Brass Measurements Don't Add Up

J

jmcasto

Guest
I'm turning Lapua brass for a .268 necked 6mm PPC.

I plan on using Bart's Ultras, with a .2435 base. Working backwards, .268 - .2435 (bullet base) = .0245.

(0245/2=.1225) so, as a starting point I turned the necks to .012 and made up a dummy round to measure total neck thickness (anticipating I would have to turn slightly more to get proper neck clearance in the chamber). Therefore, I thought my test round of.2435 + .024 would give me total neck diameter of .2675.

BUT...when I seat the bullet in the case and measure the cartridge neck diameter, I get .2665, not the expected .2675 (.2435+.024). Where did I lose .0015?

I'm using good equipment: a Mitutoyo Ball mic for the neck wall thickness and a Starrett electronic outside mic for overall neck thickness measurements. I've checked both micrometers with gage blocks and pin gauges and they're spot on.

I think my math is good and my measuring tools and technique are correct. What am I doing wrong - what am I missing? I suppose .2665 in a .268 neck is OK, I just didn't expect to get .2665 with the neck-turning measurements I have. I'd like to be able to use exact measurements when neck turning. Am I expecting too much?

Thanks.
 
This is one of those things where you really can't phone it in. Having said that, even though my freshly turned necks have a nice bright smooth looking finish, before I take the case out of the power drive adapter, I grab the case neck with a wad of 0000 steel wool and trigger the cordless drill for a couple of seconds. This planes off the microscopic ridges left by turning so that the finish is more like what I get after a couple of firings and sizings, and more importantly makes the measurement more representative of what the case will be in use. On the pressure rings, I do not clamp the micrometer on the bullet, but rather I use a drag through trial and error method, varying the micrometer setting until I get the feel that I want, including turning the pressure ring within the micrometer faces. On your math, probably a typo but your division problem to get half of the difference it should read .0245/2=.0123 (rounded). Like I said, most likely a typo. The insides of case necks are not cylindrical as they come, and even after being expanded some of the internal topography can survive. Years back a friend whose match travel buddy used to hold a record, told me that they used to resize their new .220 cases a few times with a one piece FL die (probably PPC) with expander to condition the necks before expanding and turning. I never tried that, but it might make the insides of the necks more uniform. The other thing that I noticed is that all of your measurements seemed to end in fives. This made me suspicious. Are you using an electronic micrometer or are you using an electronic caliper that rounds to either a 0 or 5 for the fourth digit? No matter how expensive or how many places it reads to calipers are only calipers.
 
It is very hard to get an exact measurement on turned brass using a ball mike. Close your eyes and take measurements 5 or 6 times then average the measurement.
The only true way to measure the turned brass is over a seated round with a good micrometer as you have done. Myself for a .268 neck I like the loaded round to measure around
.2655 to.2650. So looks like to me you are right on track with a little touch up.

Chet
 
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Keep in mind, when dealing with extremely close tolerances combined with multiple measurements, it's pretty easy to loose, or gain a little here and there.

Trust the measurement of the loaded round. It's the only one that counts.

By the way, I shoot a .269 neck, and turn my necks to just a tad under .012. You will find that a little more clearance is preferable to not enough.
 
Thanks

Boyd, Chet and Jackie,

Thanks to all for the prompt replies and ideas.

Boyd, yes, the measurement should read .1225. I'm using a Starrett electronic mic with five decimal places.

One takeaway from all the replies - nobody was surprised that the math and loaded round measurements were different. I'm thinking that must be a fairly normal expection and not at all unusual.

Jackie's comment about trusting the measurement of the loaded round makes a lot of sense. It is the one that counts.

Finally, Chet you say with a .268 neck you'd want 2-2.5 thou neck clearance? I was going to use 1-1.5 thou and now I'm wondering if that's not enough?

Thanks to everyone,
Joel
 
Not Chet, but I can point out that the recent trend has been for more clearance, although some have run it for a long time. No one that I know of has made more appearances in the NBRSA unlimited category records in the last few years than Gary Ocock. He once told me that he likes to be at about 2 3/4 thousandths. If you have not viewed the six segments of Jack Neary's presentation that are on Youtube, I recommend that you do. One of the things that he recommends is to "Go thin to win." He gives examples of why he believes this is true. What he means is that having more neck clearance has produced better results, that tend to be more visible at 200 yards. The thin that he refers to is case neck thickness, decreasing that to increase clearance. His examples are convincing.
 
I'm also shooting Bart's Ultra's. I'm using a .268 neck and turning my necks to .011, which gives me a clearance of .0025.
 
Thank You Boyd

Boyd, thank you very much for recommending Jack Neary's presentations. I watched them this evening and now have much, much more confidence. I feel like I got to peek inside the inner sanctum, learning some of the secrets of the Holy Grail.

Anyone reading this thread, I strongly suggest you go to YouTube as Boyd recommends. I guarantee you will learn something.
 
You are welcome. I am glad that you watched it. Jack does an excellent job and covers a lot of ground. There is something for everyone. ;)
 
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