How do you figure/ calculate neck tension

[radmcg]

Neck tension how do you measure/calculate it?
I have reloaded for hunting a longtime but just got interested in target shooting. I have a 6.5/284 I want to figure measure the neck tension. My redding dies have a .294 bushing. This was reccomended by the gunsmith who built the rifle. How do you measure neck tension?
Sorry for the newbie question.
AHA
Rad

 

[spclark]

Simple: measure the diameter of your ready-to-load case necks, then load 'em & measure the necks again. Loaded dimension minus unloaded dimension = neck tension.

Example: your bushing is .294 so your sized necks will probably be around .295 - .296 depending on brass temper & spring-back.

Load a bullet, measure again & the diameter becomes .298 with .002' tension, .297 with .001' tension.

Obviously your bullet choice will have an impact as all 6.5's aren't the same diameter but the concept is the same.

Did your gunsmith tell you what your chamber neck diameter is?

F-Class & BR rigs tend to run tighter necks than rifles used for other types of long range & hunting applications; I have two 6.5-284 barrels, one with a .300 neck & the other is .296.

If your chamber neck diameter is .297 or smaller you may need to turn your case neck wall thickness a bit to make sure loaded rounds chamber safely. You want at least .002' - .003' clearance for safety.

 

[radmcg]

Thanks SP. It was just stated as a no turn neck chamber. I'll email him and get the measurement. Thanks for the quick and easy lesson.
Rad

 

[spclark]

If it's a no-turn chamber, the neck diameter is probably .2980' - .3000'. A .294 bushing would give you .295' necks,OD) & loaded rounds maybe .296 - 297'.

What brand of brass you use may have a slight effect too as some have thicker necks and / or some neck wall thickness variance than others.

Lapua's is about the best & most consistent but most shooters I know still turn a skim cut off,maybe 1/2 - 3/4 of the neck circumference) just to remove the thickness variable & make the neck walls uniform. A deeper cut likely will require a smaller bushing so as to provide sufficient neck tension to hold a bullet.

When you check with your 'smith you might also enquire as to the reamer spec as far as base diameter; there are several 'styles' of 6.5-284 chamber profiles & some brass may be too large as purchased to work properly. You can read up on the nuances here in the 6.5-284 info pages:

http://www.6mmbr.com/SixFive284.html

 

[dennisinaz]

Neck tension would be the amount of force it takes to move the bullet seated in the neck, not the difference between the loaded round and the empty case. I am not sure what terminology to use for that, maybe bullet interference. To measure neck tension, you have to rig your seating die with a dial indicator and a spring to provide tension. Mic McPherson wrote a good piece on it in Precision Shooting a number of years ago.

This is important too as consistent bullet tension makes for lower SDs and ES- a big consideration in long range BR.

 

[spclark]

Yes, that's essentially true but common terminology gets confused with more specific terms for some operations.

Neck tension is most commonly referred to as the difference in diameter of an unloaded vs. a loaded case neck.

Seating force,or unseating, if you'd rather) is the other metric you're describing, and can be measured by the means you describe but is affected by many other variables than just the relative 'springiness' or lack of it in a brass rifle case neck.