Nobody I know with a K-Hornet starts with anything but standard cases, either. But in all these instances, my friends and I were or are working with factory chambers or a chamber modified by a known, reputable gunsmith who had marked his work on the gun. 

Street people say "Know your dealer."  I say "Know your gunsmith or know your chamber" -- both are useful to keep out of trouble. 

I draw the line at using standard cartridges in an unknown chamber. Why? Because a chamber modified by someone unwilling or too ignorant to mark the modification may or may not be a person fit to play with matches, much less a firearm. I might do it with a Mark V but not with a Mauser 98 derivative unless I could secure it and fire it with a lanyard. 

Life is so much simpler with a chamber cast, a digital caliper, and the SAAMI specs. This will also give you useful information about the most advantageous bullet seating depth, maximum loaded neck diameter for adequate bullet release, and several other pieces of useful information you can't get any other way. 

"Every man for himself and God for us all" but I value my eyes.

What I was suggesting is that IF a 300 Weatherby case won't come close to chambering, which it won't in a 300 H&H Magnum chamber, then I would fire a 300 H&H cartridge in the chamber and see what the extracted case looks like when it comes out.  Assuming my head was still attached...

Everything else being equal, firing 300 H&H cartridge in a 300 Weatherby (or 300 Ackley Improved, etc.) chamber is no more of a problem than firing 22 Hornet in a 22K Hornet chamber.  That's how you made brass before you could buy it...

I'd worry much more about a standard action converted to short magnum, where some people did/do some dumb/dangerous things, than I would a long magnum 300 H&H versus 300 Weatherby.

Lou

Louis Luttrell said
The Model 70s most affected are 22 Hornet, 300 H&H Magnum and 375 H&H Magnum.  Usually the gunsmith responsible for the rechambering would mark the barrel in some way, but not always.  Especially the H&H Magnums made before 1950, when the caliber stamps were only "300 MAGNUM" and "375 MAGNUM" (no "H&H" or "Weatherby").
I cannot give an accurate estimate of how often the rechambering was done and not marked, mainly b/c when it wasn't marked it wasn't marked.  But the rechamberings themselves were pretty popular back in the day.  I once took a very nice transition 22 Hornet of my Dad's to the range and after firing extracted beautifully fire formed 22 K Hornet brass!!!  No marking to indicate the conversion...
I guess my approach to these guns might be considered dangerous.  But we are talking about rechambering (using a reamer to open up an existing chamber), not a conversion, for example from 30-06 to 300 Winchester Magnum.  That conversion can be done badly, unnecessarily removing metal from behind the bottom recoil lug under the mistaken assumption that it was needed for the cartridge to feed and inadvertently creating an unsafe gun.  Even a chamber cast won't pick up that disaster in the making!!!
But, after all, the way you MADE 300 Weatherby, 375 Weatherby or 22 K Hornet brass was to fire form it from 300 H&H Magnum, 375 H&H Magnum, or 22 Hornet.  Before purchasing, I would check that 300 Weatherby or 22 K Hornet brass would not chamber (I used to hunt with a Weatherby 300 Magnum so have lots of brass), but if not I'd just shoot the gun and see what the brass looked like.  I think the only danger in this would be the situation Zeb described, where an incompetent gunsmith cut a sloppy chamber and didn't use Go, No-Go gauges...  But if the larger brass wouldn't chamber I've just assumed that the chamber was factory original.
Maybe I'm just too much of a "risk taker"...
Lou
  

Lou,  I think what you are suggesting the OP do is load and fire a 300 HOLLAND case and see if it fireforms into a Weatherby case - or a 300 Ardvaark or a 300 Mashburn Long, or a 300 Ackley Long, etc.  

I think the OP proposed to fire a 300 WEATHERBY cartridge. 

cj57 said
Congrats! Nice looking 86 in a scarce caliber 
  

Thanks. I was looking at a 38-56wcf with a pistol grip, single set trigger, 28" barrel and i'm 99% sure the butt stock was changed because it was so much lighter color than the forearm. It also had Philip screws rather than slotted in the butt plate. I almost pulled the trigger.

Bert

Bert

25-20wcf 

First one I ever seen in this configuration. Seems to be non-bubba. Any opinions?

Thanks!

I cannot give an accurate estimate of how often the rechambering was done and not marked, mainly b/c when it wasn't marked it wasn't marked.  But the rechamberings themselves were pretty popular back in the day.  I once took a very nice transition 22 Hornet of my Dad's to the range and after firing extracted beautifully fire formed 22 K Hornet brass!!!  No marking to indicate the conversion...

I guess my approach to these guns might be considered dangerous.  But we are talking about rechambering (using a reamer to open up an existing chamber), not a conversion, for example from 30-06 to 300 Winchester Magnum.  That conversion can be done badly, unnecessarily removing metal from behind the bottom recoil lug under the mistaken assumption that it was needed for the cartridge to feed and inadvertently creating an unsafe gun.  Even a chamber cast won't pick up that disaster in the making!!!

But, after all, the way you MADE 300 Weatherby, 375 Weatherby or 22 K Hornet brass was to fire form it from 300 H&H Magnum, 375 H&H Magnum, or 22 Hornet.  Before purchasing, I would check that 300 Weatherby or 22 K Hornet brass would not chamber (I used to hunt with a Weatherby 300 Magnum so have lots of brass), but if not I'd just shoot the gun and see what the brass looked like.  I think the only danger in this would be the situation Zeb described, where an incompetent gunsmith cut a sloppy chamber and didn't use Go, No-Go gauges...  But if the larger brass wouldn't chamber I've just assumed that the chamber was factory original.

Maybe I'm just too much of a "risk taker"...

Lou

I have heard that many H&H Model 70's were converted to Weatherby. I don't know how true that is, or what is meant by "many". The % that were converted and not identified as Weatherby conversions.... if that is dangerously high then I see your point. Are you saying all Winchester model 70's that are H&H Magnums should have a chamber cast to find out if they are still H&H?

My prior thoughts were if your Model 70 H&H Magnum without anything but factory script does not chamber a Weatherby magnum it can be considered a factory H&H Magnum. If my thoughts are wrong then word should get out that these Model 70 H&H Magnums should be avoided unless you take it to a gunsmith 1st. Considering that the cost of these pre-64 H&H Magnums is high, the high risk of a botched conversion let alone a well done conversion should become a big factor in any sale, especially an auction where chamber casting results aren't known. There is a lot of money at stake, and even more important, your safety.

  If it was mine I would put a good scope and mount on it and leave the wood alone. I would hunt with it. I've got a grandson with one I bought cheap because of a recoil pad. He's pretty proud of it and the nice bucks he's shot with it.  T/R