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Is 9mm Supercomp in a 38 Supercomp barrel OK?

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I am getting an open racegun this month chambered in 38 supercomp. I know that 38 super/ supercomp is a straight walled case and the SC is rimless. I have a bunch of Starline 9mm supercomp brass left over when I used to shoot this caliber in a race gun I sold years ago. I know that this is a rimless tapered case that was designed after the 9x23 Winchester case.

My question is that if I use 38 super resizing dies for resizing the 9 supercomp brass then will it be safer to use this brass in a 38 super/ supercomp chambered barrel? Will it turn a tapered wall case to a straight wall case?

Next question : Does anybody know if the case wall thickness and case capacity is the same for Starline 9mm SC vs 38 SC? The previous owner gave me extensive load data on this gun using 38 SC brass and it would be nice to be able to use the exact same load formulas.

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The 9 mm SC brass is thinner at the case mouth and will not size properly in 38 SC dies. There is no neck tension on the projectile and it will drop into the case.

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Load the 9SC brass in 9mm dies and it will fire-form to the 38SC chamber when you shoot it. Then it can be loaded for 38SC in the appropriate dies.

Goldfieldshooter is right on the initial loading, but the brass will work after fire-forming it in the 38 SC barrel. Just don't try to make major PF after that with that brass! It will be stretched a bit behind the case mouth from tapered to straight-walled.

Extractor tension will be a slight bit different too, as the rims are different.

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