COL vs base of bullet inside case measurements

[length_of_pull_1]

I have been reloading for several years now and have come to wonder why we don't think of things in a different fashion. When thinking about straight walled cartridges, wouldn't there be some advantage to calculating powder charges relative to how deep the bullet is inside the case? For instance if you take 9mm-: case .750", 125gr lrn .597", col I've been using 1.105", so, an overall calculation of 1.347" minus my col leaving .242" inside the case mouth. Wouldn't any 125gr projectile with its base seated to the same depth yield the same result in velocity? So instead of saying this particular shape bullet at this col works well at this certain powder charge and supplying data that way, wouldn't it be easier to calculate the base of the bullet inside the case mouth and realize that if it fits your guns chamber length wise any shape projectile of the same weight would work well at the same charge level? Thanks in advance guys for entertaining my kookiness

 

[Walkalong]

I log bullet lengths as well as the OAL of the loaded round so I can always do the math and compare the depth in the case if I need to.

 

[mdi]

I believe early reloaders did use "seating depth" instead of over all length. In my "Complete Guide to Reloading" by Sharpe (copyright 1937-1952) there are loads given but no OALs. For each bullet/charge there was a "seating depth" listed. For 30-06 for example, a 153 gr M2 bullet was to be seated .250" and a 173 gr, "M1" bullet seated to .420". Same with handgun rounds, listed for a 357 Magnum using a 158 gr. cast bullet seating depth was .340"...

 

[Walkalong]

I believe early reloaders did use "seating depth" instead of over all length. In my "Complete Guide to Reloading" by Sharpe (copyright 1937-1952) there are loads given but no OALs. For each bullet/charge there was a "seating depth" listed. For 30-06 for example, a 153 gr M2 bullet was to be seated .250" and a 173 gr, "M1" bullet seated to .420". Same with handgun rounds, listed for a 357 Magnum using a 58 gr. cast bullet seating depth was .340"...

Makes sense, they did the math for us.

 

[cfullgraf]

Wouldn't any 125gr projectile with its base seated to the same depth yield the same result in velocity?

There are more variables that affect velocity than just seating depth. Bullet materials, construction, shape, bearing surface, and design are a few. I'm sure there are more.

At least with cartridges for semi-auto guns, cartridge over all length can be more critical for function and seating depth of the base is just what you get. Different design bullets have a different COL needs for reliable function.

 

[Obturation]

Valid point but missing / incomplete info I think. Due to variation of chamber throat certain profiles need different seating depth. Beyond that, bearing surface will effect pressure / velocity . so assuming all things are equal and length ends up the same and the profile and bearing surfaces are the same, velocity should be close- but every bore us different too. Too many variables but like @Walkalong said, good info to have so you can determine a load that's close. Unfortunately nothing is ever the same and testing is always nessescary anytime anything changes- including powder lot.

 

[Varminterror]

Wouldn't any 125gr projectile with its base seated to the same depth yield the same result in velocity?

No.