Newbie question - COL

[Ahsan Ahmed]

Much appreciate if the experienced folks here could offer some insight on the matter ....

I created a dummy round (.30-06) seating a Hornady SST 165gr bullet based on CBTO length (which is 2.632 in).

Presently, the COL on that dummy round measures 3.263" which is obviously well under the Max COL 3.340.

However, the Hornady manual also shows a COL of 3.210 underneath the SST 165gr projectile - what is this figure? Pls refer to image below, thanks,

 

[Chas59]

Based on your pic, I would say that the 3.210 relates to seating the bullet to the cannelure, which is fine but not necessary. Do yourself a favour and get a Hornady COAL gauge. Using this gauge you can tell the max length the cartridge can be loaded to still fit in your chamber. Then decide your own seating depth. Not even sure why Hornady uses a cannelure as most don’t crimp their rifle rounds.

 

[Ahsan Ahmed]

I actually used the Hornady OAL gauge to derive the CBTO measure to the lands of my rifle (CZ550) given that bullet.

To add, from what I had noted, I could perhaps seat the bullet another 40 thou and that cannelure will still be visible over the case neck. My concern is that might seat the bullet a little too deep base on my CBTO (I calculated 20 thou jump to the lands) ....

 

[Camoman1]

Cartridge max COL will change from bullet to bullet, depending on the shape of the bullet/ogive or as previously stated for setting the cannelure for crimp. They are mentioned in the books as per SAMMI specs to be safe in all rifles, if you are measuring yours to your lands ignore the what the book says for COL, you can go over that max because you know where your lands are, this will be safe in your rifle, with that particular bullet. just make sure it fits in your mag. Obviously if you are over what the book says for COL it may not be safe in other rifles.

I believe you are using the phrase CBTO incorrectly, you are currently measuring the COL on your bullet when it is touching the lands, you would need to be using the Hornsby bullet comparators to be measuring the Ogive (CBTO Cartridge Base To Ogive). Just take your measurement of 3.263 and -.020, this will give you 3.243 for a jump of 20 tho. Ignore the cannelure it makes no difference where it is and dont set your seating die to crimp. I recommend buying the hornady bullet comparators as it is much more consistent to be measuring from the Ogive then from bullet tips, mite as well get the hornady case comparator while your at it so you can set proper head space after resizing.

 

[cosmic]

The crimp was put there as a guideline for reloaders that are not trying to chase the lands. Like factory ammo, it makes the cartridge suitable for all factory rifles of that caliber. As a newbie, 20 thou jump might be a bit tight for hunting ammo. Bear in mind that I've seen some pretty substantial bullet variations from lot to lot for Hornady bullets (and others). Hornady themselves recommend 30 thou for their secant ogives. This gives you some margin against inadvertently jamming the bullet, which will cause a pressure spike. Also, bear in mind that your ammo is now customized to that particular rifle, and should not be used in another rifle without proper survey.

 

[Chilly807]

Not sure where you got 3.340" from? Is it a generic dimension for the 30-06 in general?

The 3.210" COL dimension for the 165 SST listed in the Hornady manual is specific to that particular bullet. It should be safe in any 30-06 rifle with a SAAMI-spec chamber.

Since each bullet type has its own unique profile, the manufacturer's recommended COL will be different from bullet to bullet.

That's because the different shape puts the point where each bullet will touch the lands in a different spot in relation to the bullet tip.
As an example, look at the recommended COL for the 168gr A-Max on the same page. It's 3.220", not 3.210". That's because the longer, sleeker tip of the A-Max puts the .30-cal ogive further back on the bullet in relation to the tip.

Now look at a heavy, more blunt tip bullet. The COL is probably shorter. The Sierra manual lists a 150 gr round nose load for the 30-06 at 3.000" COL, while the 150 gr Match King load for the same calibre calls for 3.250" COL.
Again, same bullet weight, very different profile.

Also, a different bullet shape means that the lands may "move" slightly with a different bullet. You should re-check your point of contact when you switch to a different type of bullet. Don't assume it's the same. It might not be much of a difference in some cases.

