Question on neck size bushing

philcox

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Nov 27, 2018
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Auburn, CA
I have a quandary on determining the neck size bushing for my RCBS Matchmaster FL die set. Here is the situation ...
  • 300 WinMag
  • Barnes 175g LRX
  • Lapua Brass
So to figure out ideal Neck Size Die, I initially used the "wall thickness" and "projectile thickness":
  • Walls on 5 different cases were .016
  • Projectiles were consistent at .309
So .309+.016+.016 = .341. If I assume that my Mic is off, then .308+.15+.15 = .338

Taking the .002 off would give me .339 or .336. I originally bought the .339 sizing die because I trusted my mic.

Now the quandary: With the .339 bushing in the FL die, when I measure the neck of a loaded round that was sized with that die, it ends up being .335 ????

So I am wondering how I neck size down a cartridge to .339 and then end up with a loaded neck of .335? I just don't get it. I am new to this whole thing, and must be missing something. Any insight is appreciated. Does anyone have good specs on what neck bushing I would use in the 300WM with Lapua Brass and wanting a .002?

Thanks in advance.

Phil
 
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Bushing dies will drastically reduce the neck below their stated size if they have to move a lot of brass, ie the neck of your chamber is large
What is the size of the neck on fired brass?
 
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philcox

philcox

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Bushing dies will drastically reduce the neck below their stated size if they have to move a lot of brass, ie the neck of your chamber is large
What is the size of the neck on fired brass?

Fired brass is .340
New unprepped brass is .331
Fully loaded brass is .335

I just done see how the neck is shrinking down if I only have a .339 bushing in the FL resizer. That is what is confusing me.

Also, if the projectile is .308, and loaded is .335, then that leaves only .027, and this .0135 for walls, and I’m measuring .16. Something is just screwy.


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LaHunter

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If you don’t have the correct tools / instrument, it is impossible to accurately measure the neck wall thickness.
1) get a loaded / live round
2). Measure the o.d. of the neck of this round. Actually, doing this with 10-20 rounds and getting an average would be best. Record reading
3). Subtract .002 or .003 or whatever ‘neck tension’ you are wanting. Order that size bushing. I would also recommend getting one size larger and one size smaller, just in case you want to try different neck tensions

Panhandle Precision has some good videos on reloading
 
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philcox

philcox

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Here are some images

New Unprepped

928157e1d014aa126e00e49ddf94a3a0.jpg


Fired
b1732dd74dd024300642ca630fc732ce.jpg



Prepped
452c03d3c9e86fee8baebe5873784854.jpg


Loaded
dedbe5624d323051ba8e2a54f70133b6.jpg


Bushing
8efc2e919098bb8d110673daf3371544.jpg


Die parts (I am not sure what the difference in the expanders are, currently using silver)
75679191648c820ef5a262980327ee4d.jpg


Standard FL die parts
35704eb871b175fa741e1dff6bc3c8cb.jpg


I just went and prepped 10 cases (8 fired, 3 new)
- using the FL die with bushing for 1X, all 5 are .339 (as expected)
- using the std FL die, 2 1X .332, 3 New .334


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jonson25

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Nov 4, 2021
Messages
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Use a good ball micrometer to measure neck thickness. I measure multiple pieces of brass (probably more than I even need to) and spin them and measure different sides on the brass for neck thickness. I take an average of all that and use that as my neck thickness. If you leave the expander mandrel in your sizing die it will bump the necks back out slightly from your desired neck tension so be mindful of that. I personally size necks with a bushing 0.002 smaller and leave the expander mandrel in giving me approx 0.0015 neck tension. My 30 cal brass is neck sized with an inner diameter at 0.3065 and that’s what I’m looking for.
 
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philcox

philcox

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Joined
Nov 27, 2018
Messages
577
Location
Auburn, CA
If you don’t have the correct tools / instrument, it is impossible to accurately measure the neck wall thickness.
1) get a loaded / live round
2). Measure the o.d. of the neck of this round. Actually, doing this with 10-20 rounds and getting an average would be best. Record reading
3). Subtract .002 or .003 or whatever ‘neck tension’ you are wanting. Order that size bushing. I would also recommend getting one size larger and one size smaller, just in case you want to try different neck tensions

Panhandle Precision has some good videos on reloading
So, let me ask a question. If my current neck tension load is too tight or loose, won't measuring the loaded rounds just exacerbate the problem that improper neck tension might cause? This is a new load I am developing, so starting from scratch. Also, this is Nosler Brass, not Lapua. I am thinking I may just go with the standard (no bushing die) and make my life simpler/easier, though maybe not as precise.
 

LaHunter

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Mar 9, 2013
Messages
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Location
N.E. LA
Here are some images

New Unprepped

928157e1d014aa126e00e49ddf94a3a0.jpg


Fired
b1732dd74dd024300642ca630fc732ce.jpg



Prepped
452c03d3c9e86fee8baebe5873784854.jpg


Loaded
dedbe5624d323051ba8e2a54f70133b6.jpg


Bushing
8efc2e919098bb8d110673daf3371544.jpg


Die parts (I am not sure what the difference in the expanders are, currently using silver)
75679191648c820ef5a262980327ee4d.jpg


Standard FL die parts
35704eb871b175fa741e1dff6bc3c8cb.jpg


I just went and prepped 10 cases (8 fired, 3 new)
- using the FL die with bushing for 1X, all 5 are .339 (as expected)
- using the std FL die, 2 1X .332, 3 New .334


Sent from my iPhone using Tapatalk
You have the wrong size bushing
Why did you order the .339” bushing?
Looking at your fired brass neck diameter you are most likely going to have to size down the necks in 2 steps. I have read that you can run into problems if you try to size the neck down more than about .005”
 

LaHunter

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Location
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So, let me ask a question. If my current neck tension load is too tight or loose, won't measuring the loaded rounds just exacerbate the problem that improper neck tension might cause? This is a new load I am developing, so starting from scratch. Also, this is Nosler Brass, not Lapua. I am thinking I may just go with the standard (no bushing die) and make my life simpler/easier, though maybe not as precise.
With that specific bullet and that specific brass, the neck o.d. measurement of the loaded round can’t be any smaller than it is, regardless how tight you size the necks prior to seating the bullet. You can’t change that measurement. You are simply trying to minimize how much you are working the necks
 
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philcox

philcox

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Auburn, CA
PROBLEM Solved. I am an idiot. I MUST have used my standard die, and when I just did the 10 this AM, the ones with the Bushing, that measure .339, then the inside is .3115, I then grabbed the projectile, and it slid right through the hole! My "regular" FL die is .306-.3065 ID
 

Mulyhuntr

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Jun 20, 2017
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Location
CA
I always start by measuring necks with a ball mic, then order bushings .002" and .003" under neck x2 plus bullet. I then check neck tension with pin guages to be sure I'm getting desired neck tension (usually .002").
 
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