Okay, I'm a relative newbie to using a comparator for measuring cartridge OAL. I'm just starting to reload for a .270 WSM, and though I'd use my new comparator to determine the max COL for this rifle. So, using the old split-neck trick, I ran a bullet up against the lands to fully seat it into the case at what should be max COL (or longer). All's fine and good up to this point. Just for shits and grins, I tried this with a couple other bullets, both of which provided different OALs than the first (and each other). Now, maybe I'm just having a brain-fart here, but shouldn't this measurement be pretty much the same for all bullets considering you're using the ogive to seat the bullet with the lands and then measuring the COL with the same point on the bullet with the comparator? Can one of you old pros clear these muddied water and tell me why the COL would differ by .04" depending on bullet? Do differt jackets respond differently to the lands? Could I be getting different neck tensions, which cause different "pull-out" rates in the rifling? I'm not sure on these, since I get pretty repeatable measurements using the same bullet multiple times. Help me out here! Thanks.

DesertRam, The variation you`re getting is most likely caused by the differance in bullet ogive shapes and their relation to the lead angle of your rifles throat. The problem with seating a bullet in a case with the neck split is you don`t "feel" the bullet hit the lands. One bullet can be seated deeper or deep enough to stick slightly and get pulled out some when you remove it from the chamber in relation with another bullet. The best way IMHO (I`m sure I`ll hear differant)is with a Stoney Point AOL guage. The trick with it is to follow the instructions and use a dowel from the muzzle to feel the bullet hit the rifleing.
I recommend useing a fire formed case from your rifle and drilling & tapping in place of the Stoney Point one, as theirs has been resized and will be loose in your chamber. The headspace is not the same as your rifles and it affects your measurments accuracy.

Desert Ram,

Just about every bullet manufacturer Makes a bullet with a slightly different shape than the others. Hornady likes the secant ogive. Sierra prefers the tangent ogive. Sierra, for example, has a different ogive for the same caliber and weight bullets between their Prohunter and Matchking bullets. Lead tipped bullets out of the same box will vary slightly in length. For this reason, I think the OAL is a measurement that is on its way out. OAL is relevant only to magazine length. Comparator length is relevant to the lands of the bore and tell us much more than OAL. The comparator measures from the point on a bullet that is the land diameter. On a .308 Win., the usual land diameter is .300". So the comparator measures from the point on the bullet that is .300" to the cartridge face. For different shape bullet, the measurement will bu different. For some of the longer nosed bullets it maybe possible for the comparator lengtth to touch the lands to result in a OAL that is too long to fit in the magazine. Ain't this fun!!!
I hope I was able to make some sense of this for you. Good luck.

Desert Ram,

If you had a short piece of your barrel cut off and chambered with the same reamer to use as a headspace gauge, it can also be used as a comparator when seating bullets... the throat will contact the bullet at the same place the barrel does, assuming it is from the same bbl. The difference between the readings you are getting is coming from the shape of the ogive. A comparator will not be contacting the same place that the rifling does, close but not the same, this is where the shape of the bullet shows a deviation... in the comparator. If the bullet was pulling out of the case at all when extracting it while using the split case method, it would show erratic results, not consistant ones, this is using the same bullet of course. Switch to a different brand/type bullet and the readings will be consistant but, they will not be the same as the other type bullet which has a different shape. Some bullet types are real close, but they are all different shapes. Measuring for a reference distance is what's important here. Measuring off of the bullet tip and not the ogive is non-condusive to consistancy as well...

I think you guys nailed it. Must be the differing shapes of ogives. In my simplicity, I assumed that the ogive of every bullet would stop against the rifling at pretty much the same point, then the cartridge could be removed and measured at the same point using the comparator. So much for one man's dream of measuring only once and using that measurement for all subsequent loadings. I guess we can't have it all...

DesertRam,
There is another situation you should be aware of. Within different batches of bullets from same manufacturer, there can be a difference in ogive. My bullet seater (Redding) set the bullet at a slightly different depth because of the different shape of the bullet where the seater plug made contact. This happened on a new batch of hollow points. The depth was shorter by about .008 as measured by the comparator.... Nothing seems to be simple in this business.
Ron

Just measured some 178 A-Maxes from base to ogive with the Stony Point comparator, the new lot# was a whopping .030" shorter than the old ones. Would have never guessed it would have been that much, but it was... After checking the difference in seating, it proved to seat at the same OAL though. Contact on the lands was consistant with the old lot too.

When a point (ogive) form die is made, an abrasive-laden lap is used to finish the shape and provide a surface smooth enough to release the bullet after it was compressed against the interior surface of the die enough to flow the bullet material. No two are exactly alike. This would account for differences in ogival shape. Also, bullet jacket material differs from lot to lot. Bullet manufacturers go through a lot of dies over the years and goodness knows how many dies of the same general dimensions they run at the same time. Its actually surprising to me that any two bullets would resemble each other as closely as they do. The bullets available these days are capable of way more than most of us can exploit in the guns we use.

Paul