Other than 300 GNR, why no 30-30 based revolver cartridges?

based on the .220 Swift which has a small rim, and is about the same size as the .30-30. I think also a shortened version of the .375 Supermag to fit a 1.75" cylinder. Both wildcat rounds.

You can make .401 Herters from .30-30 brass, it's just a bit more involved than some others. My best .401 WSL rounds are from 7.62x39mm blown straight. Some have used .30-30 brass. I also have used modified .41 Rem. cases with long seated bullets, and modded .414 SM brass to make perfect replica factory .401 WSL (0.406 - 0.408" bullet) cases.

The .375 is interesting. The .375 Win. is a good round, much better to me than the .38-55 Win. that had wildly variations in bores. Up to 0.380"! Starline makes extra thin necked cases to compensate for fat .38-55 cast bullets. It's a boon to the Reeder designed .400 GNR round and my choice for making cartridges. .30-30 brass can vary wildly as to rim "thickness" as well as neck wall thickness. I've hit many a snag goofing around. One time I even CAREFULLY deepened my Contender barrel rim recess 0.003" to allow for these variations and make my life easier on a fancy custom barrel. Not for the faint of heart or shade tree machinist.

I once made some "rimmed" 10mm Auto rounds and made them work. But it was not easy. Pawl gears necessitate very small rim diameters (on L-frame size guns) and the case is very thick on a .30-30 that far down, requiring much inside case reaming. This experiment was to make rimmed rounds so I did not need full-moon clips. Another hurdle was headspace. I had to thin the rims to a point that they could not be used for headspace, I used the case mouth for that, and simply used the rims for ejection. But the cylinder closed and they did not drag. Easier to show than explain, but it was fun to try and make some.

between the 250 GNR on the 32 H&R case for small game, several on the 357 maximum case including the 224 GNR, 256 GNR, 284 GNR, and 307 GNR, then the 356 GNR which is one of our top 3 cartridges every year based on the 41 mag case, then several on the 445 Super Mag case including the 340 GNR, 358 GNR, 375 GNR, 401 GNR#2, 416 GNR#2, then on the 44 mag case the 401 GNR and 41 GNR, and on the 454 Casull case two very popular cartridges the 429 GNR and 410 GNR, in the smaller calibers on the 22 K-Hornet case, the 240 Banshee and 255 Banshee, then in the bigger stuff the very popular 455 GNR on the 475 Linebaugh case, the 470 GNR and 501 GNR both on the 500 S&W case, then the 510 GNR or either the 510 GNR case or a cut down 500 Linebaugh case, then the 500 GNR based on the 510 GNR case, and probably more than I can't think of right now. I came up with the 354 GNR based on the cut down 30-30 case but it wouldn't beat the 356 GNR plus it was more of a hassle with cutting down the case case and such, so I dropped it. So I think we have the revolver cartridges pretty much covered. Another reason is the money factor. To come up with a new cartridge I have to design the new cartridge, send those numbers to the reamer maker (Dave Manson), wait 6 months and when the new reamer comes back build one barrel with the new reamer and run tests on it with made up load data off the top of my head, and by jerry rigging the cases. Then if the data looks good to me, build another barrel for the full testing by Sean Harper. Send that barrel to Sean, wait several months and if Sean says it is a viable cartridge send all the case data to Hornady and order the dies (usually a dozen at a time) and wait another 6 to 8 months before getting the dies back. So in the process of all this I have spent almost $2000 and waited right at 2 years. And I have no idea if it will fly with the customers or not. So I have to make sure that new cartridge is good enough to make a lot of wildcat reloaders happy. So there is a lot more to it than just saying I am going to build a new case on the 30-30.