OGIVE vs COAL measuring, what do I need and how do I do it?

[Shooter_308]

Hey all, so last night I was loading up some .308 rounds for load testing using Hornady HPBT 168's. I was seating them to 2.800" COAL, but noticed a variance of +/- .006 per round, with a few being right at 2.800" I notice dthe jacketing at the tip was NOT uniform on a lot of rounds and this is making me want to start going from OGIVE length measurements. Now, how DO I go about doing this? Do I have to make a dummy round, then use a comparator, is there a SAAMI spec i could follow? I'm not looking to go right off the lands or anything, never had to before and was easily keeping .5 MOA at 100 in the past using amax's loaded to 2.800 COAL end-to-end.

I'm looking at the Sinclair Gauge, but i'm not sure of everything I need. Help a once-again noob understand this in laymans terms? Sorry!

 

[Letzgoracin301]

I would think the best way to go about it would be to. First measure and find the lands. The take a measurement to the ogive. Use that for your starting measurement then subtract how far back you want to seat it. Mag length will also be a factor to consider. I'm kind of a newb measuring to the ogive also, but that's my take on it.

 

[Shooter_308]

From what I understand, Hornady makes the comparator gauge with a modified case that allows you to seat the bullet you're using, chamber the "round", push the bullet into the lands, lock it. Then you can measure the OGIVE using the comparator insert from sinclair, and subtract say .010" off the lands etc.

I THINK thats how it's done.

 

[djskit]

This thread may help...

http://www.snipershide.com/shooting/snipers-hide-reloading/264029-off-lands.html

 

[Shooter_308]

My man, hookin' it up with a good thread. Thanks djskit!

 

[dauntedfuture]

.006 variation in bullet length is insignificant, especially for Hornady 168's as they are not picky bullets; there is a good possibility that there is nothing to worry about. .006 variation is not going to make a true .25 MOA combination open up to 1 MOA etc. The difference in BC is worth about 1-2" at 1000 yds or so at most. There is no reason to attempt to seat those bullets close to the lands as they will take the jump from a magazine without an issue. .006 OAL variation is based on the OAL of the bullets as NO bullets are exactly the same OAL from the forming process. Shoot a group and get some velocity readings, if its a factory rifle be happy with 2-3" at 200 yds and 50 FPS ES for TEN rounds.

Measuring OAL is from the base of the sized cast to the tip of the bullet. To measure to Ogive you measure from the Base of the case to the first .308 portion of the front of the bullet. To do this you need a comparator, its a small special tool that slips over the end of the bullet. Alternatively you just need something to give you a consistent measurement to some point somewhere from the .308 portion of the bullet to the tip and use that as a reference like a .22 case slipped over the tip, measure from the base of the .308 brass to the base of the .22 case slipped over the tip of the .308 bullet.

 

[djskit]

That being said, all the 308s I've loaded for have not been even the least bit sensitive to minor variations in oal. I start my 308 bolt loads at 2.85 and have never varied.


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[.50 Cal Cow]

Getting that difference from your COAL is likely, as you state, the difference in uniformity of the tips. Most likely if you are to measure those length CBTO (bsae to Ogive) all the lengths will run the same length (or really close) since your die seats off of the ogive or somewhere close there of. Some dies seat closer to ogive than others so depending on bullet uniformity is how likely you are to get consistent lengths.

Regardless, if you are not loading to chase lands and are only interested in loading to factory mag length than I'd say you could probably skip the measuring CBTO and spend the money elsewhere. Just pick a length and set the die so they all average out to your target length.