best thread sealant

Teflon tape. Cheap, easy to use, no mess when removing.

Permatex Ultra Copper . It'll never leak again . And also will never come off again lol

Does anyone know what the factory black sealant is on the mrod  foster? I have not found anything as good.

No.

I just use four wraps of a good plummer’s tape.

Most of the OEM's use a black teflon tape that I've only seen available in small pc's on some paintball sites. What I use on any that don't seal with plumbers tape is Permatex 518. It's an anaerobic sealant/gasket maker. The parts can be disassembled using a little extra force. 

Stay away from the stuff at box stores labeled "Rector Seal"!

I wanted blue pipe dope (teflon) and get some NASTY (guessing) castor oil based non-drying.

Did I mention nasty?

I frequently use teflon tape on the male, and then pipe dope over it and inside the female fit.

This seems to let the tape stick to the male, instead of balling up against the female's face.

Hate leaks..... hate is s strong word, that's why I used it.

Used this with great results on my PCP, HPA leaks. Got it from HomeDepot.

Turns black applied.

RTV