Its been done. Read Parker Ackley's (as in "Ackley Improved") reloading and shooters manual. There is pretty much any wildcat you can think of in there. There is even that one mentioned specifically but Id have to dig the book back out to remember. Either way, that book will blow the mind.
In addition to th 6.5mm that I never saw other that in books, there was a "7MM waters" that had a brief period of popularity in the silhouette crowd, but it was quickly trumped by the 7mm-08, that is still offered today. The 30/30 case is a little weak for the higher pressure wildcat loads. I went the other way, with a bigger bullet using the .357 Herret.
I want to see someone cut the neck off a 5.7 FN case and make a hot .25 auto (magnum) round with all that extra space for powder
To quote the Bare Naked Ladies, "It's all been Done...woo hoo hoo!"
Going back some 40 years ago, JD Jones started making his famous hand-cannons, which were very powerful cartridges designed for the T/C Contender break-action pistol. He tried making a 6.5/30-30, but soon realized that he'd be better off using the similar, but stronger, 225 Winchester for parent brass. The end result was an entire lineup of cartridges, among the most successful of which was the 6.5JDJ. This is a 225 Winchester case opened up to .264" and blown out, with a 40 degree shoulder and fairly short neck. It's a surprisingly effective deer cartridge out to 250 yards or more and is devastating on varmints. I got mine back in the mid-90's and just last summer shot a 5-round group that was barely over an inch at 100 yards. EA Brown also made his version of this round, called the 6.5 BRM, again chambered in a T/C.
The 7-30 Waters is a much more popular variant based on the original 30/30 case, but also blown out to a straighter body and sharper shoulder, but still suitable for use in a tube-fed lever-action rifle.
My brother is trying to convince me to make some 300 blackout cases. He's never reloaded, I've never wildcatted, and we don't own anything in 300 blackout. That said I still kinda want to do it just because it seems fun. I wish I had the machine tools necessary to make dies for all this stuff.
baba, Lee has regular production 300 blackout dies offered for sale. Buy the set of dies and a barrel chambered for it, you'll be ready to go. Sound like a fun close range cartridge.