don't build your own... none of the factory tolerances are equal to each other so your frankengun will not have pins line up or will have air seal or feeding issues. You'll spend more on shipping parts than you will a whole gun.
Find a gun you like the most and tweak it from there.
This isn't real ARs where shit lines up better and is cheaper to scratch build. It's made in china bullshit with "close enough" tolerances.
And by close enough, they come off the manufacturing line and the guy putting them in boxes looks at the part to see if it looks like the part it's supposed to be instead of actually assembling it or measuring it with any standard measuring device.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.