Hi All,
I'm theorizing ideas for an efficient 7mm cartridge that will function through a short action with standard bolt face and AICS magazines.
I'm already aware of 7mm-08, 7mm-08 AI and the 7mm SAW. All based on the .308 case.
Even though the .284W was originally designed as a short action cartridge, to properly fit in an AICS magazine it would need the bullets seated too deep to be a realistic contender so the .284W or the wildcat .284 Shehane is really better suited for long actions.
So I came up with the idea of a .284 Short Shehane.
The idea is simple. Just shorten the body length of the .284 Shehane by 0.200" which would allow the 180gn Hybrids to be seated properly in the case neck and keep the overall length under 2.900". Think of it as a slightly chubby 7mm Creedmoor.
I'm thinking to base it on the Shehane because the base .284W case is larger diameter than the .308 case and the reduced body taper of the Shehane adds a couple more grains of H2o capacity over the standard .284W (and also for a second reason that I'll mention further down).
I have mocked up a design in GRT and it estimates the powder capacity to be 61.0gn of H2o.
This is approx 6.0gn more than the standard 7mm-08 (almost an 11% increase). For comparison can anyone tell me how much H2o capacity gain there is for the 7mm-08 AI?
Some preliminary calculations are showing estimated velocities with the 180gn Hybrid at 2,700 - 2,750fps using powders in the H4350 range. This is with around 47gn of H4350 for 91% case fill and 100% powder burn. RE16 can deliver another 30 - 40fps.
Powders like Varget are too fast and the slower powders like H4831 appear to be too slow. They need a compressed load at 105% to reach the same velocities and only a 95% powder burn.
So is it worth effort? I'd really like to see what the 7mm-08 AI and 7mm SAW bring to the table.
The short Shehane would require considerable brass prep. I'm assuming .284W brass would need to fire formed in a full Shehane chamber before being sized down to the Short Shehane. If straight .284W brass was sized down in a Short Shehane die would the lack of side wall support cause case wall collapse or deformation?
Then there would be some neck turning and/or reaming required. I'm starting to wonder if the juice is worth the squeeze.
Lastly, (if it was feasible enough to warrant further experimentation) in an effort to minimize reamer and die cost for the first tests I have the idea to use a standard .284 Shehane reamer and short chamber the barrel by .200"
Dies could be made by cutting .200" off the base of a set of Shehane dies.
The issue i see with this approach is that due to the body taper the base of the chamber and dies will be slightly smaller. If my math is correct It will be .002". Would it be an issue squeezing the base of .284W brass down by .002"?
If the first test were successful i would then proceed with a proper reamer and dies.
I'm theorizing ideas for an efficient 7mm cartridge that will function through a short action with standard bolt face and AICS magazines.
I'm already aware of 7mm-08, 7mm-08 AI and the 7mm SAW. All based on the .308 case.
Even though the .284W was originally designed as a short action cartridge, to properly fit in an AICS magazine it would need the bullets seated too deep to be a realistic contender so the .284W or the wildcat .284 Shehane is really better suited for long actions.
So I came up with the idea of a .284 Short Shehane.
The idea is simple. Just shorten the body length of the .284 Shehane by 0.200" which would allow the 180gn Hybrids to be seated properly in the case neck and keep the overall length under 2.900". Think of it as a slightly chubby 7mm Creedmoor.
I'm thinking to base it on the Shehane because the base .284W case is larger diameter than the .308 case and the reduced body taper of the Shehane adds a couple more grains of H2o capacity over the standard .284W (and also for a second reason that I'll mention further down).
I have mocked up a design in GRT and it estimates the powder capacity to be 61.0gn of H2o.
This is approx 6.0gn more than the standard 7mm-08 (almost an 11% increase). For comparison can anyone tell me how much H2o capacity gain there is for the 7mm-08 AI?
Some preliminary calculations are showing estimated velocities with the 180gn Hybrid at 2,700 - 2,750fps using powders in the H4350 range. This is with around 47gn of H4350 for 91% case fill and 100% powder burn. RE16 can deliver another 30 - 40fps.
Powders like Varget are too fast and the slower powders like H4831 appear to be too slow. They need a compressed load at 105% to reach the same velocities and only a 95% powder burn.
So is it worth effort? I'd really like to see what the 7mm-08 AI and 7mm SAW bring to the table.
The short Shehane would require considerable brass prep. I'm assuming .284W brass would need to fire formed in a full Shehane chamber before being sized down to the Short Shehane. If straight .284W brass was sized down in a Short Shehane die would the lack of side wall support cause case wall collapse or deformation?
Then there would be some neck turning and/or reaming required. I'm starting to wonder if the juice is worth the squeeze.
Lastly, (if it was feasible enough to warrant further experimentation) in an effort to minimize reamer and die cost for the first tests I have the idea to use a standard .284 Shehane reamer and short chamber the barrel by .200"
Dies could be made by cutting .200" off the base of a set of Shehane dies.
The issue i see with this approach is that due to the body taper the base of the chamber and dies will be slightly smaller. If my math is correct It will be .002". Would it be an issue squeezing the base of .284W brass down by .002"?
If the first test were successful i would then proceed with a proper reamer and dies.
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