I was shooting a .300 RSAUM in 1k matches around a few States and Raton for about two years; to the point; I shot next to a guy in several matches that started using a 6.5 SAUM based on the .300/7mm RSAUM... Man... that thing could shoot! With that said... I had a 6.5 Rem Mag for years; it was nothing more than a little more powder 6.5 SAUM with a belt and far ahead of it's time as a Remington cartridge in the 60’s... Today'..., with all the good 6.5mm bullets and powders, I think I'd go with the “6.5 Rem Mag” if I was going to build "one more" short magnum; a very impressive and over looked cartridge which was introduce in the wrong rifle the M600
As for 6.5 SAUM reamers; you might try HTG George Vais in Boise ID
Barrel length; 26" for hunting, 30" for LR matches. And use a long action even though you could use a short, the long let's you use the long bullets in the magazine.
I have several 6.5wsm's and love them all. with 140's and 26" bbl they all run 3150 to 3250fps. I use long actions and seat them very long. Between my friends and I we have 6 all shoot very well with the same powder H1000.
I am looking to build one also. I have a SA Remington action with a Wyatt extended magazine box that will let me seat to 2.95 COL. I'm thinking it would make a great 600yrd deer rifle. I was going with a 6.5-284 but it looks like I would get more powder and less of the bullet in the case with a 6.5 SAUM. Maybe a 26" 1-8 twist Remington mag taper barrel from Pacnor with 6 flutes. I have a Bell and Carlson Alaska Ti stock and a kahles Helia C 3x10x50 sitting on a shelf. If someone has the reamer I could rent it would be great. Don't have all the funds set aside yet but will by late fall. Saving for my 2013 deer lease now. I like to have next year's lease money set before this year's season.