Hi, I've got the gold medal match reloading dies for my 308 and have the 330-336 bushings. I'd like to know how the case mouth tension on the bullet affects the physics of the shot. i.e. burn rate/pressure etc.
Advanced Marksmanship Case mouth tension and accuracy?
- Thread starter ZincChloride
- Start date
Re: Case mouth tension and accuracy?
I am not sure lot of testing has been done in this regard. However it is generaly accepted that the higher the neck tension ( grip of the bullet ) the higher chamber pressure will be because it slows up the initial movement of the bullet. With normal neck tensions that we commonly use there would be very little difference to measure . Heavy crimping might see something to measure as it tends to hold the bullet back a lot better .
I feel that getting a good even neck tension radialy around the neck is more important than what pressure it may or may not change because it affects bullet seating concentricity and concentric release of the bullet into the bore which is more important . I think any pressure change caused by say a .001 sizing change in neck tension would be real hard to measure .
There is different neck tensions required for different actions and use . So you need to apply teh correct amount for the intended use . A hand fed bolt action can have a lot less neck tension than an auto or big cal magnum would need. More a function of recoil , action bolt inertia , slam feeding , heavy bullet weights , magazine use , rather than accuracy requirements .
If you fed your big cal magnum one round at a time you could have lighter neck tension but as soon as you put rounds in the magazine the bullets in the case need to resist recoil moving them .
I am not sure lot of testing has been done in this regard. However it is generaly accepted that the higher the neck tension ( grip of the bullet ) the higher chamber pressure will be because it slows up the initial movement of the bullet. With normal neck tensions that we commonly use there would be very little difference to measure . Heavy crimping might see something to measure as it tends to hold the bullet back a lot better .
I feel that getting a good even neck tension radialy around the neck is more important than what pressure it may or may not change because it affects bullet seating concentricity and concentric release of the bullet into the bore which is more important . I think any pressure change caused by say a .001 sizing change in neck tension would be real hard to measure .
There is different neck tensions required for different actions and use . So you need to apply teh correct amount for the intended use . A hand fed bolt action can have a lot less neck tension than an auto or big cal magnum would need. More a function of recoil , action bolt inertia , slam feeding , heavy bullet weights , magazine use , rather than accuracy requirements .
If you fed your big cal magnum one round at a time you could have lighter neck tension but as soon as you put rounds in the magazine the bullets in the case need to resist recoil moving them .
Re: Case mouth tension and accuracy?
The idea of course, as with any brass prep, is maintaining consistency throughout the process. And, making sure that the amount of neck tension is appropriate for the purpose. As Country has already stated
.002" is basically where I have settled for most loads. Both out of a semi-auto (AR15) and my short action bolt rifles. Non-coated bullets.
The benefit of a bushing die is of course specifying what tension you would like to run, and be able to change that with brass changes- both brands/dimensions/neck thickness, and those that occur after repeated firing/sizing.
Unless you anneal brass, it is easy to change the tension on the neck as it begins to work harden.
Does it make a supreme difference in a loads performance? Alone, doubtfully. However, it is another tweak to chase down if you are so inclined. I have forgotten to change the bushing out of my .308 die when switching between Winchester and Lapua. The inadvertant change in neck tension was not a detriment.
The idea of course, as with any brass prep, is maintaining consistency throughout the process. And, making sure that the amount of neck tension is appropriate for the purpose. As Country has already stated
.002" is basically where I have settled for most loads. Both out of a semi-auto (AR15) and my short action bolt rifles. Non-coated bullets.
The benefit of a bushing die is of course specifying what tension you would like to run, and be able to change that with brass changes- both brands/dimensions/neck thickness, and those that occur after repeated firing/sizing.
Unless you anneal brass, it is easy to change the tension on the neck as it begins to work harden.
Does it make a supreme difference in a loads performance? Alone, doubtfully. However, it is another tweak to chase down if you are so inclined. I have forgotten to change the bushing out of my .308 die when switching between Winchester and Lapua. The inadvertant change in neck tension was not a detriment.
Re: Case mouth tension and accuracy?
Thanks for your responses. I went out to the range a few days ago with 25 rounds using the same components. The only difference is that I sized 5 groups using a different bushing. (.334-.330). (edit) I shot at the 200 yard mark.
Components: Once shot and processed winchester brass, Fed. 210 primers, 41.0gr. H4895 powder, and 175 SMK's.
The .331 offered the best Chono results but the .330 offered the tightest group. I'm going to load my next 50 rounds 25 w/ .331 and 25 w/ .330 and see what happens.
Bushing_Grouping__SD_____MAD____ES
.334:___2.85"_____22.7___18.0___53.5
.333:___2.1"______8.3____6.6____25.4
.332:___1.975"____16.7___13.9___40.2
.331:___1.75"_____6.7____4.5____18.4
.330:___1.5"______10.7___9.2____22.4
Thanks for your responses. I went out to the range a few days ago with 25 rounds using the same components. The only difference is that I sized 5 groups using a different bushing. (.334-.330). (edit) I shot at the 200 yard mark.
Components: Once shot and processed winchester brass, Fed. 210 primers, 41.0gr. H4895 powder, and 175 SMK's.
The .331 offered the best Chono results but the .330 offered the tightest group. I'm going to load my next 50 rounds 25 w/ .331 and 25 w/ .330 and see what happens.
Bushing_Grouping__SD_____MAD____ES
.334:___2.85"_____22.7___18.0___53.5
.333:___2.1"______8.3____6.6____25.4
.332:___1.975"____16.7___13.9___40.2
.331:___1.75"_____6.7____4.5____18.4
.330:___1.5"______10.7___9.2____22.4
Re: Case mouth tension and accuracy?
This should just about cover it for you. As the article below points out, neck tension measured in bushing size is not correct and consitent bullet release
I have personally witnessed different seating tension between annealed brass that was sized and loaded at different times. Example, anneal 50pcs and load 25. Two weeks later load the other 25. Seating tension was noticeably different/random.
And yes, you will absolutely notice a reduction in verticle at long range with consistent bullet release.
http://www.accurateshooter.com/technical-articles/reloading/neck-tension-not-just-bushing-size/
This should just about cover it for you. As the article below points out, neck tension measured in bushing size is not correct and consitent bullet release
I have personally witnessed different seating tension between annealed brass that was sized and loaded at different times. Example, anneal 50pcs and load 25. Two weeks later load the other 25. Seating tension was noticeably different/random.
And yes, you will absolutely notice a reduction in verticle at long range with consistent bullet release.
http://www.accurateshooter.com/technical-articles/reloading/neck-tension-not-just-bushing-size/
