If you look at a schematic you will see how barrel fits into receiver and v-block pulls barrel into receiver (take 10 seconds to see what it is you're talking about).
Some aftermarket v-blocks are a steel which will handle more torque - can pull barrel in tighter (but am skeptical that pulling it in tighter prevents droop caused by sloppy fit between receiver and barrel). Just provides you with more tolerance against over torquing (remember that screwing into aluminium though).
The Green Mountain barrels appear a little over size, in effect ream them into place with the extractor groove, no droop issue (clear extractor groove before putting bolt/extractor back into receiver).
Some aftermarket v-blocks are adjustable - set screw in top that "supports" the barrel and prevents droop if barrel fits in receiver loosely. Some debate whether a little screw can support a barrel. Can take standard v-block and drill & tap to achieve the same.