Since this is my first post I might as well say "Howdy!" from North of the Border.

I've recently completed my first AR - I built up an armalite lower w/magpul M93 stock and then purchased a 10.3" LMT upper with a Troy MRF-CX FF. I've been debating getting one of the new Noveske KX3 flash supressors but I also want to replace my gas block...

I have been told - on other boards - that the aftermarket gas blocks that use set screws rather than being pinned have a tendency of moving once they heat up and are generally not very reliable.

My goal is to replace my front sight with a flip-up sight so I can mount some optics... I guess I could always co-witness with the existing sight, but I'd rather use a low profile gas-block...

Should I be concerned?

I have an aftermarket setscrew block (with staked screws) on my IPSC rifle, and it has never missed a beat in 4 years, even with rapid fire. On the other hand, I had another rifle (a Dissipator) with a set-screw block (no staking) and that block did slip and cause malfs. For this reason, I check the setscrews/gas block security on my IPSC rifle before each match, and I will only use a pinned FSB on my SHFT rifles.

If you want a low-profile block, why not just shave down the pinned FSB ? If you want to use irons, just add a dovetail setscrew block in front of the pinned one... if it slips, the worse that happens is that your sights go off, but the rifle will still function.

Just my 2 cents worth.

Never had a problem with my Larue gas block, which has the set screws, after several thousand rounds and on different barrels. I don't plan on installing pinned gas blocks after using the Larue, it makes everything alot easier if you ever want to change things around, simply remove the screws. Something else to consider is that you can use a screw mounted block on a different barrel without having to deal with pin holes that don't line up.

Is there any merit in countersinking the bbl where the set screws line up?

I would assume some locktite on the screws would also go a long way towards preventing movement.

countersinking offers a void for the setscrew to sit it and keep the gasblock from moving or torqueing.

not necessary but it helps, i have used just red loctite in the past wit no problems and if you are really worried stake them