Neck bushing, brass spring-back and ideal chamber clearance

[SGK]

I have a 6mm Creedmoor which has a chamber dimension at the neck of .272". I'm shooting Lapua brass which I have turned to a target thickness of 13 thou with a view that I would achieve a 3 thou clearance with a loaded round (0.272 - 0.243 - 2*0.013 = 3 thou). [Quite why it's called 6mm when it's not actually 6mm is beyond me. Why not 243 Creedmoor?]

I have been using a .267" bushing with a view to achieving 2 thou of neck tension. 0.243 + 2*0.13 - 0.002 = 0.267.

However, I noticed today that when my Lapua brass comes out of my FL bushing sizing die with 0.267" neck bushing it measures 0.2695/0.270". What's more, loaded rounds are measuring about the same which I guess suggests my brass necks are a little thicker than target and I'm not getting much neck tension.

Do you think I need to move to a smaller bushing?

(I'm at the very early stages of load development for this rifle.)

 

[rijndael]

Have you measured the bushing to confirm the number matches the diameter?
Do you anneal?

 

[bugman]

after a lot of years and a lot of bushings, I can tell you that the odds of getting a bushing to size per its stamped ID is very slim. Carbide might improve your chances, but still no guarantee. Add to that sad fact.....bullet OD vary widely (measure some at the bearing surface) from brand to brand and lot to lot, and spring back varies per annealing interval and a lot of other factors.
If neck tension is important to you, start using mandrels to control the loaded clearance and tension you want.

 

[Bill Norris]

Try a group with a .266 bushing and see what it looks like.

 

[6bra1k]

not sure there is an "ideal" neck clearence.
may vary with caliber and distance shot.
lots of numbers work me thinks 2-3

 

[6BRA]

I know this defeats the purpose of the bushing....go low and open with mandrel. My runout is .0005 over 50 pcs of brass.

 

[SGK]

I reached out to Whidden as the bushing seems to measure too big. They just responded that they will send me a replacement. I would not expect brass sized to .267 to spring back to .270. I also have a .268 bushing which measures true. I understand some other options might perform better but I have only 100 rounds down this barrel and I'm only now starting to do load development on 1x fired (and annealed brass).

 

[SGK]

I know this defeats the purpose of the bushing....go low and open with mandrel. My runout is .0005 over 50 pcs of brass.

How much lower do you go? Thoughts on best starting point for my setup?

 

[PracticalTactical]

How much lower do you go? Thoughts on best starting point for my setup?

Nobody can give you a 100 percent meaningful answer. You have to make the final determination for yourself.

Annealed brass can achieve suitable neck tension with a larger bushing than a case that has been work hardened from the neck turning process or by firing and sizing x number of times.

We can give you a ball park number, but the rest is up to you based on feedback you get from the sizing process and load development.

If you feed from a mag you will want more neck tension than if you single round hand feed.

For all out accuracy with single feed I find that the less neck tension I have, the less variation in neck tension I have... Therefore most consistent velocity.

If I could offer you a process for finding the right bushing, start with a bushing that is too big and size a case... Test if a bullet can be moved in and out of the case.

If a bullet is sloppy, try the next smaller bushing on another case (not the same case)... See if a bullet still fits lose... if yes.. drop a bushing and try again on another case.

Once you find the bushing that can hold a bullet, you have your base line number. How much smaller than that you go is up to you. For F Class I go one size smaller, for PRS I go two sizes smaller.

 

[SGK]

Indeed, and why I asked for a suggested starting point. I mostly single feed although this rifle has a Defiance Deviant Elite action with controlled round feed and mag.

 

[PracticalTactical]

Indeed, and why I asked for a suggested starting point. I mostly single feed although this rifle has a Defiance Deviant Elite action with controlled round feed and mag.

Sounds like you have a great set up. Controlled round feed does not put dents in your necks at ejection, so your reloads wont have much runout.

 

[6BRA]

How much lower do you go? Thoughts on best starting point for my setup?

.004 under, anneal every firing, mandrel barely feels anything final .002

 

[stevenpoplin]

See what your target tells you with a 267, 266, 265, and 264. if your best groups are with a 264, try 263. my guess is you'll land somewhere in the first three. if you change anything retest.

 

[YBPS]

I have been enjoying my new mandrels from K&M. I just called them and ordered a few different sizes to experiment with. I got .003, .002, and .001 for a .25 cal. They also included pin gauges.
I highly recommend.

 

[SGK]

.004 under, anneal every firing, mandrel barely feels anything final .002

So let's say you're targeting 2 thou of tension around a .243" projectile, i.e. an ID of .241 at the neck. You're suggesting a useful starting point (I understand that later one might wish to try either side of this starting point) is to size with a .237" bushing (4 thou under target) and then enlarge with .241? Or is that bushing a .239" (4 thou under projectile)? I think you mean the latter but I want to make sure. Plus with spring back neither the downsizing nor the enlarging will end up where the bushing or mandrel measures.

 

[6BRA]

.238/.239 yield .239/.240 open up with mandrel allow for reverse spring back. .002 to .0025 under bullet diameter. If anneal every firing/spring back will be very minimal....all bushings are not the same. You had mentioned chamber to neck clearance...I am at .004 to .005 on paper...but not actually. bushings dont get the entire neck..shoulder junction is within .001 of chamber reamer...runout... is .0005.

 

[SGK]

But the bushing needs to account for brass thickness. Given I have the 21st Century neck turning lathe I already have an expander mandrel of 0.241". So, presumably, as a starting point I should try a bushing of .239 + 2*0.013 = .265"

(Understood re clearance)

Given the amount of working of the brass neck, are you guys lubing the necks ahead of sizing (as opposed to just the case body)? I presume you lightly lube the expander mandrel. Preferred lubricants?

I can't help but find it partly ironic that over-reducing with a bushing and then expanding with a mandrel is what should be achieved with a (better) expander ball in the resizing die. Maybe dies should have a selection of (redesigned) precision ground expander 'balls' or 'pins'.


EDIT: 21st Century says their lathe expander mandrel is "about 0.242".

 

[Dave M.]

So let's say you're targeting 2 thou of tension around a .243" projectile, i.e. an ID of .241 at the neck. You're suggesting a useful starting point (I understand that later one might wish to try either side of this starting point) is to size with a .237" bushing (4 thou under target) and then enlarge with .241? Or is that bushing a .239" (4 thou under projectile)? I think you mean the latter but I want to make sure. Plus with spring back neither the downsizing nor the enlarging will end up where the bushing or mandrel measures.

Make sure to measure the bearing surface pressure ring on the bullet you use. Don’t assume it’s .243. More than likely it will measure .2435

 

[JFrank]

I believe guys can over think this stuff, I use a mandrel on virgin brass only.
Measure the OD of a loaded round then subtract the desired amount for a starting point bushing then test both sides to determine what’s ideal rather then targeting a particular setting that your combination may not like. I found that heavier NT shoots more consistently in mine without any annealing.

 

[JFrank]

Testing spring back ?
Measure the ID of a bushing than size a case and measure the OD of the neck.
mine doesn’t change after sizing ( no spring back )