Do you know a source for 416 Rigby-sized basic brass?
A one word answer would be no.
But then i dont own a gun using those cases either.
My brother in law does however, and ive shot his gun numerous times.
His velocity is about 100 fps faster than my sons 338x378, both built by Bruce Baer.
His barrel is 6" longer however, and that no doubt makes a difference.
There is no comparison between the case size of those cartridges and the Big Baer case, which is Baers version of the Cheytec.
He furnishes the cases for both the Rigby and the Big Baer, but other than from him i would have no other source.
Of coarse the 378 Wetherby cases are available where ever.
For about 25 years i used a 36" barreled 30x378.
And frankly if i had my time to do over i still would be.
My barrel was never a really good one, and it got to where i wanted to replace it. And at the time it seemed logical to me to go to the 338 Big Baer.
There is no doubt that the 338 is a better choice for the longer distances of say beyond 1500 yards.
But them how many animals are shot beyond 1500 yards?
How many days during a season are the conditions such that we can we even hunt at those kind of distances? Especially in places like PA.
So fact is that we live in a dream world for the most part as for hunting.
But, if you want to shoot at say steel targets at the longer distances then the large 338s would be a good choice.
And by large, i mean large, and not one that sends a 300 gr less than about 3000 fps.
Sit behind them looking thru good tripod mounted optics as they are being shot, and what you will see will need no explanation.
It would be best to talk directly to those who build them, and not just be listening to people like me on sites like this.
Id highly recommend you talk with Bruce Baer, also Kevin Cram at Montour Rifles and get their opinions.
www.longrangehunting.com
Now if got somebody else to pack for you that's a different story. 
