Answer from DelTon
 

I read some old posts and noticed some other places like del ton and cmmg used to sell 300 whisper uppers I wrote del ton recently and they quickly responded : "This does not exist in our inventory. I'm not sure about the situation overall with other companies. I know conversion kits started becoming scarce about 2 years ago and were almost non-existent after the election. We would like to carry the 300 Fireball again, but the demands are not that high which doesn't put it high on our list. We plan on offering more conversion or calibers by the end of the year, but I have not heard if the 300 Fireball will be a part of it. Thanks for your inquiry" Does anyone have a Del Ton made upper that would like to give a review? Just wondering about the specs.

Del-Ton made their rifles using Olympic barrels in 300 Fireball. Since Olympic quit making barrels in this caliber, Del-Ton no longer has a source.
Olympic barrels were known for their accuracy... specifically in their SUM broach cut barrels.