I am needing to downsize my primary. After that, I would like to carry a J 38. They won't be the same chamber; hmm, now I need to really think about this!:banghead:
I was thinking about this. Why does it really matter if they are not the same chamber? Unless you plan to carry 2 firearms with interchangeable mags? (I know the J 38 is a revolver, not the point.) Point is, unless you intend to carry a back up that uses the same mags, or you just intend to carry a pocket full of loose ammo, you're probably not gonna be interchanging the ammo anyway.
Think through the average defense scenario. One that would have you lose your primary carry. Are you going to empty your backup into the BG then take the time to unload a primary mag and refill your backup's mag so you can keep shooting? Probably not as most defense gun fights are less than 1 minute total.
I could see it for only stocking one type of ammo, sure. But otherwise it probably won't matter much if they are different.
BTW, as it happens, I have actually carried my G19 OC and my G26 CC on a IWB. Guess what, the mags are actually interchangeable. But I wasn't planning that ahead of time.