The 300 jarrett is a proprietary cartridge from Kenny Jarrett at Jarret Rifles, www.jarrettrifles.com.
It is a 8mm Rem Mag necked down with 35 degree shoulder and pushes a 180 gr bullet at 3150-3200fps. He stocks dies for this at his shop if you decide to go with it. He started the SC "beanfield rifle" craze about 10-15 yrs ago and has aptly capitalized off it. He builds very good guns at Rolls Royce prices compared to other gunsmiths. Just my opinion.
However many other improved and standard 30 caliber cartridges do the same thing with better brass. Such as 300 Ack, 300 RUM etc. Unless your gunsmith just happens to have a absolutely great deal on a new/like new 300 jarrett barrel already chambered, I think there might be better ways to go. For example a standard 300 weatherby with short throat for example cut with reamer for minimum saami chamber and can still shoot factory ammo if you want. Might be 50-100 fps advantage to 30 jarrett in worst case and even in best case. Will that really matter to you?