For those who have built them, other than opening the bolt face, what additional work needed to be done to get the 404 to function in the MRC action? I am assuming the ultramag mag box is used. How many rounds in the magazine as well. I know the one I built my 375 H&H on would hold 5 down.

I have not done a 404 on an MRC, but MRC did attempt to build me three 9.3x62 chambered rifles. After arriving 5 months late none of them would feed from the magazine, eject smartly, And the extractors were so poorly fit they were not CRF. Montana did not acknowledge the problems but said if there was grisly a problem they would make sure the parts were within specification. I was supposed to leave for an Alaskan bear hunt in 3 months. My nephew was dealing with MRC at the time thrying to get his rifle fixed. It was already approaching 6 months for the repair with no delivery date forthyfrom MRC. Thus I contacted a friend and well respected gun smith to fix them at my expense. It required: new magazine boxes, milling the action to accept the mag box, new mag spring, fitting Mauser followers as MRC’s were too soft, fitting Winchester extractors, adding an extractor clearance cut to the back of the barrel, cleaning up the feed rails, and adjusting and polishing the feed ramp. My friend got them working and we had a great hunt.

In the end my nephew ordered 6 MRC rifles for the shop. One was right from fact. All needed work. He is still dealing with MRC almost 12 months after sending his and the stores two rifles back to be fixed.

It might take a lot of work to make a 404 from an MRC.

If you can't make a 9.3x62 feed on the first try then you need to stop building rifles.

My 404J is on a LH MRC action. I really can't tell you what they did to make it function as I ordered a barreled action from them. It feeds slicker than snot. I'm assuming it has the Ultramag box but don't really know.
It did give me problems with misfires when I first got it but I changed out the firing pin spring to a stronger one and have had no further problems. It holds 3 down and will almost take a fourth.

Sounds like they haven't improved over time. I had one of the early ones when they were using the narrow screw spacing for the rear scope mount holes, and my receiver cracked through the rear ring. I was hoping they had improved, but I'll be avoiding them completely.