I'm interested in getting some 20rd magazines for target shooting but I'm afraid I don't know enough about what's legal and what's not.  
I've seen some 20rd magazines that the seller said were originally blocked 5rd magazines.
Is it legal for someone to take a blocked magazine and remove the block if the magazine was manufactured before 1994?
What is the difference between the straight bodied 20rd mags and the curved 20rd mags?  Other than one is curved and one isn't.
And are there brands that are more preferable, as far as USGI mags go?
Thanks for any light you can shed on this.
DCS

The MAG FAQ will answer most of your questions...read it, learn it, embrace it!

[url]http://www.ar15.com/forums/announcement.html?b=3&f=17&id=26[/url]

As for the mag block, it is legal to remove it as long as you don't live in a State that restricts Mag capacity.

USGI is the way to go. IIRC, the 20 round USGI were no longer manufactured after something like 1971, so no worries whatsoever with whether they are preban or not.

Manufacturer of USGI stuff doesn't matter, except that you don't want 30 rounders by Cooper (less than 6 spotwelds). Some will pay more for Colt mags, but Colt didn't make them, so unless you're a collector, there's no point.

Colt still sold (and still sells) 20 rounders after the early 1970's. They are the same as USGI. There are post-ban Colt 20 rounders but all USGI 20 rounders are pre-ban.

CRC

Thanks Guys for all the information.  It was very helpful and I appreciate how quickly you responded.
DCS