Case Volume & Velocity

[ashland]

After reading all of the posts on the stainless steel media thread, I wondered how important it would be to have the inside of the cases cleaned as well. I've been tumbling with a vibrating tumbler and am on the 5th firing on my brass. The insides are not clean and my question is how much does cleaning inside the case affect the velocity. Does it increase the velocity by a certain fps per each firing? Anyone done any research on this?

Thank you.

 

[ainnamor]

Re: Case Volume & Velocity

There should not be a very substantial buildup in the case so your volume change is very small. However, decreasing the case volume and using the same powder charge will increase velocity. You have the same volume of gas created in a smaller space. That leads to higher pressure and more velocity.

 

[ashland]

Re: Case Volume & Velocity

Well hell, what do you all do? Should I clean the insides of the cases everytime or should I compensate for lower velocities of brass being fired the first time and then expect different velocities on the next firings. I did chronograph this brass on the first firing and the last firing and there was about 100 fps higher difference on the last firing. However the lower velocities were shot when it was 20 degrees cooler and I have not annealed the brass since the first firings. What I didn't want to do is order more equipment like the Thumbler and steel media if cleaning the inside of the case didn't matter. Have I answered my own questions and need to clean the insides every time? I know I need to anneal and have gotten all the things needed to do it.

Thanks for your help.

 

[gcoulsey]

Re: Case Volume & Velocity

I use a sonic cleaner every 4-5th firing, but have never noticed any difference.
I just like the idea of getting the insides clean now and again.