I "inherited" a rifle and die set for a 300 Wby Short wildcat. Called RCBS with the shop order on the die set. Referencing their records they told me that it's a 300 Wby. shortened to 2.50". I can't seem to get 300 Wby brass to form in the sizing die. They want to buckle. My options are to use 7MM Wby brass but it is slightly larger ahead of the belt. As a trial I ran a 338 Win case through the die and it formed perfectly and chambers fine.

Anyone have experience or hear of this round? Any advice would be helpful.

RCBS probably sells the forming die, or dies.
I had a similar problem forming .30 Carbine into .17 Pee Wee. I was using new brass and the case was just too soft to withstand the pressure. Once fired .30 Carbine commercial brass worked fine. Military surplus worked even better.
In your case, you might want to try using some Imperial Sizing Wax and run it through the seating die before you use the sizing die. Remove the seating stem, of course.
Regards,
HockeyPuck

10at6
Welcome to the forums.
Don't take this the wrong way.
I would find a good gunsmith and see if you could have the barrel set back a turn. Then have it chambered in 300win. That is basically all it is any way. Then your brass worries would be over.

There is no way you are going to be able to squish 300 Weatherby into "300 weath. short". At .325 difference something has to buckle.
Your best bet for brass would be to see if factory 300win would head space on the belt properly.(There again you will need a good smith.) If it does then every time you pull the trigger you have one more "new" case.

Another reason for a rechamber to 300win would be free bore. If the guy who had this thing built was such fan of that radius shoulder he may have also went for the famous Weatherby free bore. I don't even think Weatherby people like it any more.

Good luck and God Bless
Shawn

Sounds like a step in the forming operation is missing, such as a forming cut off die where the excess length protrudes from the case hardened top of the die and is then cut off with a hacksaw and file finished. Might take a look at the old standby, P.O. Ackleys handbook for shooters and reloaders Vol.1 he has a section dealing with forming these short magnums. The excessive freebore is a good point. sounds like you have a 30-338 with a radius shoulder, which even Roy W. admitted, didn't do much. good luck

Yes, The capacity is nearly indentical to a 30/338. I think the idea was to use necked up 270 or 7MM Wby brass. Yesterday I resized 7MM Rem.Mag brass and them fire formed with Bullseye and Cornmeal. The finished brass are about.025" short, but nicely formed. I believe they will work fine, but I'm going to start with very moderate loads. Thanks for the input.

If you resized and fireformed 7 Mag brass and it was just a tad short, then 338 Win Mag brass run through the same process will probably come out just right. Since the 7mm is being opened up you're generally going to get a little shorter, whereas you're necking down the 338 and should gain just a smidge of length in the process.

10at6

I'm not trying to mess with your parade, but bluetick pointed out my first thought when I saw this topic. Actually, I thought the topic might be a clever pseudonym for the 300WinMag.

If you're looking for something weird to play with, and have the cash to do it right you should go balls-out on this. If you're not sure about it then the 300WinMag makes more sense.

Again, I realize "making sense" has no place in a discussion about wildcats. Enjoy whatever you do.

I guess I wasn't very clear. I "inherited" a nice FN Mauser and die set in this caliber (broken stock), but no data in terms of brass forming, etc. I called RCBS and they told me it was a 2.5" shortened 300 Wby and the brass should be formed from 300 brass. That didn't work. I have been trying to figure it out without a chamber casting. By the way and as advised above it is a duplicate of the 30/338 in terms of capacity. 338 Win or 7 mag brass seem to work just fine. I would never build anything like this, but hey the price was right. BTW I installed a Boyd's JRS laminated stock on this and it looks great. For 70 bucks with a pad these are a great deal. Very nice lines and inletted much closer than most. Thanks Again.