First, don't put all your eggs in one basket. Keep the arms you buy on the yellow sheet separate from those purchased through private sales, and when they come, decide to surrender them or not.
Obtain others of the same caliber through private sales and keep them separate, but at least one or two 'handy'. (Storage units in your name will be likely searched as well, so get creative.) Any purchases of ammunition which you make via credit card or check are likely traceable as well, so the recorded arms will cover for ammo for the ones which are off the books.
Do not order ammunition in quantities so large that you cannot claim to have shot it up, and be at the range often, if even only for a magazine or two, to bolster claims of having used up ammo and to keep your skills fresh.
Better to buy twice as much as you use, pay cash, and salt the 'extra' away. Spread the (cash) purchases around several stores, if you want to accumulate ammo faster--they can use the business, and you don't trip any flags that way.
If your jurisdiction requires some sort of record of ammo purchases, you might as well buy in bulk, it is cheaper and it will be recorded anyway.
Look at history, there is always a fresh supply of minions out there, always. Also, evil builds its own momentum and that's what we're experiencing right now, the incessant bombardment of ever more hateful rhetoric, unbridled rage that is never challenged, well, you see it.