When I ship Firearms which is all the time. I never put the words "Firearm" or "Gun" anywhere on the label or box. For instants if the name of the place receiving the gun is Bills Gun Shop, I will put Bills Shop or Bills G shop. To put Gun or Firearm just lets everyone know what is in the box. There has been a lot of theft and damage cause to firearms through USPS, FedEx and UPS. The reason UPS and FedEx decided to ship handguns by overnight only is because of theft.
You can easily custom make a box to fit your rifle. Just get a flat piece of cardboard measure it to size score it with a narrow blunt object and fold it. Keep doing that until you have the size box you want. I would overlap the first and last flaps. Then tape it. Make it long enough for the end flaps. Make enough room for padding in the bottom and top, the two most critical areas that padding is needed because long boxes are always being dropped on their ends. I learned that the hard way by a cracked butt stock on a Winchester 94. Make the box with just enough room for your gun and the packing material. The tighter the better. You could make a small box to fit the gun and have a box within a box.
GC