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Measuring OAL in precision ammo...

 
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Old 03-14-2013, 10:44 AM
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Measuring OAL in precision ammo...

I've often wondered how others do this -- particularly those who are building precision ammo. I'm talking about BR quality ammo (and yes, I have a rifle that will allow me to see the variance on paper).

When I measure loaded ammo with my Starrett calipers, clamp-on comparator, and comparator base, my setup allows for very repeatable, precise readings. I use Redding competition micrometer seating dies for all bullet seating. I hold the case flat against the comparator base, and I center the bullet in the comparator before lightly lowering the comparator onto the ogive with consistent light pressure. Nevertheless, I will often get variance from round to round in terms of the OAL to ogive after seating. I attribute this (perhaps incorrectly) to variance in the bullet dimensions. In an effort to deal with the variance, I wind up seating all of my rounds to something greater than what I'm shooting for, and then "walking" down the OAL a thousandth at a time on each round until I get the reading I'm looking for. To say that this is painstaking is an understatement.

I'd love to just set the seating die, confirm the first round, and then load 30 rounds. However, when I do that, I invaribly wind up with a few that our out of spec by a couple of thousandths -- some lower, some higher. For normal shooting, who cares. When I'm doing OAL testing, however, it matters.

How do the rest of you tackle this issue? Most importantly, what's causing the variance? Is it bullet dimensions, or is it undulations in the case head causing it to sit less than level on the comparator base? Is it errors in measurement, and am I just not thinking about it correctly? I've fine tuned my measuring technique enough that I'm pretty confident it's not me, but I've been wrong before. Those of you who nitpick like I do, what are your thoughts, and how do you deal with this issue in your ammo?
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Old 03-14-2013, 12:26 PM
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Re: Measuring OAL in precision ammo...

You are probably right in thinking it is variation in bullet deminsions. I've seen as much as .007" variation in suposedly match bullets. Try measuring the lenght of the bullet from base to ogive with your bullet comparator and caliper. Then seperate them in groups according to length. Load and shoot the same length bullets as groups of 3 or 5. You'll find that about 50% will be the same length with the others being longer or shorter.
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Old 03-14-2013, 12:44 PM
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Re: Measuring OAL in precision ammo...

You are going to find a variance in Ojive dimensions, it's the nature of the beast. what I do is to measure the bearing surface of my bullets, and sort or not depending on my specs for that load.

As for your seating depth variance you see with a bullet comparator, you need to keep in mid that where your bullet contacts the lands and where your seating plug contacts the bullet, and your bullet comparator contacts are going to be at least 2 different places and likely 3. So you will get varying readings, because it's very difficult for the Ojive to be made the exact same from one bullet to the next. What you can do is to order a seating plug from Redding to fit the bullet your using. then maybe drill/ream out your comparator to contact the Ojive in the same place as your new seater plug.

Another thing you can do is to shoot Walt Berger an email and ask him this question since BR is his bread and butter. I'm sure he's forgotten more than most of us will ever know collectively.
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Old 03-14-2013, 01:52 PM
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Re: Measuring OAL in precision ammo...

Hmmm... I didn't know that Redding would make a seating stem to fit a particular bullet. Not sure I'd want to mess with that, as I use MatchKings and VLDs, and I certainly don't want to be changing out seating stems every time.
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Old 03-14-2013, 05:29 PM
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Re: Measuring OAL in precision ammo...

I think you're fanning the wind over nothing. Properly developed charges and seating depths are more tolerant of small variations than many think.

Challenge you to a test; load five each at your desired jump to the lands, load another batch 5 thou longer and another shorter. Shoot the three groups blind, meaning let someone else hand you the ammo so you won't know which batch is which. Then see if there's any significant difference on target.

I'd bet you can't see any difference unless you're loading on the ragged edge of the good shooting window and, if so, only one of the three groups will be blown but not directly because of the difference but because your seating depth isn't proper to start with.
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Old 03-14-2013, 05:43 PM
Zep Zep is offline
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Re: Measuring OAL in precision ammo...

boomtube - you have a very good idea regarding shooting tests blind as I find myself shooting my test loads with a bias.
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Old 03-14-2013, 07:45 PM
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Re: Measuring OAL in precision ammo...

I usually load a whole batch long. I then measure each with a comparator, adjust my Redding micrometer and seat them a second time individually so that they all have the same length to the ogive. I find this minimizes variation.

Thanks to Shawn Carlock for this tip on his DVD:

Defensive Edge - LRH Reloading DVD
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