When you built your dummy round, does your CBTO dimension of 2.632" leave the bullet touching the lands, or does 2.632" include seating the bullet 0.020" deeper to give you 0.020" jump?

If you measured to the lands in YOUR rifle using the Overall Length Tool, then seated your bullet 0.020" deeper, you should be ok as a starting point.

If you had loaded to the book value of 3.210" COL, the result should also be safe in your rifle based on the numbers you've provided. You would have at least 0.053" jump to the lands, or as much as 0.073" jump depending on the answer to my question about the size of your dummy round.

What the OAL length tool (and measuring to the ogive in general) does, is give you the ability to safely build your ammunition to a custom length for best performance in YOUR specific rifle, rather than relying on a COL dimension that will chamber in any rifle.

The downside of that is you can't safely give your buddy with a different 30-06 a couple of your custom-length handloads to try in his rifle. They might be fine, or they could be jammed into the lands and potentially cause an unsafe condition.

 

[Ahsan Ahmed]

I believe you are using the phrase CBTO incorrectly, you are currently measuring the COL on your bullet when it is touching the lands, you would need to be using the Hornsby bullet comparators to be measuring the Ogive (CBTO Cartridge Base To Ogive). Just take your measurement of 3.263 and -.020, this will give you 3.243 for a jump of 20 tho. Ignore the cannelure it makes no difference where it is and dont set your seating die to crimp. I recommend buying the hornady bullet comparators as it is much more consistent to be measuring from the Ogive then from bullet tips, mite as well get the hornady case comparator while your at it so you can set proper head space after resizing.

I hope I'm not misunderstanding your post.

Actually, the measuring process and all the tools you mentioned were all used correctly (glimpse of my notes attached below to offer some more clarity on my initial question).

The 3.263 figure is the COL (not CBTO measure) of the finished dummy. I just needed to check-off that last remaining box wrt my confusion in terms of the mention of specified COL for 165gr bullet in the manual.

Not sure where you got 3.340" from? Is it a generic dimension for the 30-06 in general?

The 3.210" COL dimension for the 165 SST listed in the Hornady manual is specific to that particular bullet. It should be safe in any 30-06 rifle with a SAAMI-spec chamber.

Since each bullet type has its own unique profile, the manufacturer's recommended COL will be different from bullet to bullet.

That's because the different shape puts the point where each bullet will touch the lands in a different spot in relation to the bullet tip.
As an example, look at the recommended COL for the 168gr A-Max on the same page. It's 3.220", not 3.210". That's because the longer, sleeker tip of the A-Max puts the .30-cal ogive further back on the bullet in relation to the tip.

Now look at a heavy, more blunt tip bullet. The COL is probably shorter. The Sierra manual lists a 150 gr round nose load for the 30-06 at 3.000" COL, while the 150 gr Match King load for the same calibre calls for 3.250" COL.
Again, same bullet weight, very different profile.

Also, a different bullet shape means that the lands may "move" slightly with a different bullet. You should re-check your point of contact when you switch to a different type of bullet. Don't assume it's the same. It might not be much of a difference in some cases.

When you built your dummy round, does your CBTO dimension of 2.632" leave the bullet touching the lands, or does 2.632" include seating the bullet 0.020" deeper to give you 0.020" jump?

If you measured to the lands in YOUR rifle using the Overall Length Tool, then seated your bullet 0.020" deeper, you should be ok as a starting point.

If you had loaded to the book value of 3.210" COL, the result should also be safe in your rifle based on the numbers you've provided. You would have at least 0.053" jump to the lands, or as much as 0.073" jump depending on the answer to my question about the size of your dummy round.

What the OAL length tool (and measuring to the ogive in general) does, is give you the ability to safely build your ammunition to a custom length for best performance in YOUR specific rifle, rather than relying on a COL dimension that will chamber in any rifle.

The downside of that is you can't safely give your buddy with a different 30-06 a couple of your custom-length handloads to try in his rifle. They might be fine, or they could be jammed into the lands and potentially cause an unsafe condition.

Exactly, that's what I did.

Those reloads are for my own use in one specific rifle. No intent to share with any anyone.

Much appreciated all the feedback, was very helpful. Many thanks everyone!