The XM177s also had an 11.5" barrel too, right?
So yeah, replace that 14.5" outer barrel, grab one of the flash hiders, slimmer hand guards if you'd wish, and cover it in burlap to hide features of the upper, and you'd be able to pass the distance test with your M4. Thanks to China, this stuff is pretty easy to come by if you look hard enough.
I have no idea about those Black-ops walmart bbweapons, so I have no idea about upper-lower receiver compatibility. Read they're APS rebrands or something.
So if you want to also pick up the A2 upper you'd need to find a G&P or CA lower/upper set to make sure it works. That is, if you're set on Vietnam reenacting.
Or just pick up the G&P XM177E2. Again, because being dead set on reenacting. And quality.
http://shop.ehobbyasia.com/gandp-xm177e ... zr3ucldXms