Help me understand question on case capacity

[8185854610]

I am loading .308 in both WCC 07 MIL SPEC CASES AND WINCHESTER COMMERCIAL CASES. THE CASE CAPACITY BY H2O TESTING OF THE WCC IS LESS THAN THE WINCESTER COMMERCIAL CASE. I HAVE ADJUSTED THE LOAD FOR THE WCC 07 SO AS TO NOT OVER LOAD THE CASE.

MY QUESTION IS: If I want to duplicate the load in th WCC 07 case, using a winchester commercial case that has more capacity than the WCC 07 case. Can I use the same amount of powder and achieve the same FPS , ACCURACY etc. or do I have to increase the powder percentage to match the percentage of volume Increase for case pressure? Or can I use the same amount of powder with same results.

Thanks

[Fordtrucks]

That's gonna take a chrono to figure out. That's a complicated physics problem their, maybe Quickload can u close but the only realy way is a trusted chrono device.

[Mike402]

This is a great example of how helpful Quickload can be. Without it, you'll just have to experiment with a Chrono to figure out the right charge weight to adjust for capacity difference.

One thing - make sure to measure water capacity in fired cases out of your rifle.

[CSACANNONEER]

You'll need to use a crono. Therory is one thing but actual measurements will prove that a certain load fired through a certain barrel at a certain point in that barrel's life will result in ........

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The basic premise is that with a case with lower H2O case capacity, you are generating higher internal pressures within the case with an equal powder charge versus another case with a higher H2O case capacity, and with a higher internal pressure, you get slightly higher FPS. This is pretty much Bernoulli's Principle at work.

Quickload does help in experimenting with these slight differences in charges, but really the only way to truly be sure is to try out the loads under a chrono.

[tamalpias]

This is why if it is hunting are target ammo I advise to just use all the same case brand. If it is plinking ammo then I would not even bother adjusting

[bigedp51]

"The most important thing to remember about military brass is that the internal capacity will probably be less than commercial .308 Win brass, because military brass often has thicker webs or casewalls. Montana Marine reports his fire-formed milsurp cases hold 56 grains of H20 on average compared to 58 grains for fire-formed Winchester. Given the reduced capacity of military brass, you should reduce posted max loads by 1.5 grains when loading with Lake City or most other milsurp brass. However, the IMI MATCH brass is closer to commercial brass in internal case capacity (ICC). Kevin Beggs reports: "Fired, my IMI Match brass runs with an ICC of 55.0gr and bumping the shoulder back .001" will net me an ICC average of 54.0gr. LAPUA runs an average of 54.4gr and Federal runs an average of 54.3gr ICC."

.308 Winchester Cartridge Guide

http://www.6mmbr.com/308win.html

Below is the Lake City 5.56 case and the opposite of Lake City 7.62 cases which are thicker to make them stronger in military chambers with larger diameters. The 5.56 case if made thicker would not hold enough powder so they made the 5.56 case harder than any other .223/5.56 case made.

The charts below show the exact same powder charge of 55 grains of H335 in each case. The top case is a Lake City cases with 30.6 grains of H2O capacity.



The bottom chart is an old lot of no longer made Lapua cases with 28.0 grains of H2O capacity and there is approximately 6,000 psi higher chamber pressure using the smaller capacity case.



Because of the size of the .308 and 7.62 NATO cases the chamber pressure difference will not be as great as the .223/5.56. And on average the powder charge difference will be 1.5 grains less powder in the Lake City cases for the same "APPROXIMATE" chamber pressure.

Bottom line, pick a brand of .308 cases and try and stick with them and not have to worry about large variations in case capacity using mixed brass. And Winchester cases are harder than Remington or Federal so Winchester would be my vote for the cheapest of the off the shelf American brands.

"BUT" if you can get large amounts of Lake City or any commercial military contract 7.62 cases these will last longer than any other brass because they are built Ford Truck Tough.





Bazinga

[J-cat]

Add 1-1.5grs more powder for the commercial case to get the same velocity.