Best seating measurement?

[Skeptic123]

What do you think is the most critical measurement for consistency when seating bullets? Accuracy One feels that measuring from the datum(shoulder) to ogive is the most accurate in that when the firing pin strikes the primer the shell is moved forward so that the shoulder is in contact with the corresponding part of the chamber. The bullet's ogive is moved forward varying amounts depending on how much shoulder bump the case has. Obviously this would change the seating depth relative to the lands. Most other guages (Hornady, Whidden, Redding etc.) measure from the base of the case to the ogive and don't factor in the datum at all.

 

[Alex Wheeler]

That is the most important number. Probably more important for using factory ammo as Im sure theres some variation in base to shoulder datum length. If hand loading your base to shoulder lengths should be within .0005" or better. If not then you should address that first. Thats a good number for base to ogive also. So at worst you shoulder to ogive measurements will have a .001 window.

 

[Bugs]

Here you go. . .

Seating Depth Comparator

The Accuracy One Seating Depth Comparator measures bullet seating depth, ensuring consistency. Compatible with 22 to 30 caliber cartridges.

bullettipping.com

 

[RegionRat]

The uncertainty of Cartridge Base to Ogive (CBTO) versus Shoulder to Ogive (STO) can be affected by your case sizing habits. This is a similar issue with how trimmers work. Some go from the base and some go from the shoulder.

Like Alex points out, imagine an extreme example like a 303 Brit where the unfired shoulder datum is often 0.030" - 0.045" short of the chamber. On the first firing, it would be a big difference to reference from the base instead of the shoulder. Not that the old SMLE was ever accurate enough to know the difference off the rack, eventually the accurized designs for competition with heavy floating barrels proved his point.

As long as you manage the sizing of the shoulder there is not really a big difference in the target outcome between going from the base or going from the shoulder. However the opposite is also true, in that if your sizing process on one batch ends up -0.002" +/- 0.002" in one session, and then is -0.004" +/- 0.004, then the extra uncertainty in that sizing and shoulder work ends up affecting your seating depth measurements. Like Alex points out, if your case shoulder is held to less than a mil, you will have less uncertainty all around.

I'm not able to think of an actual seating die that sets the depth on the shoulder rather than the base, but that shoulder sizing issue would add into the seating depth if there were one.

If there was a conclusion to be drawn, it would be that the case sizing should be well controlled such that if it is used for anything else, like trimming or seating depth, then it too should be well managed. If that case shoulder is well managed, the difference between the two methods of measuring the ogive won't matter much.

It does some good to think about it and discuss it out loud, since the way a cartridge behaves in a chamber upon firing is a reality.

 

[Ccrider]

That is the most important number. Probably more important for using factory ammo as Im sure theres some variation in base to shoulder datum length. If hand loading your base to shoulder lengths should be within .0005" or better. If not then you should address that first. Thats a good number for base to ogive also. So at worst you shoulder to ogive measurements will have a .001 window.

If your bullets are not .0005 consistent, you never get there.

 

[D-4297]

Don't forget to add the variation of Headspace into the equation , which is different in every rifle . Variations of case bump length , bullet base to ogive lengths , and you could have a variance of .003 - .005 , even when doing it all correctly . I think we all try to find that consistent "mean" point to minimize those variations , and adjust our seating depths accordingly .