When determining the OAL (inches) aside from the listed book value, is this based on the projectile length, power type / charge weight (some powders fill the case to higher levels) or other factors? One example would be the below differences according to my Lyman digital caliper.

180gr XTP length – aprx 0.625
180gr IDP # 5 TCFP – aprx 0.630

Any help on the reasoning behind the different OAL would be great
Thanks

Ken

I usu error on the long side ( not past specs for saami ) that functions 100% in the gun

But much also depends on the shape of the bullet so it can feed properly ( exp; flat heads are shorter than round due to needed tipping angle )

I imagine you could write a book on the topic.  However the shortest chapter would be for semi auto guns.  OAL has a lot to do with functioning.  Too long is bad.  Too short is bad.  It varies some by gun and the book value is essentially a best guess....  an educated guess but still a guess.  SAAMI standards help a lot with that but it's not written in stone (especially with revolvers). 

Bullet design has a lot to do with LOA based on how the bullet is seated to length vs pushing into the lands.  I have found that round nose bullets can generally be seated longer than flat nosed bullets.  Another reason is my Glock's will not feed/function WFN bullets seated to "full length".  I have had more feeding problems in my 10 mm Glock's with truncated bullets but very very few with WFN bullets.  But there are enough variables out there that somebody will probably jump in and express the opposite view. 

I think it is fair to say that too long is generally worse than too short.  Reload first with a primary eye on safe repeatable function of the gun.  Worry about velocity and accuracy after that.  Getting it just right has always been much of what keeps me coming back to reloading.  48 years and counting. 

Thanks, I assumed the primary reason was feeding reliably, but was also thinking it might also have something to do with how  the powder is being compacted when using larger flake powder along with a heavier charge

Reliability may be a reason, but any gun I have that won't feed rounds loaded to 1.260" goes on the "needs improvement" list.  And I have a few.

For me, it is simple physics.

I want the average pressure during the firing cycle to be as high as possible.  A larger initial powder space filled to pressure means the average pressure will stay higher longer. In a closed system, P1V1 = P2V2.  if I increase the initial volume, and maintain the same pressure, then the final pressure will be higher since the final volume is fixed.  This increases the average pressure, resulting in more velocity.

First is tipping angle and bullet nose shape, as the cartridge strikes the feed ramp, it is sliding up till it makes contact with the upper chamber.  The position of the slide as it is pushing and its return speed with relationship to the tipping angle as the casing head starts to slide up the breech can be affected by damaged extraction rims.
The tipping angle changes slightly with the COL changes.

I load at 1.2500" for most everything, and 1.2450" with the WFN.

Thanks Shadow, I am learning a great deal thanks to all of you. I hope to be able to help others as I learn more

Ken