I'm trying to finish my first lower build. I started with a double D lower and PSA lower kit. Things were going well until fastening the pistol grip to lower.
The screw (bolt) PSA's kit had was a 1/4-20 x 2" with a hex head (seems kinda crude for a firearm) I finger-screwed it in until resistance about a half-inch down then backed it out and internal threads on lower looked finer than 1/4-20. So it looks like either DD or PSA was wrong.

I loosely tried 1/4-28 and some metrics I had around but still didn't seem to go smoothly.
I have metric and SAE taps, just wondered which I should use.

Suggestions?

Thanks