And actually (not convinced that 4 screws out of the original 15 is adequate to hold the barrel & hand guard together on the receiver)...
I know what you're thinking and I had the same concerns at first, but when you tear apart the OEM handguard assembly, the only screws attaching the barrel to the receiver are the 4 that get replaced. (Marked with green arrows)
The screws marked with red arrows just hold together the pieces of the assembly. (I missed two on the lower rail, but those only help hold the lower rail onto the handguard assembly) This accounts for 9 of the 15. So realistically, its like going from 6 to 4. Except it's a bit more complicated - 2 of the 6 don't really contribute much.
The two on the bottom of the trunnion (orange arrows) only help attach the bottom rail to the handguard. Sensibly, if the barrel is secured to the handguard via that rail, and the handguard is secured to the receiver, then the barrel is secured to the receiver, right?
As it turns out, not really, since removing the 4 green marked screws means the entire assembly, handguard and barrel, is attached via only 2 of the red screws (the front 1 on each mounting plate that has 1 red and 2 green). Those two screws mate with holes in the receiver that are rather loose, and provide little retention. In fact, its the mounting plate which secures the lateral motion of those screws. And the mounting plates are secured by the 4 green screws.
Hence, if you delete all the screws but the green ones, nearly all of the force which secures the trunnion within the receiver is retained. You do lose the support of the bottom attachment via the handguard, but being that any support it provides is primarily dependent on the 4 primary screws anyway, it does little to add anything but weight to the design.
Which comes full circle to why this thing has 15 screws in the handguard anyway (16 if you count the one holding the sling swivels!). Its a vestige of a crappy redesign towards modularity on the part of CZ. The military Bren 2 models have the same 4+2 screws attaching the trunnion to the receiver and the 2 in that design provide even less support, since their attachment is leveraged at end of the lower rail, giving even less support to securing the barrel. The bottom screws are only there to provide an attachment point for the bottom rail, else CZ would have extended the receiver down to be bolted into directly.
Good call on the nordlocks by the way, those things get chowder'd so fast, HBI should really ship new ones. HBI does give you 4 new, much nicer screws though.
EDIT: Forgot an "s" and a comma. Also, split paragraph for emphasis.
