There used to be a ATF "loophole" as far as magazines went, that has been since closed.
Here is how it used to work, but it is now ILLEGAL.
First you bought a pre '94 AK-47 hi capacity magazine for around $5 to $10 bucks at a gunshow.
You then found a source for "replacement magazine tubes", springs and followers.
You replaced the "defective" AK-47 magazine tube with the "replacement magazine tube", then put in new springs and followers. Then destroyed the "defective" magazine.
Here is where the "loophole" comes in, the "replacement tube" didn't have to be for the same weapon as the original "defective" magazine. Thus you could get a H&K USPc hi-cap replacement magazine tube, to "replace" the AK47 magazine.
And it was LEGAL.
But the ATF has since ruled that the replacement magazine MUST function in the same weapon as the original magazine. So this doesn't work anymore.
A man with a pile of "defective" AK-47 magazines, and a good source for "replacement magazine tubes" could pump out legal hi-capacity magazines for as little as $40 to $50 each, $10 for the AK mag, $20 for a replacement magazine tube, and another $15 to $20 for springs and followers.
This is how many pistols that were manufactured after the ban, have a pile of legal hi-capacity magazines for them.
People just learned to read the "fine print" in the legislation, to get around the ban, as cheaply as possible.