ihmsa forums

I am having a 300 Whisper Rimmed barrel made and have made up some brass from 357 Max. Got to checking several load manuals and Internet info to find the correct trim to length for the 300 Whisper and have found five different lengths. Anything from 1.368 to 1.400.
I wonder where all these different versions are coming from. Tried to look at JD Jones website but they don't give specs on the case.
I have emailed MGM to ask them specs for their chambering reamers.
I have cut my brass to 1.360 and figure it will shorten a little once the case and shoulder blows out.
Anyone out there have the "real" 300 Whisper trim to length?

My brass for my BF 300/221r is 1.390. There are many versions, slight differences in all. The 300 Blackout was supposed to standardize to a SAAMI standard.
Also, I make my brass from Starline DW360 brass. Not as much trimming, and you don’t get into the thick part of the brass like you do with 357max brass.
Good luck with yours. It is a great little cartridge!
Melvin