Help me really grasp C.O.L. and Ogive please

[bikemutt]

I'm trying to gain a better understanding of why Catridge Overall Length (COL) is not as important to seating depth as knowing length to the ogive of any given bullet. Maybe the best way to ask my question is to state what tools I have now and how I use them. This way, if I'm overlooking something, misunderstanding something, or just plain doing something wrong, it should be easier for the experts to help me get on the track.

I've got a Hornady OAL gauge and one each of their specially threaded brass cases for every rifle cartridge I load for. I also have a set of Hornady bullet comparator bushings.

For any given rifle and projectile, I use the OAL gauge and measure both the COL and the length with the comparator bushing which should tell me the length to the ogive.

I subtract the amount of bullet jump I want to start with, say .020", from these lengths in order to come up with a seating depth. I use the same projectile which was used in the OAL gauge to seat the first bullet.

Here is where I run into the weeds. To my way of thinking it really doesn't matter if I seat the bullet to a measured COL length computed above, or to a measured length to ogive computed above, after all, I subtracted the same bullet jump from both numbers. In other words, if I seated the bullet using either length, the other length should should also be met.

What am I missing here Team?

Thanks

 

[Walkalong]

The ogive is what hits the rifling. You can't get any meaningful measurement using the tip. SAMMI establishes OAL's to be sure cartridges fit magazines and are a reasonable lengths.

On good bullets the ogive is very consistent, where the length may not be. Hollow points generally are not, and since the tip is not important for accuracy, it didn't matter. Nowadays with everyone trying to shoot long distance, the tip is more important, not for raw accuracy, but for consistency in long range trajectory, which makes hits easier.

Unless you are loading to fit the mag, the OAL of the cartridge is not important as long as it can chamber without jamming into the lands.

 

[otisrush]

I struggled with the same question just a couple of months back. After getting my bullet comparator I pulled out a box of my ammo and confirmed:
* When measuring COL measurements would vary +/- .005" or so.
* When measuring Base to Ogive using the bullet comparator *ALL* cartridges kept within +/- .002"....and the vast majority were within .001".

Measuring to the ogive showed me it was a MUCH more consistent measurement. And gave me confidence the variability in my ammo was not as large as I feared when I'd been measuring to bullet tip.

OR

P.S.: It's my understanding that variability in the manufacturing process is what causes inconsistent tip measurements.

 

[Sunray]

The length to the ogive is about the off-the-lands stuff. You can forget the off-the-lands stuff altogether until you have worked up a load. It's a load tweaking technique that makes little to no difference. It's also a 100% trial and error thing as all rifle chambers are slightly different and everyone of 'em prefers a different distance off the lands. There is no math formula to find what length a particular chamber likes either.
OAL's given in manuals are SAAMI spec and have nothing whatever to do with ogives. SAAMI spec is from the pointy part to the flat part only.

 

[bikemutt]

I struggled with the same question just a couple of months back. After getting my bullet comparator I pulled out a box of my ammo and confirmed:
* When measuring COL measurements would vary +/- .005" or so.
* When measuring Base to Ogive using the bullet comparator *ALL* cartridges kept within +/- .002"....and the vast majority were within .001".

Measuring to the ogive showed me it was a MUCH more consistent measurement. And gave me confidence the variability in my ammo was not as large as I feared when I'd been measuring to bullet tip.

OR

P.S.: It's my understanding that variability in the manufacturing process is what causes inconsistent tip measurements.

Fair enough otisrush, I totally get that measuring a set of handloads to the ogives will most likely show less variation than to the tips.

From a practical standpoint though, I don't measure every round once the bullet is seated, I take for granted the press, the die and operator together will make the rounds sufficiently similar to be accurate enough. I do check the first few rounds and there I can see where using the ogives may be a more accurate measure helping to ensure I'm where I want to be.

 

[spitballer]

Our moderator hit the nail on the head, I can only add that with a bolt rifle you might get by without some of that equipment by using the lands to seat the bullet (not a useful method for a rugged autoloader).

 

[Paladin7]

Here's a great video on it, using Hornady products for the example...

 

[otisrush]

Fair enough otisrush, I totally get that measuring a set of handloads to the ogives will most likely show less variation than to the tips.

