Don't get yourself in trouble needlessly.
Anyone who sells [i]individual soldiers[/i] Law Enforcemen Only magazines or weapons is breaking the law as is the individual soldier who has no right to own them. There is no exemption for individual military members, only [i]individual civilian policemen.[/i]
Units can buy whatever they want and ISSUE them to soldiers, but there is no individual purchase or ownership allowance other than for civilian policemen.
The fact that individuals have done this, via uninformed sellers, doesn't make it legal.
Your attempts to illegally purchase restricted items will get you into trouble. Does it make any sense? Of course not, it's Federal Law.
Your unit can buy 120 Beta-C magazines with government money or via a government purchase order. Your unit can buy NFA weapons. Unit members can be issued any and all of these as part of their issued equipment. But you'd better have the hand receipt for 'em. Under Federal law your unit can issue a M16A2 if it wants. AR, NGR, and WINGRs (you're WIARNG, right?) will cover if the unit can do this under military regulations, of course, but it's legal otherwise.
BTW, the same circuit court in CA which ruled that "under God" in the PoA, and the 2d Ammendment has no individual rights to bear arms also ruled that the police have no more right to any arms than the general population -- that is that exemptions for high capacity feeding devices are just as illegal for police as for other civilians.