Depth of Bullet

The big difference in published oal's for 9mm v.s. 45 acp, is because the 9mm is a much smaller, much higher pressure cartridge, thus very sensitive to variations, accordingly. With the low pressure 45 acp cartridge, one may not even notice a difference in pressures with a .010" - 015" oal variation, which is not the case with 9mm.

And although I can appreciate your extensive experience reloading (40+ yrs.), it appears you may not have much hands on experience with small high pressure AL rimless cartridges such as 9mm.

GS

No disrespect taken, nor intended. If I did offend you, and I apparently did, I sincerely apologize for having done so.

But with respect to 9mm, in my Speer # 10, they deliberately seated a bullet .030" deeper in a tested 28,00 cup load, pressures more than doubled @ 62,000 cup. This is why it's paramount with cartridges such as 9mm, to account for variances in press flex or olgive variation by seating to the longest oal the firearm will accept / function at by using the plunk test to find zero to the lands. And to start work ups from a safe, low charge from the tables. I don't have as much experience as you do, and I respect that, but I do have over 30 yrs. at reloading. But in that time I have yet to come across an AL firearm that was throated shorter than published oal's would allow for, thus I have always been successful with the above method of locating an appropriate oal in this respect.

And to reiterate an earlier reply I made, I completely understand what you are getting at, and it does make complete sense as well. But there are other factors, as have been discussed and outline by others here, that can and do impact pressures. This is why published oal's are a good safe and standardized reference point to utilize. If we could effectively and accurately measure how much of the projectile is inside the case, and with certainty, I'm certain our reloading books would function on that platform, but they don't.

Many of the books I have introduce the 9mm as being extremely sensitive to seating depth variances of as little as .010", so unless we have the expensive pressure testing equipment the manufacturer's use, we really shouldn't try to second guess or reinvent the standardized methods of determining appropriate seating depth measurement of "Cartridge Over All Length" at least not without an increased risk of a catastrophic event. And although I can and have thought of an accurate way to do it, it still requires taking an OAL measurement to determine such, which is very redundant in nature.

GS

Do your own testing

Here is just a small example of 9mm bullets I have reloading data for. I have several manuals to cross check data with and also have many notes I added through my testing with 3 different 9mm pistols, all with a different length chamber.
Sierra 125gr FMJ OAL= .574-.576, bearing surface(BS)= .290-.292 (Base to ogive length)
Hornady 125gr HAP OAL= .571-.573 bearing surface= .306-.309
Nosler 125gr JHP OAL= .543-.547 bearing surface= .324-.327
Sierra 125gr JHP OAL= .545-.548 bearing surface= .339-.341
I just received a new shipment of Sierra JHPs and BS lengths are longer now at .346

My testing over a chrono has proven different bullets of the same wt, same charge, primer, etc does not = same velocities. There is a big difference in BS between these listed bullets, especially the two Sierra bullets. Looking at the OAL of the two Sierra's, the FMJ is about .030 longer but seating depth is only .015 difference in the manual. This would lead you to believe the base of the S125 JHP is seated .015 deeper in the case than the S125 FMJ, thus acheiving the same or close to the same velocity in the manual.

I did the plunk test and the S125 FMJ are seated to 1.165 in my Glock. For the S125 JHP to pass the plunk test, seating depth = 1.122. This is because the BS is extended much closer to the point of the bullet than the FMJ. So bullet profile does affect velocity based on the BS length.
1.165 case length has given the FMJ the best accuracy where the JHP is seated to 1.090.

I may have missed this in all the replies, but;

The 9mm case thickness increases quite a bit as you get seated down to a depth around .240 or so, depending on the headstamp. The internal case volume is NOT linear.
A very small increased seating depth can cause a very high pressure increase.

HACKSAW a 9mm case vertically down the center. Look at it and measure the thickness of the brass. Different headstamp cases will show different results.

Just a consideration worth mentioning.


Another thought about pressure being the only key to bullet speed. Not really so. As the powder level drops, so does the "amount" of gas produced. Yes pressure can be the same, but in a smaller space there will be less gas. When the bullet travels, the pressure will drop off sooner. To get the same bullet speed using less case volume will take more pressure.

.030" deeper in a tested 28,00 cup load, pressures more than doubled @ 62,000 cup.

Click to expand...

When I first read about this, I could have sworn is was 0.30" deeper. I wonder what powder could make this much difference with 3 hundredths of an inch difference?

Did they seat one 190 gr bullet to 0.83" and one to 0.80" ?

Speer didn't specify what the seating depth was, but assuming you have an original depth of say 1.115 producing a nominal operating pressure of 28,000 cup, increasing the seating depth .030", thirty thousandths, .03, or three hundredths, would result in a depth of 1.085" with an operating pressure that more than doubled. They were demonstrating that .030" isn't much, yet such a small deviation created a very significant increase in operating pressure. The specifics of the load weren't discussed, other than the fact it was a 9mm cartridge, so a 190 gr. projectile would not have reflected a realistic load combination.

And yes. .03" can be referred to as three hundredths of an inch, but for the sake of reloading we often work in thousandths, as seating depths are published utilizing the third digit behind the decimal. This is due to relevance, and necessity.

GS

Here is some data i collected at the range over time. It shows that with all the rest being the same and reducing OAL that Velocity does change thus pressure.

I really want to study that data collection Lefty, but I can't seem to open that file. I live in a rural area, so my provider limits how much I can down load with sort like an on, and off peak hours of electric providers / service does. Yesterday they sent me an e-mail and test informing me I had exceeded the day time limit. If you can post it as regular attachment, I'm fairly sure I could open it. But then again, I'm pretty computer challenged and it could just be I'm not doing something correctly?

GS