The .300 came from the need to make a short-action round feed in a long-action receiver. I tried and tried to design a shell that would use the base datum diameter of a 300WSM, but that is one of the largest ODs short of .590" shells that there is. I ultimately had to have a new barrel made and design the shell from Square One. It took me several months to finalize the dimensions because I just did not know what I wanted versus what could be cold-formed. I eventually decided to use the '06 case length, shoulder angle and neck length because it's a design proven over 100 years and several billion rounds fired in WW I, WW II and Korea. Everything nowadays is short & fat with steep shoulder angles and near-zero case taper. I wanted an easy-feeding rifle, so I used the gentle shoulder angle from the '06.So you just do this for the halibut? Cool hobby, amigo. Wish I had that kind of time and scratch available...
The rifle was a half-minute gun before the rebuild from 300WSM to 300NDM. It's now a quarter-minute rifle, and five-shot groups in the .200s are certainly attainable. The best it has done is five shots having a 0.135" center-to-center from 100 yards. The rifle is equipped with a 6-24X x 50mm Vortex Viper PST scope. One of these years I will get my act together and make it hit that 10" steel plate from 1000 yards. It's a long walk out and back, so I'm just not all hyped-up to do it. I have thought about bringing my bike out to the desert and riding it out to the target and back, but my purpose-built shootin' pickup (see below) hasn't run since November 2015. I had that top custom-built. It's five feet high inside, has a shootin' bench in there and lots of other things you'd need when you head into the desert for a day of shooting. The top provides all the shade you'd ever need on a hot & sunny day. You could even shoot when it's raining. The rear has two swing-out doors that provide all the access you need to get in and out of it.
The .358 was done because I wanted to see what the 300NDM would look like as a .35-caliber. I started it in July 2012. Eleven months and $4500 later, I knew. The .358 Sierra Stomper is a sledgehammer. I like to say it will kill anything on the planet short of elephant, hippo and rhino. I'll never know for sure because I will never go to Africa and hunt with it. I'll never hunt anything on the North American continent, either. It was built in an attempt to get 4000 foot-pounds of muzzle energy from a 250-grain bullet and to deliver two MOA accuracy at 250 yards, which is about the effective limit of the .35 Whelen. The recoil of the Stomper is so vicious, I can only shoot it about a dozen times and I've had enough for the day. The rifle for the 300NDM weighs nineteen pounds; recoil is about what you'd expect from a .243 Winchester.
To bring these rifles to fruition I had to have Hornady Manufacturing and Lee Precision make forming dies (4 by Hornady), reloading dies (2 sets by Hornady), neck-only dies (2 sets by Lee) and Factory Crimp Dies (2 by Lee). Each die is around 75 to 90 dollars; can take up to a year to have them made. I also had special-length trimming mandrels made to get consistent trim lengths. I started out by turning the necks on the .300 to get 0.002 inches of clearance, but there is a total neck clearance of 0.001 inches when the necks are not turned, so I don't bother to do it anymore. The necks of the .358 are not turned; there is 0.005 inches of clearance there. Cases were formed from Hornady .375 Ruger Basic brass until Hornady quit making it. I now use the factory-formed .375 Ruger case. If Ruger ever quits making the rifles for that case, I'm SOL.
I have the time and scratch available to do this because I am not married. I am also fat, bald and uglier than sin-- which is why I am not married. Some would say being afflicted as I am is a curse. I praise God for Him having made me as He did. Sure, I want a hot blonde and all that jazz. I'll never have it, but look at the money I'm saving! No wife, no kids, no ex-wife, no ex-kids, no college tuitions, no braces on the kids' teeth, no house with five bedrooms and three bathrooms-- the list goes on and on. I can spend my money as I see fit. I will never have to say "I'll have to ask my wife" about anything. If I see a rifle I want, I just say "Wrap it up! I'll take it."