rookie question

P

PHammer

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Will someone explain to me what is a good neck size bushing for a 6mmBR with a .270 neck chamber.how much does neck tension affect downrange accuracy? I'd also appreciate if you can tell me what would be a good case trim length measured off the case oal chamber length. Thanks in advance...
 
I am shooting a 6mmBR with a .2704 neck chamber. If I use a .268 bushing with Lapua blue box brass, the bullet is barely held in place and I can move it by hand. So my bushing of choice is a Redding .267. This holds the bullet firmly and cannot be moved with finger pressure. I estimate I have about .015 neck tension. Down range accuracy and neck tension can be a system with bullet, powder, twist, and everything else coming into play. I achieved very good results in the BR as well as my .223 with minimal neck tension and I stick with it.
I trim cases to 1.551". Had to edit this, looked at my chart wrong.
 
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With a tight neck chamber there are a few factors that come together to pick the right bushing.

In order, the pieces come like this
Neck diameter
Bullet diameter
Desired Neck Clearance
Neck Thickness after turning


In the typical 6PPC case it works like this...
.262 neck - .243 Bullet = .019 for the brass
.019 -.002 (.001 per side --> minimum clearance) => .017 (or .0085 per side is the approximate max neck thickness)
.019 -.005 (.0025 per side --> max clearance) => .014 (or .007 per side is the approximate minimum neck thickness -- well below what I like)

.258 and .257 bushings are the middle of the acceptable range. A .259 bushing will work on a .0085, and .256 may be necessary on the other end.
Put it all together and you see that for a .262 neck, the normal neck bushings run from .003 under (.259) to .006 under (.256), but the most likely fit will be in the middle (either .004 or .005 less than the neck diameter).

In your 6BR it would work the same way
.270 neck - .243 bullet = .027 for brass
Min Clearance: .027 - .002 => .025 neck: turn necks .0125 => likely bushings: .269, .268
Max Clearance: .027 - .005 => .022 neck: turn necks .011 => likely bushings: .267, .266
with .267 and .268 being the two middle range choices.

BTW: At the .005 clearance figure you run a big risk of split necks and cases that allow some blow by -- I would target .003 a more realistic maximum clearance)

The actual tension on the neck is a nebulous thing to try describe in terms of thousandths of an inch. Bullets can vary from a little under to a little over the .243 mark and have a pretty drastic effect on the 'tension' that holds the bullet. With this (and seating depth) you are varying the amount of pressure that is required to build up in the case before the bullet starts to move. 'Normal' tension and 'bullet not jammed' has a figure of roughly 3500psi to start bullet movement. Same tension and 'jammed into the lands' has a pressure that can push 11,000-12,000psi before the bullet moves. The difference between those two number can have a huge impact on the peak pressure and the shape/duration of the pressure curve inside the chamber and barrel as the bullet starts its joyful journey. With the same exact powder charge you can vary the peak pressure by 15-20% with these factors. Hence the need to always start 10% below 'Max Load' and work up.

Case length comes into play as when the cases grow too long and the chamber starts to crimp the mouth onto the bullet. This can lead to even more initial pressure and further affect that curve thingy. The max case length is usually well defined by the case designers and adhered to by reamer manufacturers. I consider .010 to .012 shorter than max as the desired length to trim to. When the cases stretch to within .004 to .002 of max I trim them back again.

An erratic pressure curve is the root of all evil in accuracy. Nothing destroys groups faster than a single long case, thick neck, or oversize bullet. Any of these can lead to a 10,000psi or more difference in the peak pressure inside the case. If the load is already near Max, that can shorten case life very quickly or even damage the rifle.


Rod
 
CubCouper said:
With a tight neck chamber there are a few factors that come together to pick the right bushing.

In order, the pieces come like this
Neck diameter
Bullet diameter
Desired Neck Clearance
Neck Thickness after turning


In the typical 6PPC case it works like this...
.262 neck - .243 Bullet = .019 for the brass
.019 -.002 (.001 per side --> minimum clearance) => .017 (or .0085 per side is the approximate max neck thickness)
.019 -.005 (.0025 per side --> max clearance) => .014 (or .007 per side is the approximate minimum neck thickness -- well below what I like)

.258 and .257 bushings are the middle of the acceptable range. A .259 bushing will work on a .0085, and .256 may be necessary on the other end.
Put it all together and you see that for a .262 neck, the normal neck bushings run from .003 under (.259) to .006 under (.256), but the most likely fit will be in the middle (either .004 or .005 less than the neck diameter).

In your 6BR it would work the same way
.270 neck - .243 bullet = .027 for brass
Min Clearance: .027 - .002 => .025 neck: turn necks .0125 => likely bushings: .269, .268
Max Clearance: .027 - .005 => .022 neck: turn necks .011 => likely bushings: .267, .266
with .267 and .268 being the two middle range choices.

BTW: At the .005 clearance figure you run a big risk of split necks and cases that allow some blow by -- I would target .003 a more realistic maximum clearance)

The actual tension on the neck is a nebulous thing to try describe in terms of thousandths of an inch. Bullets can vary from a little under to a little over the .243 mark and have a pretty drastic effect on the 'tension' that holds the bullet. With this (and seating depth) you are varying the amount of pressure that is required to build up in the case before the bullet starts to move. 'Normal' tension and 'bullet not jammed' has a figure of roughly 3500psi to start bullet movement. Same tension and 'jammed into the lands' has a pressure that can push 11,000-12,000psi before the bullet moves. The difference between those two number can have a huge impact on the peak pressure and the shape/duration of the pressure curve inside the chamber and barrel as the bullet starts its joyful journey. With the same exact powder charge you can vary the peak pressure by 15-20% with these factors. Hence the need to always start 10% below 'Max Load' and work up.
Click to expand...

Rod, When you are calculating bushing size are you figuring it using an expander or not? I would think that regardless of what bushing size you use ( within reason) the expander will push the neck out to the same dimension.
 
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