Seems like a lot of work to gain nothing. Why not get a set of dies and download the 7.62x54 if you want less recoil? I have played with quite a few single shot and bolt 30-30's. Even using spitzer bullets and loaded to the max it is far from a "400 meter" round. I would say 250yds is the farthest I would consider the 30-30 good for on deer sized game. With 150gr spitzers and loaded above the max to ~2500fps at 400 meters your down to only about 1600fps. Most bullets need more velocity then that to expand reliably. If you’re keeping to standard 30-30 pressures you’re going to be down to about 1400fps at 400 meters. Most bullets are too hard to expand at those velocities. I have shot deer with 150gr spitzers from a 30-30 handgun and have found that they expand poorly at less then ~2000fps. In talking with a couple different bullet manufactures about this most recommend around 2000-2200 fps at impact for proper expansion in their 150gr bullets. I use 125gr ballistic tips loaded to just about the max and get 2500-2600 fps depending on the gun and barrel length. The 125gr ballistic tips are not as tough and should expand fine out to about 250yards. Another thing to consider is while the nagant is a strong action and can handle more pressure then a Winchester 94 the 30-30 case cannot. Keep it to normal book loads as the cases can't handle the pressure. Tougher cases can be made from .375 win and you can load hotter but then you’re just trying to turn the 30-30 into a .307 Winchester. If you’re looking for a rimmed .30cal round to shoot 400 meters your better off just starting with the .307 Winchester or .308 Marlin. Their basically a rimmed .308 with the 30-30 rim diameter. A nagant in 30-30 does sound like a fun project, but lots of work to get it to work right and have a magazine function. It would be a great deer rifle once finished as long as you take into account the limitations of the round. It is far from a humane end ethical "400 meter" deer round with any bullet. If you do decide to do it be sure to post it as I would like to see it. I really like bolt action 30-30's even with their limitations.