Bullet seating depth / airspace question.

[Bazoo]

With 45 acp I'm loading a Lee TL452-230-2R and a Lee TL452-230-TC all the same but the TC seats deeper and takes up more case volume. With all else being equal, if both have an airspace between the powder and bullet base, what are the effects?

This is a query I have in mind for other calibers such as 38 special and 44 special as well, so please don't limit it to just the 45 acp.

Thanks for looking.

Bazoo

[metricmonkeywrench]

There are a bunch of technical explanations and that weird math that uses letters and numbers to come up with an answer.

To my simple mind the variation in space makes the combustion chamber smaller the deeper the bullet seats. Less space means the highest pressure for that powder charge will be reached sooner

As a practical experiment if you have a chronograph seat something like a Wadcutter further out in the cases in steps and see the velocity drop off.

[Bazoo]

I know essentially the smaller the airspace the greater the pressure. But I'm interested in how much it's really effected. I could probably keep myself busy if I has a chronograph, but I've yet to acquire one.

[JonB_in_Glencoe]

I moved your thread as requested ;)

What I've learned in my 10 years of reading this forum daily, when it comes to reloading cast boolits, is that there is never a condition that is "With all else being equal".

The biggest other factor to answer your question, which you didn't mention, is which powder?
Each powder has it's own characteristics on how it reacts to differing amounts of fill percentage.
Some are case fill neutral, some will get wonky.
Some prefer 100% fill (with no air space), some prefer to NOT be compressed, while others don't mind it.

[Larry Gibson]

Results of seating depth test with 357 magnum, TL358-158-SWC and 5 gr green Dot.

Cartridge OAL...…...PSI as measured via Contender barrel and Oehler m43 PBL

1.632 24,500 psi [bullet driving band jammed into leade]
1.582 23,900 psi
1.532 28,400 psi
1.482 32,500 psi
1.432 35,700 psi

Similar tests with the same bullet and load (for cartridge) but with different seating depths in 32 S&WL, 32 H&R, 9mm, 38 S&W, 38 SPL, 44 SPL, 45 Colt and 45 ACP cartridges all demonstrate the same results. The deeper a bullet is seated the higher the psi.....nothing new there as it's been known for a long time. Also the smaller the case capacity the greater the psi increase will be with a given deeper seating depth change.

[Hickory]

Years ago it dawned on me that there are tens, if not hundreds of thousands of combinations of loads you can get with different bullets, powders, primers, charge weights, seating depths not to mention different combinations of boolit alloy, that a person could spend a life time seeking a accurate load while wearing out your gun(s) doing it. I realized that my life would be one disappointment after another in search of what? Perfection? That won't happen in this lifetime!!!

So, I embarked on another tack, plinking.
Less frustration, less record keeping, more fun, no self condemnation for poor shots, did I say it was a lot more fun and relaxing.

I have already found the boolits and loads the work for hunting and they are the standard for every gun I own, so there is no guess work or load development when the season comes around.

Plinking, it's what old men do for fun.

[mattw]

I do not have pistol caliber data to back up what I am saying... but airspace or lack thereof can and does make a huge difference. My latest confirmation of this was with 9mm rounds. I have several full size 9's and an SCCY CPX-1. I load the Lyman Devastator hollow points for the 9's that may see defensive use. This is a fairly long bullet and my standard 3.5 gr. of 231 that I use in my full size guns works great. But, in the Sccy they need to be seated deeper to be reliable, the chamber in this gun is a little loose and not really well supported given how thin the barrel is. A load that was great in the full size guns, became what I considered not safe in the compact. I only shot 5 for testing and found that the cases bulged and the primers flattened significantly more in the compact. No data... my daughter shot my chronograph the day before or I would have had the data.

[Norske]

Quickload, the reloading software that uses chemical engineering to predict pressure and velocity has proved to be quite close to chronographed velocity in my rifles and handguns. and velocity is directly proportional to pressure. There is no such thing a a "fast" barrel, just a chamber and bore that makes for more pressure.

[downzero]

Pressure = force / area

[charlie b]

With 45 acp I'm loading a Lee TL452-230-2R and a Lee TL452-230-TC all the same but the TC seats deeper and takes up more case volume. With all else being equal, if both have an airspace between the powder and bullet base, what are the effects?

This is a query I have in mind for other calibers such as 38 special and 44 special as well, so please don't limit it to just the 45 acp.

Thanks for looking.

Bazoo


This is why it is important to start with reliable load data. It must reflect the type of bullet that you are using. As you point out, just because a bullet is 230gn does not mean it is the same length as a bullet of another shape/type.

DO NOT rely on data from a forum. Go to the powder or bullet manufacturers' load data. Caution: note what weapon or barrel they use to come up with their data.

[Bazoo]

Thanks for moving my thread sir. The powder I'm using is WSF currently which I know prefers a full pressure load over a light load. But my query is more general for my own understanding than for my specific load development.

I know seating depth affects pressure, and I'm wondering how much when there is an airspace. So I guess it always affects it, but more or less depending on a host of other factors.

With the load I'm using, both bullets leave an air space. The RN leaves more, and with the same charge has only the slightest difference in less perceived recoil. I don't have a chrono yet but I sure need one to satisfy my curiosity.

I appreciate all the responses on this matter in helping me understand things better. Thank you Larry for your response, very helpful.

[charlie b]

FWIW, there will almost always be an air space. And, some powders like less air space than others. Sometimes strange things can happen when you get below ~60% load density. A lot of us will shoot plinking loads in our rifles that have a very small amount of powder in them. The results can be surprising and very accurate.

If you are going to reload a lot and have this kind of curiosity then do get a chronograph. Sometimes the results are surprising. It also helps you to refine your reloading process. When you see a velocity spread drop from 100fps difference to 10fps difference there is a lot of satisfaction that you are doing things well. Larry is one who has taken the extra step and uses strain gauges on his barrels to measure pressures as well. There is also the radar chronograph where you can also measure aerodynamics of your bullets. I wish I could justify the expense of both of them :)