jakester
If your interested in that type of cartridge there are a few others you may want to look at. There is the .38 Casull and the 40 Super. Like the .400 they are bottleneck cartridges.
As far as I know the .38 Casull is only available as a complete gun (1911 type) from Casull. I believe that a factory gun in .40 Super is being made by STI. Like the .400, 40 Super conversions/barrels are available from numerous sources.
FWIW.