Is there any legal AND practical way to convert HK mags to M14 mags?  I've seen HK mags for REAL cheap lately and was thinking "hmmmm $5 for HK plus a little sweat vs. $50 for an M14 mag...  hmmmm"

The problem is it still has to function in the HK.  You could fit and weld an M14 mag top to the bottom, replacing the floorplate, which would be legal, but not that easy (or cheap in any way) for some.

Does it really have to function in the HK?  I don't own one so don't care one way or the other.

Yes.  It is only for legality reasons.  It is my understanding that you can modify a magazine any way you like, as long as it still functions in the original gun.  If it were not to function in the origional firearm it was designed for, them you would have constructed a new high cap mag.  And obviously unless you are LE/military, you (or I) cannot be in posession of a post ban, high cap mag.

This is all for our own saftey of course.  Cough, cought, bullshit, bullshit.

And man does that suck balls.  Just when we found we could take these old surplus Soumi 9mm drums, and convert them for use in MP5/HK94's, Macs, and UZI's - ATFE finds out and shuts the party down.  It also killed off the Ceiner .22rimfire mags.  DAMN!  It was nice while it lasted.

If the above is correct, then how do companies offer modified sten and uzi mags that have welded blocks so the work in pistol cal carnines?

Oly and DOLPHIN offer converted mags and they don't work in the original gun, correct???

There are only two ways.  One - companies offering modified mags that do not fit and function in the original firearm they were designed for, are selling mags that were already converted before the ban came down.  Two - the use of magwell adaptors.  Some mags can be somewhat modified, but still work in the original firearm - such as M3 converted to MAC10/45 mags, or M14 converted to AR10 mags - they fit and function, but you have to hold them in with your hand.