Mega forged lower
Mega forged upper
Vltor A5 buffer system
Sprinco green rifle action spring
Criterion core series barrel
BCM MCMR-15 MLok
Sionics bcg and lower parts kit
Bcm gas block
Trajectory arms gas tube

Looks good. Trigger? Optics?

Looks good IMO, just going to more or less echo the other poster and suggest picking a good trigger, optic, and maybe BUIS.  

Make sure not to skimp out on the proper tools either.  Follow all manufacturer instructions and have a good solid build tutorial on hand if you haven't done this before.

Should impress your friends....

OP: be sure to get an armorers tool that’s compatible with a free-float barrel nut.

When I set out to do my first build, I bought a Mil-spec armorers tool and quickly discovered it was impossible to get all five pins engaged fully on a free-float barrel nut.

The buffer spring will be the first spring to go bad so I prefer standardization on buffer assemblies. You can buy a bunch extra online I guess, but I’ve never seen A5 stuff at any gun shop. If shit ever hit the fan you now have a rifle with a major compatibility issue.

In the same price range, the CMT UHPR rails have a much superior mounting system, and built in steel QD sockets. I’m not getting rid of my BCM rails, but I’m definitely not buying anymore as long as CMT exists.

Curious as to your optic choice/purpose of the gun.

I believe the Sionics kit includes their enhanced mil-spec trigger. It's a pretty good one. One of my builds used their lower parts kit, only thing I replaced was the safety for an ambi radian raptor. Kinda regret that safety

Definitely get an optic, or some quality irons like the DD A1.5 set. Other than that, looks good.