Contact MM and tell them your quandrous: "ball ammo feeds, the MM ammo will not". If they are worth a pittance of the salt they ask for their wares, they will send you some new, pronto.
I have two 7x57mm mausers. I prefer to neck size, and once in a while, I grab the wrong ammo, cases formed in the other chamber. My Deutsche Waffen chamber is tight(er) and it takes some effort to bring the bolt down the last 30°, as compared to my Yugo in 7x57. But, I know the load is modest, and once fired, the case slips right out.
IF you have already fired this MM ammo in another gun, and have not experienced high pressure signs, then I suspect your new gun is a tight chamber, and the ammo
may be over size a bit as well. Without gauging the MM ammo as
Steve Marshall suggested, it is hard to say.
If you have not fired the MM ammo period, due give MM a call and tell them you want to be a satisfied customer. We will be here to back you up
. Let us know how things turn out.