I am using a Hornady Bullet comparator to measure cartridge length to the bullet ogive.

What part of a bullet first engages the rifling? Is it the ogive? I am guessing not, since if that is the case all bullets should have the same cartridge length (base to ogive) where the bullet first engages the rifling.

My measurements for different bullets, while close, are different.

Thanks for any feedback.

David

The ogive is the curve of the bullet not a specific dia or point on the bullet. Several different types. Tangent ogive the curve is tangent to the point. Secant ogive is where the curve is secant to the curve (more straight to the point) Hornady SPs comes to mind. Sorry been to long since I had geometry to give a better example.

A bullet will engage the lands at the point the OD of the bullet equals the ID of the lands. A quick check is take the bullet stick it point first in the muzzle and turn it. The mark should be the id of the lands and indicate where the bullet will engage the lands.

Depending on the shape of the ogive the line would move forward or rear from the point of the bullet. My guage will tell me the distance from the base to the point on the bullet that equals the lands. Yes this will vary by bullet type.

quote:

What part of a bullet first engages the rifling? Is it the ogive? I am guessing not, since if that is the case all bullets should have the same cartridge length (base to ogive) where the bullet first engages the rifling.

Ramrod is correct.

Comparators (Oxford Dictionary: an organization, activity, etc. that is used to judge the performance of another similar organization or activity); so it compares; is not a measure, since you are obtaining a "value" for comparison.

A Comparator affords a "value" for a single individual bullet type provided the ogives are all the same. Once your "value" is established for that cartridge/bullet the loads can then be measured +/- @ to obtain an O.A.L., other factors notwithstanding.

The end of the Leade or Freebore of the chamber engages the bullet when they are both of the same diameter, not at a point on the ogive of the bullet which is all about "curves".

Not all bullets are created equal since the ogives are in many instances different from brand-to-brand, weights and calibers, plus there are so-called two diameter bullets, too. Lots of variables; so YES, you will always have different "value" when using a comparator on different bullets.

What a comparator actually does is remove the "tip" of the bullet from the length equation; since lead blobs, points, plastic tips, burrs on Hollow Points and unequally or poorly swaged bullets can all lead (pun) to dissimilar measurements. When using a comparator you have to take the Maximum Cartridge O.A.L. into consideration as well since you are basically only obtaining a similar "value" of each individually loaded cartridge at a given place on the same bullet that should be more consistant than using the variable of the tip(s) of the bullet(s).

Thanks Paul and Gerry for clarifying the ogive for me.

Ok - so if the bullet comparator measured the length to the bullet diameter where the bullet engages the rifling, shouldn't this length be the same for all bullets (since that diameter should be a constant)?

Paul - you mentioned that your gauge does this. How can the lengths be different for different bullets? I understand that the final cartridge length will differ due to different bullet types.

Am I missing something?

David

quote:

shouldn't this length be the same for all bullets (since that diameter should be a constant)?

Nope
Take a speer point, round nose, flat, etc Each of those bullets will have a point at which the bullet od matches the barrel ID.

You can set your length from base to OD to be the same for a bunch of bullets of the same weight. The CAOL will vary depending on bullet shape. Toss in different weight bullets and number of different COAL grows.

There are the SAAMI chamber dimension there will be a point on the throat where the bullet will engage. When a bullet maker in their data post and oal that should be a point that in a normal chamber their bullet will have what they feel is their minimum jump based on the shape of their bullet. Different manufacturers different minimum jump. Throw in the std mag box length into the mix.

If you read a Nosler book it will state a std max oal then go on to state that best accuracy will normally come from an longer OAL (shorter jump)

quote:

shouldn't this length be the same for all bullets (since that diameter should be a constant)?

No.

Because the only constant for a given caliber of bullet is the shank, that is the main body of the bullet that actually engraves the rifling. Everything forward of this point is the ogive.

Bullet body or shank length varies from bullet to bullet too, boattails are a good example as are round nose bullets.

It depends

If you are comparing the length of the same exact bullets from the same box you might get repeatable results.

If a bullet mfg has multiple dies spitting our bullets of the same design and the different bullets wind up in your box you may find variations even within the same box. You can detect this by measuring just the bullets' lengths to the ogive.

In a third case you can compared different bullets of different manufacturers but the same general design. There is very little chance you will get the same measurement or leade engagement location.

quote:

Ok - so if the bullet comparator measured the length to the bullet diameter where the bullet engages the rifling, shouldn't this length be the same for all bullets (since that diameter should be a constant)?

Ok guys thanks again for the explanation.

For now assume all bullets of the same type are identical and also that the shank diameters are all the same. According to what was written in previous posts the bullets will engage the rifling right where the shank begins. Is this correct?

If so, then forget the bullet for a moment and only consider the chamber. The distance form where the chamber starts to the rifling is a constant, so how can the cartridge length to this point be different?

I do get that the final cartridge length to the bullet tip will be different, but not the measurement above. I also understand that if bullet comparators are using a different bullet diameter (different from where the shank starts) as a reference then things can differ.

Thanks again for the discussion.

David

First off the lands will engage prior to the shank. Assuming the lands are in the range of .004" deep adding both sides says the ID is .008" smaller than the shank. So the lands will start cutting prior to the shank.

The error is the assumption that the distance from the base to the point on the lands within the chamber are always the same. They will be within a range. Just like a chamber is cut within a range. Even rifles made by the same manufacturer will not be exact. If the chamber is cut custom all bets are off.

I ALMOST never use a factory throat dimension. I have my throat cut to more or less a specific bullet loaded to fit my magazine. Different bullet will probably give me more or less jump.

I'm sensing confusion from the original poster.
Col and distance to lands are 2 different things AND will be different in the same cartridge in the same gun w/ different bullets of the same weight.
I do not use a comparator. I work up my loads(for 35 + rifles) as per the Nosler manual.
Resize a case. No powder or primer. Seat a bullet much longer than the published col. Color the bullet w/ a black or blue sharpie. Try to chamber the bullet. If it will not chamber, seat it a little deeper and try again. Do this until it will chamber w/ pressure. Remark the bullet w/ the sharpie and carefully rechamber. Then carefully remove the cartridge from the chamber. Look at the bullet and examine it for land marks. Measure the col. Seat it .005-.010 deeper. Recolor the bullet. Carefully rechamber and then carefully remove from chamber. Again look to see if there are land marks on the bullet. Do this as many times as necessary until you do not see land marks.
This distance to lands is the starting point for this bullet in this rifle. Usually you want the bullet .010-.050 off the lands depending on the bullet used and this rifle's desires. Then make sure it fits in the magazine. It may need to be shortened more giving you a shorter col. Good luck.