I, too, buy ammo for guns I would like to have, in addition to the ones I already have. Also parts and mags.
A firearm is a system. In addition to the firearm, you need mags, ammo, bore brushes, etc. Without any one of these, the system is useless. Which component you buy first may be a matter of priority, but ultimately, you need all of them.
Right now, Fudd guns are reasonably priced and extremely available. So are odd caliber ammo for them. The Fudd guns will likely not be banned, so there is no rush. But the ammo may run out, because the factories are all focusing on .223/5.56 and 7.62 right about now. If I were considering a .25-06 or a .243 or a .300 WinMag, I would start buying the ammo now, and getting the gun second.
In the case of "assault weapons," mags are the most important component. I have 100 mags for the G3/HK91/PTR91, but no rifle to go with them. They were $1.97 each (I missed them at $0.97 ea.) 2 months ago. Now they're $10.00. Once the rifles survive the ban, I'll get one. Or three.