ogive vs coal

[Ryan500]

I see lots of people talking about measuring to the ogive, not coal. Now, where are you guys finding what you lenght to the ogive should be? My manual shows COAL, are you getting your ogive lenght by measuring to your lands and placing appropriately?


2 questions on that, will having a longer COAL or COL due to seating to your lands give you higher pressure for a given load than going by the COL in your manual.

and does it help accuracy?

 

[Boomer]

The importance of knowing the ogive length is so that you can seat the bullets to a specific length from the rifling. The way I determine the throat length and ogive length is as follows:

Close the bolt on your rifle and run a cleaning rod down the bore until it stops on the bolt face. Mark the rod at the muzzle with a piece of tape, magic marker, etc. Take a short wooden dowel and push a flat base bullet base first as far into the chamber as possible, and hold it there with the dowel. If you hold the rifle muzzle up with the dowel longer than the butt so it rests on the floor, that will work. Now drop your cleaning rod down the bore again, and mark it at the muzzle. You now have two marks on your cleaning rod. The distance between these two marks is equal to the distance from the bolt face to the rifling.

Another method is to simply push a flat base bullet into a resized cartridge and chamber it into your rifle. When the cartridge is ejected it will precisely reflect the chamber length of your rifle.

To get the correct COAL . . .
Take one of the bullets you intend to load, and press it firmly, point first into the muzzle of your rifle. Then with moderate pressure on the bullet, turn it between your fingers; this will etch a line at the forward edge of the bearing surface. Make up a dummy cartridge that you can keep as a reference. If the length of your dummy cartridge (from the cartridge head to the etched line on the bullet) is equal to the chamber length measurement, the bullet has zero jump to the rifling. From this point you can precisely gauge the amount of jump to the rifling you want by making small adjustments to the seating stem length. A full turn with the seating stem of an RCBS die (28 turns per inch) equals .036".

When I work up a load for maximum accuracy, and I am not concerned with feeding from a magazine, I work up the load with the bullet in firm contact with the lands. Once I have worked out the maximum load I begin to tweak it downwards until I find the sweat spot. Every adjustment I make in the powder charge after the maximum charge has been determined reduces pressure. Once I have my powder charge dialed in, I then begin adjusting the COAL. Again, every adjustment now increases the bullet jump and decreases pressure.

 

[punkrockboy]

2 questions on that, will having a longer COAL or COL due to seating to your lands give you higher pressure for a given load than going by the COL in your manual.

and does it help accuracy?

I have not experienced the mystical pressure spike. As far as improving accuracy, its amazing how my groups shrink the closer the ogive is seated to the lands in my 700P. My best loading, I seat them .005 off the lands. I use 168gr matchkings, there are even some bullets that shoot better touching the lands.

 

[Jordan Smith]

That's true, but there are also plenty of bullets and rifles that shoot best a ways off the lands, like 0.100" off even...

Boomer's instructions are pretty darn stellar. Another way of doing what he described, is by just inserting the bullet into the chamber, pointy end first, and measure the OAL. That is the max COAL with that particular bullet touching the lands. Now, seat that exact same bullet (even bullets of the same make and model may have slightly different lengths) until the dummy cartridge is the same length as the distance between the 2 markings on the cleaning rod. Now you can use a bullet comparator like this one

and take a length measurement. After subtracting the length of the comparator tool, you have a max OAL (with the bullet touching the lands of the rifling) for that type of bullet. If the hole in the bullet comparator happens to be the exact same diameter as the distance between the lands of your rifle, then you have a max COAL for every type of bullet in that caliber.

 

[Ryan500]

well got my measurements, and turns out the 2.900" Sierra manual calls for is about .020" off the lands. I will run at 2.900 COAL (well on my dummy round anyways).

Found the Speer Hot cors i have are different by up to .008".