question about 700 ultra mag actions

HH .338

Active Member
Dec 14, 2002
hey guys i have a quick question. are all the ultra mag actions (7 rum 300 rum and 338 rum) the same dimensions? are the mags that come with the 300 rum and the 338 rum the same size (will they both allow the same max oal for a loaded cartridge?).
reason i'm asking is because i intend on purchasing a 338 rum first, using it and when i have more money i would like to put a long tube on it and a mcmillan stock. when i do put a new tube on though i might as well go with the 338/300 rum imp version rather then a standard 338 imp version. will i have any problems doing this on a 338 action?

The factory actions for these 3 cartridges are the same dimensionally. The only problem with your goals is that if you want to use heavy and long bullets, the mag will not allow for seating these bullets out.

You are forced to stay within the factory max cartridge lengths which is too short for the 300gr MK or similar LR bullets. In this situation, single feed is the only alternative.

Brownells has a mag box you can fit yourself with very little modification to the rear of the action. It comes with instructions on what to file off, where, and how much. The length on it is 3.91" I believe. My Ultras box is about 3.675".

You will not have any problem doing what you want with the 700. This is where the 338 Lapua case has an advantage, it makes up for some of the case capacity that the long bullets seated deep eat up, as it is larger in diameter and a bit shorter.

OAL on the 338 Lapua Imp is usually around 3.9" with the 300gr SMK. On the other hand, my brother seats his out to 4.1" on his 700 338/378wby and has to single feed them.

[ 02-26-2003: Message edited by: Brent ]
Sure wish I would have heard about the Brownells magazine box before I ordered the reamer for my 7mm RUM.

Spec'd the reamer so that magazine length catridges would be 0.005 inches off the lands.

Bet the magazine box is cheaper than the reamer and the barrel work. I did specify a tight neck also though, so at least I will have that going for me.
Dave Kiff at Pacific Tool can hook you up with a throat reamer for $66.00 if you change your mind and decide to extend it and get the long box. An extra 70 bucks at this point, but it still might be worth it to you if the barrel is still off. Just a thought.

[ 02-26-2003: Message edited by: Brent ]
I wish they had something like that for the detachable mag versions. I have a .338 SS-DM that I would love to convert to rebarrel to a .300 Ultra...
I just tried an experiment and two loaded .300 Ultra cartridges fit inside my detachable mag box with ample clearance. They look like they should feed okay, but perhaps not??
I have not been able to get a hold of him today, but we will be going over to his place in a little while so I'll find out then.

The 4.1" box is for a model 70 Winchester magnum action though, a bit longer than the 700 box. The 700 box, I'm almost positive is 3.91", but I cannot remember if that is ID. or OD. for sure.

I just got off the phone with my brother, he told me his Wyatt box for his M70 has a huge 4.1" internal opening.

I have Brownells old catalog, my father has the new one. He has a paper that has the dimentions on it for all the new mags that came with his. He is going to call me in a while. It is a Wyatt box though.

I'll let you know after a bit what the dims are for sure. You want the one with the triangle window cuts on both sides though.

I wasn't able to measure the box my brother had for his M70 that he says is 4.1", it is at the gunshop. I wonder if he has the 3.91" confused with 4.1" because I don't see one on the sheet here for one "that" long.

The box it 3.91", like I thought, but this is outside dimentions, so you ca subtract a total of .100" for the likely .050" thick steel at each end. Now we are at 3.81", which happens to be about .130" longer than my 300 Ultra box. I wondered if 3.91" was the outside dimention, and it was. I would call Wyatts and see if this is the largest they have, my dad said they don't show one for the Rem 700 in the Brownells catalog he has, he just found that they did when my brother got his winchester box and it had these others on the paper inside, maybe they have even more too.

Here is a copy they send of what needs to be done to fit one in and the mags available at the time. Good luck.


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