Another option would be UPS used to sell a 6x6x48" cardboard box, if that's long enough. Rifle in a soft case, packing material around the soft case. Used that method a number of times with good results.
Ditto, and sometimes I skip the soft case. Wrap the rifle in a couple of layers of bubble wrap, you can get long rolls at Walmart or several other places, then use enough padding of whatever to keep it centered in the box. You can buy 6x6x48" boxes from several sources. I've been using heavy brown wrapping paper as padding, you can get rolls of it again at Walmart. When it's wadded up it has has a fair amount of crush strength to use around the rifle.
Shipping by private parties is getting ridiculously expensive but if you separate the stock and barreled action you can put both into a 6x6x36" box and save $10-15 on shipping. Plus with each one bubble wrapped they help keep each other stabilized in the box.
U-Haul will sell individual boxes on line, most everybody else sells them in 25 packs, but you might laugh at how they ship them. I bought a half a dozen 6x6x36 and another half dozen 6x6x48 and they came in a huge flat box 59x48x3" but shipping was still really reasonable. I cut the shipping box into two flat panels and now use one to put under my motorcycle for oil changes to keep the garage floor clean with the other panel as a spare.
