I bought a new 15" Stihl bar for my 026, and a new 16" Stihl bar for my 250 together with 2 26RMC and 2 26RSC chains, and the purpose was to be able to switch chains between the two saws (the bars are actually the same lengths). I also got a new clutch drum and rim setup for the 250 and a new rim for the 026.
However, I was close to going berserk when trying to tighten the chain, it just wouldn't do! If I go back to the old bars, chains and drive sprockets the problem is gone, so whatever it is, it does not seem to be the saws.
I was just out in the shed checking on the 250 and the difference is quite big: When the chain is the most saggy the whole drive tang comes out of the groove, and when it's the least saggy the chassis of the chain are flush against the bar rails. I'm pretty sure both saws were the same, but for now I only confirmed on the 250.
As I couldn't find an explanation to this I decided to just don't care, I've tightened the chain so that it still rotates freely from hand power, and it hasn't been thrown yet. Don't look too good but since it cut's wood I'm just ignoring it.