Cartridge Overall Length - a definitive explanation please

[ColinR]

I have been reloading for years, pistol then rifle, all good accurate stuff, but I could have sleepless nights over COL.

I use RCBS Competition dies in .308 and .338LM mostly with Sierra Matchking. Measuring the length of SMK's varies considerably due to the often malformed metplat. I usually adjust the die by easing the bullet down in increments and measuring COL until I get to, say, 2.800" (.308) and leave it at that. Due to differences in bullet length everything (COL) subsequent is plus or minus a few thousands of an inch, but presumably the ogive is in exactly the same place.

Given these inaccuracies in bullet length why do manuals all refer to COL when base to ogive is a far more accurate and definitive measurement?

Although I have the Hornady kit for measuring to lands and base to ogive I rarely use them and rely on getting a basic COL measurement and working from there. Depending on the length of the first bullet used for setting the die the ogive may be a few thou forward or a few thou back from the previous batch. Having said that there seems to be no discernible effect on accuracy in my AI so I have no need to worry, but the question still niggles.

[User702]

I had the same problem with my loads for .308 and was forever fiddling with the seating die to try and get things consistent. When I mentioned this to Ovenpaa he handed me a 2 piece comparator like you use and told me to use that instead. That works.

As for why people still use COL, dunno. Maybe it's just there as a rough measurement or an aid to Quickload-type calculations.

[phaedra1106]

Base to ogive is more accurate but as bullets have a huge variety of shapes and sizes measuring to the ogive could give you a round which is longer than the Saami COAL. so may not fit in certain magazines or rifles.

Using either A-Max or Nosler CC bullets my Steyr likes a longer COAL, 2.857", this puts the ogive at 40 thou off the lands. However, while my Howa will chamber those rounds fed singly they won't fit in the magazine. In my case, rather than load at two different seating depths I just load to Saami spec at 2.8", the Steyr is quite happy with them but doesn't group quite as tight as it does with the bullets set .057" further towards the lands.

[ovenpaa]

Base to Ogive is the most accurate way of measuring your own loaded rounds once you have determined what this measurement needs to be. The downside is this measurement will vary between measuring systems so manufacturers published figures are based on the cartridge head to meplat (Over All Length)

[DW58]

Article from Berger Bullets (PDF):

Effects of cartridge over all length (coal) and cartridge BasE to ogive (cBto) By: Bryan litz

This may help.

[ColinR]

Article from Berger Bullets (PDF):

Effects of cartridge over all length (coal) and cartridge BasE to ogive (cBto) By: Bryan litz

This may help.

Wow! That is absolutely the best explanation I have read. Makes superb sense and clearly explains the shortcomings of COL opposed to measuring base to ogive. It is pretty much what I think I already knew, but a very elegant explanation nonetheless. I am grateful to you for ensuring my sleepful nights until the next niggle invades my mind.

[DW58]

Glad it helped - I only found the article on Monday evening and saved the PDF, knew it would come in handy but not so quickly. I learnt a huge amount from reading it and plan on implementing it when my .300 WSM rifle arrives and I start working up loads for it.

[ColinR]

Rob, thanks again, a good find and an article all reloaders should read. I makes me think I should take more measurements, but I think sometime it is easy to miss the plot chasing the impossible when you can already load rounds that outshoot the shooter. Colin

[DW58]

You're welcome, but it's really down to Berger Bullet and Brian Litz.

[ColinR]

Reading the article Rob provided and the reference to different gauges (comparators) giving different results can someone explain the dimensions of the comparator calibre gauges i.e. is the hole in the gauge that engages the ogive based on the dimensions of the lands and why would they be different for gauges of the same calibre from different manufacturers - is there an optimum?