changing guns/barrels for NMC

Is is permitted to switch to iron sights for the NMC either by changing pistol or barrel(M41)?

Yes you can shoot one gun for SF and a different gun for the NMC. I think you are allowed up to 4 changes in a 2700 match. 

I know a guy that shoots a 208 only for SF because it is finicky but crazy accurate. 

However, you can not shoot one string of SF and use a different gun for the 2nd string unless you have a mechanical problem.