Now that I have my KKM barrel I've been practicing making dummy rounds that headspace good  with 357sig dies while I wait on the 9x25 dies. I've noticed that underwood 9x25 brass is .985". The problem I'm having is that after I form 10mm brass, it stretches to >.990" which causes the brass to headspace off of the mouth instead of the shoulder. I'm having to trim the 10mm brass .007" to keep the formed brass less than .990". Is this normal? Maybe this wont be a problem when I use the right dies. Thanks for any replies.

I have never had to trim any of the 10mm brass that I have formed using the Dillon 9x25 Die set. 
Are you sure that the barrel is not short reamed, and needs to be finished reamed after the barrel is fitted to you gun?
I will say that with the Dillon Die set it was possible to over size the cases which pushed the shoulder down/back too far.  So dies adjustment is critical to proper head spacing.  If there is too much head spacing you will likely see case neck separation or splitting or light primer strikes.

Thanks for the quick reply. The barrel is a drop in fit. I guess i'll find out more about the headspace when I get the dies in.

Snapper what are you using for the neck sizing expander?  I use the RCBS 9mm neck expander because it has a better reach before the case mouth flair starts.  It provides a straighter neck section for bullet insertion even with cast bullets.

I've been using the Lee universal expander. So far it's been alright. The bullets still rock a little and you have make sure they're straight.

I just got the Dillon  dies in and they solved all the problems I was having with the 357sig dies. I'll have to give the rcbs expander a try. thanks again

You're welcome and good luck! 

Well, I must have messed up 10 or so cases trying to seat bullets. I'm ordering the rcbs expander now. I've tried the Lee universal and the Lee sig powder through expander with no luck.

9mm rcbs expander die has been order along with a lot of brass to practice with. I also have some Underwood ammo on the way. That way I cant try out new barrel until I learn how to hand load this round.

Take your time as you size the brass, don't screw the sizer all the way down to touch the shell holder or plate, go slow and check case fit in your barrel before expanding the case neck.
Once you get the case to fit in the chamber flush to the barrel hood, then expand and recheck the fit.

These are pics of the ammo flush with the barrel hood of my LWD barrels as chambered and also fit the S&W1006 Bar-Sto conversion.

Thanks for the pictures Shadow. I'm getting good headspace when I size the brass now. But after I use the expander it wont fit  back in the chamber. I am probably gonna have to wait on the rcbs expander. Right now im using a lee 357sig powder thru expander that I am sanding down to get it to work. The ID of a  formed case is .333"at the neck which seems way too small    I've got about 2 out of about 30 dummy rounds to chamber for me so far.

Well the rcbs expander die has made a world of difference. But it did change the headspace a little. I had to resize and bump the shoulders back . I guess that is normal?

It could be that the expander is opening the neck area deeper down, thus the radius changed slightly.

Thanks for all the help. After a lot of practice forming and seating bullets and reading all your posts , I'll have to call my first trip to the range a success . Fun to shoot, accurate and zero malfunctions . I loaded up 40 125gr HSTs and 15 125gr gold dots over 10.5grs longshot. I was at an indoor range so couldn't use my chrono but the brass (sig sauer) looked good.

Sounds great, glad I was able to help out...