I wanted to know if there are any major problems in rebarreling a magnum bolt faced Remington 700 to a 300 Ultra or any new Ultra ??? I've heard that there are longer magazine boxes if needed, but was wondering more about feeding problems and magazine capacity as is. Any insight?

A magazine box is availble that has a triangle shaped hole cut in each side. That allows the rifle to hold 3 of the larger diameter U'mag cases. Brownell's has it and maybe it is longer than regular mag box, I dunno.

feed rails are more of an issue than the mag box. The belt on the mags is near the same as the dia of the RUM's. I built a 338RUM on a wthby MK V action that was originally a 257wthby mag,..and the RUM feeds from the magazine,..even with it seated to touch the lands

Jesse,

I built one early on, can't remember how many years back, but bought one of the first sets of dies from RCBS, been playing with if for a while.

I took a std rem mag action and had dewight marshal a very good south texas gunsmith cut the rails, he tried to make it feed using factory follower, it feed but not all that great. I lowered the sping pressure on the follower and it feed pretty good. But only 2 down. I tried to get Rem to sell me the follower they use with the factory rifles, but you whould have though I was trying to buy a hootus for a hobbie horse, they told me they didn't have one, so I bought a HS sgl stack mag/floorplate and it now holds three down and feeds like a champ. The HS mag is a pain in the butt but it's better than what I had.

I would buy a factory 300 ultramag if thats the way you want to go. But if I had it to do allover again I would not have built one, not sure you can live on the difference between a 300 wby for all the powder you have to burn.

Here is my pain in the ass 300 ultra mag, but it does shoot good, Montana Rifleman barrel and Muz brake. Boyd bro stock, still havent fitted recoil pad sinse I took it out of tupperware stock.