Figuring Out Hull Length?

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Figuring Out Hull Length?

Postby turkeyassassin » Sat Jul 31, 2021 11:41 pm

How do I know what length to trim my hulls to for roll crimping. I understand that these inertia drivin guns can be a little picky with shell length when cycling. I will be loading 3” 20 gauge. Any help will be much appreciated

Dave in AZ
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Re: Figuring Out Hull Length?

Postby Dave in AZ » Wed Aug 11, 2021 12:50 pm

You don't trim the hulls... the pressure test results assume the full length of the hull specified. The resulting airspace inside the shell is critical for the pressure. It gives gas somewhere to expand into on initial burn before wad starts moving. There may be 1/4 in of "open space" in there, if you trim and reduce it to 1/8in you cut that volume in half. That may raise peak pressure many thousand psi. Heck, even crimping a fold 0.09" vs 0.055" deep gave me a 1500 psi pressure rise, some from firmer crimp but also volume reduction.

The only major difference between hulls and all those many different pressures is their internal volume! You should be able to see that trimming 1/8in off a hull is like going from a cheddite or rio, down to a Remington gun club, which will virtually always result in overpressure.

Trimming hull down will increase pressure.

Look at the saami specs, they say exactly how long a roll crimped shell should end up from a 3in or 2.75in hull. Heck, in European reloading docs they dont even specify the hull type, only the final reloaded length after rolling or folding! That final length, and more importantly the enclosed open volume to mitigate initial gas burn peak pressures, is critical.

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