I own many MNs and tried many different brands of ammo. There is a yellow/silver tipped 181 gr. load (copper washed case) that proved to be a bad choice for all my MNs - it seems to be loaded
too hot and makes it very difficult to open the bolt after shot is fired. But it's quite accurate. This was a case for all my MNs w/o exception. I heard that it is intended for use in machineguns. May be you should stay away from this "yellow/silver bullet" ammo.
On the other hand, Hungarian 147 gr. ammo from AIM was just about perfect, not a single problem. Also, 203 gr. SP Russian hunting ammo proved to be very accurate for practical purposes. Both have steel cases, by the way...