are longer than others so OAL needs to be adjusted accordingly to keep the pressure within spec. For example, a 9mm hollow point recipe will most likely have a shorter OAL than a FMJ using the same powder and bullet weight. If the recipe called for 1.07 OAL on a hollow point, you wouldn't want to seat the FMJ to the same OAL if the FMJ is longer. The same OAL would cause the bullet to seat deeper in the case causing higher pressure.

 

Agreed... I always try and seat longer than a recipe calls for... and also don't substitute FMJ recipes for HP bullets. I am totally a stickler for detail and safety is my my primary concern. I am typically using 1.125" for my 9mm loads, which has been working out pretty well in my Beretta 92.