Can anyone point me to a good source of information for Ultra-mag based wildcat big bores? I've found some on the 416 Ultra / 416 Buff, but have found very little on the 458 version and nothing on a 475 or 500. I'm sure people have tried them so if you know where I can find any info i'd appreciate it. Thanks!

Here's a source from Fred Zeglin's site Z-Hat Ultra Mags. You may be able to get some info there.

http://www.b-mriflesandcartridges.com/default.html

Clintsfolly has a 458 AR (Accurate Reloading) that I shot yesterday in the Michigan get together (see sticky on the hunter's campfire) and it is a 375 RUM necked up to 458 and trimmed back a bit for a standard length action. It has a bit more capacity than the Lott.

Sweet...

so its like a 458 RUGER?

I suppose there are more ways to skin that cat, yes!

The 458 Ruger - if based on the standard 375/416 Ruger case - would have less capacity as it's max diameter at the head is 0.532". The the RUM's head diameter is 0.550" so even cut back to standard length case dimension (~2.5") the usable volume would be greater.

Bryce Towsley made up two, the 358 UMT and 416 UMT on the Ultramagnum case. He wrote up the 416 UMT a few years back in "Shooting Illustrated," using a Remington 700 action, Banser stock--with mercury recoil reducers installed--and a drop box magazine for single column feeding. Interesting cartridges.

It's known as the 458AccRel in Australia as our Firearms Registry felt AR is Assault Rifle ... and that's just a scary name. Case capacity is 4% greater than the 458Lott. It's very similar to another wildcat called the 458AfricanExpress. Ruger MkII and Hawkeye actions just feed it straight from the magazine box with no alterations.
Cheers...
Con

Go over to the Accurate Reloading forum and look it up in the big bore sub forum. what i have is a 458AccRel which is a 375 rum case run into the FL sizing die,fire form with powder and cornmeal, trim to 2.550 then go shoot and have fun lott power but lower pressure and as Con said will fit in a standard mag rifle(7rem meg ,300win mag,338win mag) with little or no feeding work. a lite fast handling stopper that is easy and low cost.Every one that has shoot my rifle just grins a says but it didn't kick as bad as i thought! Hope this Helps Clint

The AR wildcats are trimmed to 2.5", 416, 458, 475, and I don't recall if jeff built a 50 or not.

As far as a 500, the case is the on the borderline of providing a sufficient shoulder to headspace on.

Yes he did ... it's the 500AccRel in Oz and there are two getting about. Based on 416Rigby brass.
Cheers...
Con

Same here ... it's odd but I built my Ruger MkII to weigh like a CZ550. I've owned both the Ruger 458AccRel and CZ550 in 458Lott. The AccRel kicks less even though I'm running 'mild' loads of 500gr pills at 2230fps. There are 4 in Australia now ... and we're all either in the process of or have already lightened them up. I've got my mates here to regulate some open sights he fitted ... he chopped from 24" to 20" ... looks just like a blued Ruger Alaskan in an African stock. Very pretty rifle. He's also managed to get it to 3+1 where the unmodified magazine is 2+1.
Cheers...
Con

Thanks for all the information guys. I've had a blast reading up on things! Looks to me that the AccRel cartiridges got a jump start on the new Ruger case. SOme small differences but real similar concept. I'd probably be more interested in the full length 416 UMT type cartridge and will keep looking for more info on it. Anyone know where I can find a copy of that Towsley article?

Just checked and the trim length is 2.550" on the 458AccRel Clint