How Vital/Dangerous is out of spec COL

[petemacsydney]

Hi

If your load data states you need a Cartridge Overall Length of 2.750"... is it ok to do longer?.. for example 2,800" or 2,810"?

Obviously I'll need to keep it to a length that fits the mag, but I'm just curious about why a hunting projectile (SST 308) vs a match projectile (Nosler CC's) recommend two different max COL's?

Maybe there is a safety factor? (although not being that experienced I don't know what that could be!)

Nosler CC is 2.815" max from memory and Hornady SST is 2.750" (according to ADI load data that is). cant seem to find data on Hornady site.

Last thing I want to do is damage my rifle, so thought i would check with those in the know???

Cheers,
Pete

[Aster]

Projectile shape plays a massive part in determining COAL.

I've done a quick mockup demonstrating.

(The black lines at the top are meant to be the barrel if that wasn't clear... it's a crap diagram but took 5 minutes, what do you want from me )

bullets-and-coal.gif (3.92 KiB) Viewed 6405 times

As you know ideally you want the bullet ogive just off the lands. The left shows a hunting profile bullet compared to a match profile bullet on the right.

This is how/why COAL varies and why the ideal length can be significantly over the stated max as written by SAAMI or whichever source you're referring too. They're done using 'average' bullets which doesn't cover every scenario.

And so both in the above mockup are 'correct' yet completely different.

Look up Berger VLD and Nosler partition bullets for a good comparison in potential differences. COAL for a long, sharp ogive bullet like the Bergers VLD will always be longer than a short, round ogive bullet like Nosler Partitions.

[Carter]

Are you loading .308 Pete or just using as an example?

[petemacsydney]

Are you loading .308 Pete or just using as an example?

yup, SST 308's

[petemacsydney]

Aster,

Cool, thanks mate. I think I follow that.

Just to confirm I understand correctly though... there should be no problem with having it longer from a danger point of view, but performance may be better or worse dependant on how the rifle/bullet combo works for its COAL?

Is that correct or am I lost?

Cheers
P

[Lorgar]

If your load data states you need a Cartridge Overall Length of 2.750"... is it ok to do longer?.. for example 2.800" or 2.810"?

It's quite common for hand loads to be longer than cartridge specifications state, as Asters diagram explained.

7mm-08 (.308 necked down to .284) COAL is also 2.800" according to the books. My hand loads are 2.820" and I was forced to stop there because that's the longest round my Tikka's magazine will fit. There is still room for me to go longer to get closer to the lands but then I'd have to hand feed cartridges which I don't want to do.

I stopped hand loading for my .308 Ruger a while ago and I can't actually remember what my .308 coal was, but it was above the 2.800" specification by a few hundreds of an inch for the same reason. And I was shooting 150gr SST's

[Tiiger]

Just to confirm I understand correctly though... there should be no problem with having it longer from a danger point of view, but performance may be better or worse dependant on how the rifle/bullet combo works for its COAL?

Is that correct or am I lost?

That's correct, with one caveat.

As covered, seating depth isn't absolute.

(Maybe Aster will do another diagram for this or if you can imagine... I'll do my best to explain well)

As in the diagram, to have both the hunting bullet and match bullet just off the lands the match bullet needs to be seated higher.

If you say the hunting round on the left there is the stated 2.800" the match round on the right will obviously be a lot longer. If you seated the match bullet lower so that the COAL was 2.800" the bullet would be far back from the lands and the bullet would "jump" through the void in the chamber before it reached the lands and went through the barrel. Get what I mean? This is not good for accuracy.

So in this case it's better for accuracy to have the bullet seated higher to be just off the lands and remove the "jump". Even though it's longer than spec it will shoot better and is safe.

Here's the but... Going higher to get closer to the lands is fine, but you don't want the bullet to touch then. And you definitely don't want it too high that it jams into the lands so much the rifling pushes the bullet back into the case.

When the bullet is just off the lands, when the cartridge is ignited the bullet just has to overcome the pressure of the brass neck holding it in place. It's already moving before it touches the barrel and continues through.

When the bullet is jammed into the lands and the cartridge is ignited, the pressure inside the case has to get higher before it's strong enough to push the bullet though the lands which are holding it in place. This increases the pressure inside the cartridge which is a safety concern.

You with me?