From a practical standpoint though, I don't measure every round once the bullet is seated, I take for granted the press, the die and operator together will make the rounds sufficiently similar to be accurate enough. I do check the first few rounds and there I can see where using the ogives may be a more accurate measure helping to ensure I'm where I want to be.

Understood....and agreed. The situation was just bugging me. I couldn't figure out why there was variability of upwards of .01" in a batch when I wasn't changing anything: The fact this was happening first made me question my equipment, or bench setup, or something else that might be problematic. From a practical perspective that variability won't change much, if anything, noticeable.

 

[RecoilRob]

A good bullet seater will also push the bullet from a point on the ogive...not the tip. (rifle ammo only) Thus the OAL can vary a bit depending on the very tip of the bullet but the important part...the ogive....will be very consistent round to round.

 

[pert near]

I've read a lot of discussions where people post how many thousands of freebore they seat off the riflings. These thousands of an inch tolerance are close to impossible to keep consistent from one cartridge to the next. Due to brass spring back alone there can easily cause a variance in headspace of a few thousands in a batch of ammo. As an experiment, especially if you are shooting hunting bullets, use your comparator to measure a few bullets from butt to ogive. You might be surprised at the variance. Luckily the ideal bullet seating depth sweet spot is a "range" & not so critical once you have the load worked out.

IMHO...FWIW

 

[bikemutt]

I've read a lot of discussions where people post how many thousands of freebore they seat off the riflings. These thousands of an inch tolerance are close to impossible to keep consistent from one cartridge to the next. Due to brass spring back alone there can easily cause a variance in headspace of a few thousands in a batch of ammo. As an experiment, especially if you are shooting hunting bullets, use your comparator to measure a few bullets from butt to ogive. You might be surprised at the variance. Luckily the ideal bullet seating depth sweet spot is a "range" & not so critical once you have the load worked out.

IMHO...FWIW

pert near, I'm really trying to steer clear of the "close to the lands" ship on this one, after all, we know what happens when a ship gets too close to land

I'm not really concerned with bullet jump, distance to lands or whatever we call it. I'm wondering why, no matter what the length, is the ogive more important to calibrate the press/die to than COL when I'm using the same bullet I used to measure both lengths in the first place?

 

[Hardtarget]

I've read much about this...and still feel like I'm in the weeds. Most of my loading lately has been for my AR. No matter what or how I measure the cartridge cannot be more than mag length...right? So, I've been measuring the tip because the tip is what will rub against the magwall. I don't load to max length and I don't load to max powder charge. I'm not in the competition group so I need my gun to fire and function.and be reasonably accurate. So far I haven't come home from the range crying. I know...many will laugh at my reasoning here but this has been working well for me. Maybe one day I'll get into something that will push me further into the real effort of high-end loading. I'll have fun with it as is for now.

Mark

 

[Laphroaig]

I'm wondering why, no matter what the length, is the ogive more important to calibrate the press/die to than COL when I'm using the same bullet I used to measure both lengths in the first place?

Your setup logic seems sound to me, as long as your test bullet is representative of the lot.

To my way of thinking it really doesn't matter if I seat the bullet to a measured COL length computed above, or to a measured length to ogive computed above, after all, I subtracted the same bullet jump from both numbers. In other words, if I seated the bullet using either length, the other length should should also be met.

There are three points of measurement going on, 1) where the bullet ogive contacts the rifling, 2) where the seater plug contacts the ogive, and 3) where the bullet tip ends up after it is seated (COL). Variations in the shape of the ogive will result in slight differences in these 3 points of reference.

What measurement is most important depends on what you are trying to accomplish. If its mag length, than COL rules. If you've found some magical sweet spot for jump to the lands, then the ogive measurement is most important. But you are seating the bullet using a third point of reference controlled by your seating plug. In the final analysis, if you realize the bullet shape is not always perfect, there will be slight differences between the 2 depending on your point of measurement.

Unless the bullet is total junk, the difference will be so small its not worth worrying about, IMO.

 

[bikemutt]

Laphroaig, I probably should use the OAL gauge with more than one bullet to get an idea of what sort of variation to expect, then maybe take a average. Furthermore, I suppose I should thrust the bullets into the lands with a pressure gauge of some sort, right now I use my Kentucky pressure meter which seems to come up with the same measured lengths every time.