My 400 Whelen is a deluxe R.F. Sedgley Springfield with the Griffin and Howe chamber (very important).

The shortcut to all the 400 Whelen knowledge is to buy the book WILDCAT CARTRIDGES (which,in the interest of full disclosure, I published) by Fred Zeglin. It has a whole chapter written by the late Mike Petrov on the 400 Whelen featuring loading data, new rifles etc. etc. Available from Amazon.com

http://www.amazon.com/gp/product/B003TT5OBO

That's my 400 on the cover, left side.

It is very accurate and took this Elk @250 yards with the now (sadly) discontinued Barnes 325 gr X. (I have a hoard)
The lighter TSX also works well as does the 400gr Woodleigh. It is also a great cast bullet gun as you are right at the velocity crossover point from cast to jacketed bullets.

A Banner Mauser is a great find, do you know who built it ? Could you share some pics ? If you ever decide to sell it, put me first.

Slug the bore and cast the chamber. Typically any .410-.411 bullet will work. The quick way to get started is to buy a set of 41 S&W dies from Lee. If you can get the basic brass run it into the FL sizing die until it will chamber, check OAL, prime, add 55 gr of 3031/4895/4064 and a 41 caliber jacket revolver bullet seated backwards just deep enough to firmly engage the lands. Pull the trigger and you'll have a case fireformed to your chamber. (They are remarkably accurate at 50 yards)

Check your cast and case against the G&H measurements in Petrov's chapter. IF you have a G&H chamber, you're good to go. If not and it's just an 06 necked up, I would get it recut to G&H specs. There are a ton of verions of the 400 W like the 400 Brown-Whelen, 411 Hawk and so on.

Once you know what you have go the "three fired case" route to the diemaker of your choice. Dare I say Lee makes as good custom dies as anyone, especially for a non-critical round like the 400 W.

Excellent purchase. With bullets you cast from scrap wheeleights, You can shoot it cheap as today's price for 22 rimfire.