Short version, assuming that's all clear... If you need to seat your SST's to be 2.810" instead of 2.800" so they're just off the lands, that's perfectly fine.

[Westy]

Projectile shape plays a massive part in determining COAL.

I've done a quick mockup demonstrating.

(The black lines at the top are meant to be the barrel if that wasn't clear... it's a crap diagram but took 5 minutes, what do you want from me )

bullets-and-coal.gif

As you know ideally you want the bullet ogive just off the lands. The left shows a hunting profile bullet compared to a match profile bullet on the right.

This is how/why COAL varies and why the ideal length can be significantly over the stated max as written by SAAMI or whichever source you're referring too. They're done using 'average' bullets which doesn't cover every scenario.

And so both in the above mockup are 'correct' yet completely different.

Look up Berger VLD and Nosler partition bullets for a good comparison in potential differences. COAL for a long, sharp ogive bullet like the Bergers VLD will always be longer than a short, round ogive bullet like Nosler Partitions.

Obviously we don't need a apology as you have already done that!!!!

[Aster]

I didn't apologise... I just said it was crappy diagram

[petemacsydney]

Hey guys,

Thanks to all for the info, this has really helped me!

Cheers
P

[sbd3927]

I was reading something relevant to this last night...
http://www.ballisticstudies.com/Knowled ... +COAL.html

I'm just starting to reload, and picked up some Hornady Interlocks SPBT, being cheapest I could find, practise re-loading and handling the new centrefire at same time.

To get the projectile on the lands is 2.822". The ADI data for Hornady SP is 2.75" This gets the cannelure just inside the neck, most of the neck in contact and the boat tail inside the cartridge, whereas my 2.822 only has about 1/3rd of the neck in contact with the proj. According to the link above, for boat tail proj's better neck contact has more benefit for concentricity and accuracy than getting it closer to the lands.
I'll experiment with that too I guess, but for now I'll take the recommended 2.75 coal and work up some loads with that.

308win 165gn Hornady Interlock SPBT, Starting with 42gn ADI 2208, stepping up 0.3 gn, Fed primer

[petemacsydney]

very interesting sbd, i may have to do some experimenting on that as well

[Khan]

A tip for setting the seating depth on your dies to make life easier too...

Get an empty case and drill the primer pocket out.
Once you've got the seating depth right seat a bullet into the empty case with the pocket drilled out.
Pour a bit of epoxy through the primer pocket so it sets behind the bullet locking it in place.

In future when you need to set the seating die again, put the dummy round in and screw the seating die down to touch it.

Lets you get the depth right pretty much straight off the bat instead of screwing around finding it again each time you swap dies when reloading.

[petemacsydney]

hey Khan, gr8 tip. i'll definitely do that!!

[Khan]

[headspace]

I ignore the COL and seat according to the bullet I'm using and the length of the throat. I like them just off the lands and in a hunting rifle, the right length for a magazine.

Bullet ogives vary wildly it's better to check physically with the rifle than assume anything.

JD.

[brett1868]

I use a Hornady OAL gauge and made my own cases to fit it cause 416 / 50 are not available for the tool. With this tool I can measure OAL then with the Hornady comparator I can measure base to olgive, take 1 thou away and then setup my seating die.

I measured each of my 416's and 50's and was surprised at the difference in OAL between them. Major pissoff having to do 3 different seating depths in 50 and 2 in 416 but it is what it is...

[newsteadvic]

(Maybe Aster will do another diagram for this or if you can imagine... I'll do my best to explain well)

Here is an image to support your statements Tiiger - from the Hornady Reloading book, 3rd Ed


The other thing to say about avoiding a COAL that is far too long is that when the over-long cartridge is chambered it may push the bullet back into the case, increasing pressure. If an over-long cartridge is chambered but not fired and then extracted, if it is so long that the projectile is engaging the lands the projectile can get stuck there as you extract the unfired case, trickling powder through the action and stopping the shooting session until you knock the projectile out with a cleaning rod! (I have done this a few times with cast bullets where it is common to have a fair bit of engagement of the lands, quit different to jacketed).

[Xerox]

I ignore the COL and seat according to the bullet I'm using and the length of the throat.

That's my practice too.

[Mich]

I measured each of my 416's and 50's and was surprised at the difference in OAL between them. Major pissoff having to do 3 different seating depths in 50 and 2 in 416 but it is what it is...

Ever given Meplat trimming/uniforming a go?