United accepts one item of shooting equipment per customer as checked baggage. One item of shooting equipment is defined as one hard-sided shooting equipment case containing up to five firearms, with or without scopes, 11 lbs. (5 kg) of ammunition and articles used in the firearm sport.

Firearms will be accepted only from a customer who is 18 years of age or older.
International firearm regulations vary by destination and transiting country. Contact appropriate consulates or embassies to obtain specific entry requirements applicable to destination(s).
Firearms are not accepted to and from Bahrain, Denmark, Israel, Nigeria, Qatar and Turkey.
For travel to and from the United Kingdom, pistols, rifles and shotguns must be packed in a hard side rifle case.
Customers traveling to or through Amsterdam, Netherlands (AMS) with checked firearms/ammunition must obtain permission from the Netherlands Consulate/Embassy in their country of origin prior to departure. Weapons are subject to confiscation in Amsterdam unless the owner can show all required permits. Customers traveling on military orders with checked firearms may do so without prior consent from Dutch Customs. Checked military ammunition is forbidden.
Customers traveling to Guatemala City, Guatemala (GUA) with checked firearms and/or ammunition must be in possession of a gun import license or a temporary transportation permit, which must be certified and validated by a Guatemalan Consulate. Customers traveling with checked firearms and/or ammunition through Guatemala City do not require a permit as long as their bag does not leave the airport controlled area. Customers must ensure they have all the necessary documents required at their final destination.
Curbside check-in of a firearm is not permitted.
The firearm must be packaged in a hard-sided container capable of being locked. The container must be locked and the key or combination must remain in the customer's possession. If a hard-sided container is needed, see the container section of this site.
Handguns must be packed in hard-side lockable luggage. Baggage containing handguns must be locked at the time of acceptance by United Airlines and the key or combination retained in the passenger's custody.
The firearm will be transported in a section of the aircraft that is inaccessible to the customer. Proof of registration is not required.
Firearms carried in addition to the free baggage allowance will be assessed at the current excess baggage charge.
No more than 11 pounds of ammunition may be carried. The ammunition may be packed in the same container as the firearm or in a separate container. Ammunition must be packed in the manufacturer's original package or securely packed in fiber, wood or metal containers. The ammunition inside the container must be protected against shock and secured against movement.
If applicable, the first or second bag fee applies to checked firearms.

Originally Posted By RenegadeX:

I fly American with a nanovault in my regular luggage all the time, looks like the skies are as friendly on United.

Originally Posted By GunDisaster:
I've traveled with my handgun while flying with United before. I have a small hard pistol case with a built in lock on it. I put my unloaded pistol in it. I put the pistol case in my luggage bag.which also has a small lock on it. When checking in I tell them quietly that I will be flying with my pistol. They take me and my bag to a quiet area where I show them my gun is unloaded and locked in a case. They put a note on the gun case confirming they checked it. Then they take my bag to be put on the plane. When I arrive at the next airport my bag is usually just in the normal location with all the other checked baggage.

Originally Posted By SharpCharge:
You can also have your ammo loaded in a magazine in the same case as the gun. Put the magazine in a mag pouch because the ammo has to be completely encased. Also, no loose rounds. Or you can have an empty mag and a box of ammo in the same locked case. Either way, no issue.

Originally Posted By justmatt:
You should pack your gun separately.
You should plan ahead, read your airline's particular rules (because they're all just different enough to be irritating), and communicate with them if you're confused.
If something seems off, you should ask again, and again, and ask the people at the desk, not the people on the phone. Here's an example:

I was planning a trip, and taking my Pelican 1750 rifle case, which exceeds the oversize measurements of most airlines. The two main airlines that flew where I wanted to go were Delta and USAir, neither of whom had I flown before while taking my 1750. USAir had the cheaper fare. Both have oversize fees, but Delta has a stated blanket exemption from their oversize limits for "Special Items," which explicitly includes rifle cases. USAir has no such stated exemption. I called USAir customer service to ask them about this. Four separate customer service agents on four separate phone calls all gave me an unequivocal "No exceptions" answer, so that would mean an additional $175 oversize fee each way. I went ahead and booked the ticket with Delta for $80 more before rates increased further.

I'm not far from the Orlando airport, so a couple weeks later I went over there, with my empty Pelican 1750 in tow, to verify with Delta that one rifle and three handguns would be acceptable to them, as their website was sorta vague. It was, no problem. While I was there, I walked down to the USAir counter, which was slammed. I finally got someone free, and explained the situation I was in, that the four phone agents had expressly and separately told me that I would be oversize. His response, before I even finished my explanation was (pointing at my rifle case), "That? We don't care, we'd just take that as it is." Without being rude, I pushed a little, stressed that it was technically outside their size limits, and I just wanted to make sure to avoid any surprises. He responded (nicely) that yeah, it probably was oversize; no, they didn't have a stated exception for rifle cases; and if I wanted him to break out the tape measure he could, but that, "We wouldn't worry about it. We'd just take that case as it is."

The lesson is that even the airline employees frequently have no idea what the hell they're talking about, but when in doubt, ask the people at the counter, because they're at the pointy end, and will actually be making the decision that affects you, unlike the disembodied voice on the phone.