Besides what Monk said spend a bunch of time researching what works because he's pointed you in the right direction.
And when you're done, read some more.
Two of the most important things you'll want are a barrel and trigger, don't compromise.
With that there are a handful great Barrel makers out there.
I've had really good luck with Criterion Barrels.
Triggers are your choice, again read up
I have Geisselle triggers on most my stuff but that doesn’t mean you need to spend money like that when a LaRue may do the job for you.
I have one and they're okay.
For a good debate-
I don't care for carbine length gas system when a mid length can do so much more and I feel it's more reliable.
I also like Slash's heavy buffer as it makes things tame and reliable.
Don't get stuck on one manufacturer and don't let anyone tell you that the holy grail to buffer systems come from Armalite because it's total bullshit.
Again read up because the inside length of the buffer tube makes a difference.
BCG buy a Toolcraft as parts are everywhere.
And EVERYTHING you add to your gun will add weight, Quickly !
The biggest thing that will screw you over is gas tube length.
This one below is to short.
Wilson Combat are notorious for causing what you see below because they have to do it their way.
Buy your stuff from people that build large platforms for a living And AVOID anything prattitary
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The biggest thing to keep in mind is this is going to be YOURS and you should be enjoying yourself.
