Straight copy and paste from www.jarrettrifles.com/calibers.html
The .243 Catbird was conceived to be an ultra-long range 6mm varmint round, but also works quite well for a deer cartridge. It is a .270 Winchester case necked down to .243 and blown out with a 35 degree shoulder. This case is delivering some impressive velocities. Our original goal was to get 4000 f.p.s. with a 68-70 gr. bullet. However, we surpassed this goal with 4100 f.p.s. We were able to achieve 3800 f.p.s. with an 85 gr. bullet and 3500 f.p.s. with a 95 gr. Nosler Partition. This cartridge packs plenty of punch for long range varmint hunting or medium range deer hunting.
Cases for this chambering are quite easy to form. You simply run a .270 Win. case through a Catbird full length sizing die, load it with a reduced load and fire-form it by simply shooting it through your gun. There are two versions of this cartridge. One of them has a tight neck and the other has a standard neck. The cases for the tight necked version must be neck turned before they can be loaded and fired. Once cases have been fire-formed for either version they can be reloaded as any other cartridge. While this cartridge may not be for everybody, many people will love the high velocity and the accompanying flat trajectories that can be achieved. This round may be the fastest 6mm cartridge ever developed. Of course, you have to give something to get something. She is a “barrel eater” — expect about 1500 rounds of good accuracy.
Definitely not my cup of tea.. er my rifle cartridge of choice.
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