any of you guys have used jp's adj. gas block on your ar's

Try Zak-Smith I would think he has.

MT

my friend uses one on his dpms lr-308

The adjustable gas block was meant to be used with JP lightweight bolt carrier (competition use only) for fine tuning of bolt speed.  However, it is not something the standard 20" AR with std bolt carrier should use.  My JP CTR-02 upper comes with one, but I don't use the adjustable feature (it now runs wide open) since I found out that it causes FTF's with 40 rds or 30 rds. full magazines.  

As bravo5two alluded, the adj gas block was designed to allow use of a lighter bolt carrier.

Even in this application, opinion is split as to whether or not it's worthwhile.   In my CTR-02 with the low-mass carrier, I run with the gas "wide open" (full gas) in order to reduce the cycle time as much as possible.  It is possible to have this setup run reliably as long as I don't load more than 28 rounds in a mag, and I keep the gun clean.   If those two conditions are met, it's worth it in competition.   (ETA: the third condition is good mags.  USGI bodies, newish springs, and Magpul followers work well.)

Others believe in reducing the gas pressure so the rearward "jolt" as the buffer bottoms out is even less, but the cycle time will be more normal then.  

I would not use an adj gas block in a gun that was to be used for self-defense.  If the adjustment screw falls out, you are screwed.

-z

Great info in this thread.

Thanks Guys.

MT