This will be my first time painting a rifle barrel.

I ordered a stainless barrel that was bead blasted satin finish that I want painted black. It's a 16" barrel going under a 15" Geissele MK16 rail, so the barrel will be mostly covered. Mostly doing this for appearances, but also functional as 416R stainless can rust.

Do I need to mask off the entire gas block area, or just the gas port hole itself?

Also, I was going to go buy some high temp paint at some local store. Anyone have brand recommendation to look for?

When I was doing Cerakote I would mask off the ring around the gas port.

As far as high temp paint, something like Duplicolor Engine should do the trick. It’s usually available in matte, satin, gloss, or just about any finish you want.

All I've used so far is just the Krylon camo paint.

If I wanted to use high temperature paint I'd check out the auto parts stores for some high temperature ceramic brake caliper paint.

Be sure to degrease the barrel well prior to painting.

I use VHT paint. Comes in flat black, if you follow the baking instructions it adheres very well. A 16” barrel should fit in a standard sized oven.
I have done many small parts with it and complete barreled actions ( I have a large powder coating oven).
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Baking a finish isn't going to be an option for me. I don't own the oven, so I don't really want to risk any accidents.

So far the rustoleum high temp is looking like my best option. It's advertised as oil resistant which should help. How well does the krylon camo paint hold up?

I have used the flat black engine enamel on various parts; handguards, uppers, lowers, etc; that have off colored ano and that paint is very close to a deep propper ano.  It is durable and easy to apply.  It has held up to cleaning solvents, gun oil, etc.  Good stuff.

I have had very good luck with the Brownells Alumahyde coatings.  Much more durable than the standard Rustoleum & Krylon aerosol cans, and it holds up very well to cleaning chemicals & moderate heat.  You can bake it for a few hours to cure, or let it air dry for 10-14 days, so it allows flexibility if you don't have access to an oven.    

The temperature resistance you need will depend on how you plan on using this barrel.  If you're doing a lot of rapid fire drills, the high-temp BBQ spray or automotive coatings will hold up well.  But if you're planning on using this for rapid fire use, then maybe a chrome lined barrel might be a better option since it's more durable at the high temperatures from sustained rapid fire.  If it's going to be used to fire in short strings for precision target shooting, with an occasional burst, then any standard paint should be just fine.  

Either way, I'd definitely tape off the gas block journal from coating, or simply coat the barrel with the gas block in place so that coating buildup doesn't interfere with the fitment.  Even a light coating (0.001" to 0.003") could cause interference to the gas block fit, or could cause the gas block to scratch the coating as you slide into place. This happened on one of my heavy-profile barrels -- which wasn't the end of the world -- but I had some extra touch up to do afterwards.

Aluma Hyde.

HERE

Yep, my first choice as well.

eta, I would only paint the gas block area if my test fit revealed a somewhat loose fit bwtn the two.  I would also test fit the barrel extension for the same potential sloppy fit.

I used aircraft-grade heat shrink over my stainless ar barrel.

Easy application, and fully cures in 30 seconds.

It depends on the taper of the barrel in front of the gasblock.  I usually tape off the gasblock journal area and then touch up any stainless showing in front of the gasblock once it is in place.

If the barrel is heavy enough, you won't even be able to get the gasblock on after its been painted.  I have also had to sand paint off to get gasblocks back off when changing a rail or something up.

Is it worth going for alumahyde over some rustoleum high temp? The barrel will be mostly under the rail. I just figured it’s easier to use normal paint instead of some thicker ceramic base that sounds easier to mess up.

The barrel is a Rainier Arms Mod2 ultramatch fluted 223 wylde.

I've always just used high heat engine paint in flat black.  I run pretty long rails though, so the chances of it getting messed up is fairly small.  I also don't care that much about little nicks and scratches in general though.  I think it works well and looks pretty good.  It's hard to tell its been painted if you take your time.  And has held up great so far.

Tech-Nickel Inc. in Benton Harbor, MI will salt bath nitride a stainless steel barrel for $50 the last time I used them. Clean it thoroughly before you send it and again when it comes back. Beautiful deep black finish when done.

This.

Install and then tape off the GB or you will regret it.

Some have said use alumahyde.
That stuff is nothing more than appliances paint. Made by the same people.

I've restored over 20 old police cruiser 870s using VHT.
Stuff is tough and resists oil and cleaners.

At first I didn't have my craigslist oven in the garage  and used the one in my wife's "office" when she was out shopping.

To my surprise it didn't stink up the whole house and I was safe by the time she got home.

IMO, definitely.  

They're nitriding the barrel with the extension in place?

I bought some Rustoleum High Heat 2000F flat black paint and just finished spraying it. This paint dried very fast, almost instantly. Very easy to get even coverage.

Right now it looks like a fresh phosphate barrel. It's exactly like how my phosphate BCM low profile gas block looks. After I let it hang in the sun for a couple hours, I'll try to post a pic of how it turned out.

Pulled the masking tape off and it's looking good. That is a phosphate gas block, TiN barrel extension, and nitride gas tube for reference.

Lookin' good.