Why different OAL for the same caliber, same bullet, same powder?

[Dannix]

I finally got around to buying some powder to start loading. Got some Universal.

I'm noticing, though, from different sources different overall length numbers for 9mm with 115gr JRN. Why? I can understand a reloader trying to find the longest OAL a mag will feed to bring the bullet closer to the rifling, but why would one manf. go with 1.09, another 1.1, another 1.125 for manual publication?

To further cloud the issue, the two factory 115gn 9mm I have around aren't at these numbers:

• Winchester White Box 9mm, 115gr: 1.16"
  
• Hornaday Critical Defense 9mm, 115gr: 1.08" (not round nose, but somewhat similar profile polymer tip)
  

Thoughts? Why would I choose to load to one OAL than another? (Assuming corresponding levels of a given powder to maintain a constant pressure value.)

[G-ManBart]

Most folks aren't loading longer to get the bullet closer to the rifling, they're doing it to get the nose of the bullet closer to the feed ramp so it's less likely to nose dive when feeding.

Keep in mind that two bullets of the same weight can have significantly different profiles/shapes and what works with one might have the other hitting the rifling (more common with heavy weights). The factories do a lot of testing and one might find better accuracy, in a variety of weapons, at one length while another company using a similar bullet might get different results...they're also likely using different powders so that can cause one to like a little more or less jump than the other.

There are also other factors like how much of the bullet makes contact with the case walls, how thick/thin the case walls are, the exact diameter of the bullet etc....all of those impact neck tension and have to be adjusted for so that setback during feeding is less likely.

All told there are a lot of variables in play and it's hard to say exactly why there are differences without talking to the folks who're coming up with that particular load. R,

[Dannix]

Ah, better feeding makes sense.

I'm talking about for just the standard 9mm 115grn jacketed round nose though, not comparing HPs, FNs, etc, and for a particular given powder, Universal. So same bullet type, same powder.

Good point about perhaps it being an accuracy thing. I just noticed the Lee manual lists a different OAL for each power option inside the domain of a bullet type, while Hornaday lists a constant OAL, regardless of powder choice, inside the domain of a bullet type.

Hogdon/Winchester lists only one load. For WSF. : 1.169" OAL. Maybe this is the WWB equivalent? The OAL would seem to suggest this. Guess if I wanted this OAL (WWB has been feeding flawlessly in my G19), with Universal, I'd simply have to invest in a chrony.

Edited May 9, 2010 by Dannix

[Sarge]

Dannix,

You will eventually want to get a chronograph. I learned shortly after beginning to reload that the chrono is a MAJOR piece of the puzzle. You don't have to spend big bucks either unless you just want to. But if you are going to shoot a sport that requires power factors be maintained then you need a chrono.

Also, +1 on accuracy. While not always a huge difference accuracy can change by changing oal.

[Dannix]

Yeah, it's interesting. Less powder, shorter OAL, more powder, greater OAL. I'll have to get the chrono and start tinkering.

Thanks!

[njl]

Also keep in mind, when you seen an OAL in published data, that's generally the minimum recommended OAL for that data. OAL affects case volume which inversely affects pressure. If they publish 4gr of some powder at 1.100" as their "max load", that just means if you load 4gr of that powder, and go any shorter, you're likely to go beyond the desired limit for pressure for that round. You have the option to load longer, which is going to reduce pressure, likely reducing velocity.