So they could trade up to pistol or a Mauser rifle.
Let's assume there are about 100 million gun owners in the country. Of that number I fully expect 90% of them to roll over, show their bellies and beg the TPTB (The Powers That Be) to take their guns and please don't hurt them or their families. Such is the nature of our society today.
The good news is that still leaves a deadly ten percent (Ten MILLION) of hard core believers who will not roll over and crawl. Of this number there will be a goodly number of practitioners of the dark and deadly arts, many with combat experience and many of the special operations variety -- that is folks who know how to train and equip an army from the populace with the goal of the overthrow of the tyrant and his (or her) minions. There will be a decent percentage of officers, noncommissioned officers, warrant officer and enlisted. Former, active or retired with combat experience ranging all the way back to WWII (fewer of those everyday), Korea, Vietnam, and the conflicts since. More than enough to drive any standing military force crazy. Remember it only takes a single flea to drive a big dog crazy.
Look at Eric Rudolf. He was only a pfc infantry type but he drove 400 + agents and military nuts in a 40 square mile area of western North Carolina. Yes he had civilian assistance and so will the new militia. After all that portion of the militia isn't called "Special" Forces for nothing! So Eric Rudolf drove the law nuts for months. Imagine what a force of say 150 truly PI$$ED OFF SEALs could do running around DC with the nearby woods for safe haven.
These hardy few will already be well armed but they will always endeavor to practice the tried and true SOF doctrine of "Trade Up" and it goes something like this:
If you have a knife, you can GET a (real) assault rife.
If you have a (real) assault rifle, you can GET something Belt Fed.
If you have the Belt Fed, you can GET a (name one) mortar, Stinger, M1A1 Abrams Tank, artillery piece (self propelled) whatever. If you have one of each, you can get many more.
So they could trade up to pistol or a Mauser rifle.