A good firearm that wouldn't be traced back to the project would be something old, not new. I would say Repeating Rifles like the marlin 30-30. Large bullet good for hunting or defense, easily produced rounds, and easy to mantian and clean and very cheap to procure or manufacture.

Your argument is valid for older weapons. But there is more than one way to disguise something. A common way is to make it look like lots of other things around it. In that spirit, if an area were to have a high probability of foreign ground troops, having weapons from that military would serve as a misdirection. So if I were to try to hide in the Pacific NW where there is a possibility of sweeping down the coast from Alaska or in Texas with the Brotherhood in the S, AKMs AK-74 and AK-47s would all serve to look like scavenged weapons from foreign troops. Near US Federal courts or other Federal properties and near large metropolitan areas with special weapon teams, you may see P90s and Glock 18s. Caches of these weapons would exist for USFPS, DEA, Secret Service, IRS, BATF and other law enforcement agencies. People in these area might have even seen them in regular use prior to the war in person or on the news. Careful selection would allow these modern weapons to be placed in the manner this thread suggests by blending into the crowd.

Yes to both of these points, depending on the location. A security asset at a rural area would have the appropriate rural weapons, a factory might have a more paramilitary issue. (My campaign had Morrow Industry with a Haliburton style war zone rebuilding division to acquire knowledge and techniques for the Project).

i could easily imagine something like this to be supplied to freindly communities that need a way to run radios and such that won't be traced back to the project.

Everyone needs a little helper to get around with all this "non-Project" gear:

http://sherp.ru/en/production-sherp-pro/