Gas block material

Hello, not sure if this is the right place to post so move if needed. But was getting parts together to finish my upper and was talkin with my local gun smith, and he brought up that when pairing a barrel/gas block I should try to change the materials. For example if it’s a nitrided barrel I should try to get a phosphate gas block. But he didn’t really seem like he wanted to go into any detail as to why. Just wanted to reach out here to see if anyone could elaborate if this be a true statement. He mentioned that and that he always likes one of the materials to be nitride. Not sure if it’s for seal or heat expansion or what. But just wanted to gauge the room. Thanks everyone.

 

Doesnt really make sense to me.

In the automotive world, mismatched materials, say steel bolt in aluminum block, will cause corrosion so anti-seize must be used.

 

He didn’t want to go into detail because there are no details which substantiate the asinine claim.

 

I know he said different finishes, but suppose we had an aluminum gas block and steel barrel. There would be a different rate of expansion between the metals and a gap could form at high temperatures. Which means a leak.

 

Thanks for the feedback, I guess I should have said the finish on it instead of material. I usually just do SLR low pro nitride gas blocks or BCM but when I told him I was putting the nitride SLR on a nitrided rosco barrel he got all kinds of upset and started saying to get a phosphate or different material but got “busy” before explaining why lol. Thanks again.

 

Material or finish, completely irrelevant.

 

Because contrasting colors is the spice of life my friend! I will say a steel lightning rod should not be directly attached to a tin foil hat, that one I believe has some science behind it.

 

Well done, sky... super stealing that one

 

Do not let that local gun smith assemble your gun or repair anything you care about.

That being said, I've switched entirely to ion bond/DLC Coated Titanium Gas blocks for my builds. V Seven is about the only source for them nowadays since Battle Arms discontinued their very excellent Titanium Gas Block. They always had super snug tolerances...

Theoretically, steel will expand more than Titanium as the barrel heats up leading to a better seal but my reasoning is actually the weight savings up front. It's about an oz & a half but it still makes it handle just a bit nicer.. YMMV.

 

Minor changes in Temp Co (Temperature coefficient of expansion) show up in larger objects at higher temperatures, or in smaller objects that may have a significant temperature difference.
It is what makes a typical mechanical thermostat work.

Two different metals (often copper and steel) are bonded together, often than coiled to create a larger bond area and increase the area.
The older classic design used a small glass vial with a pair of contacts at one end and nothing at the other, and a tiny drop of mercury.
The movement of the tiny brass-steel mounting coil made the drop of mercury be at one end connecting the contacts, or the other end with no contacts.
This is the reason behind the thermostat requiring a level mountain position on the surface.
Modern solid state electronic thermostats do not have this weakness.
They typically us a diode junction.
The leakage across the junction is very well defined as a function of temperature.

 

@brickeyee - as an engineer myself, thermal expansion coefficients are not lost on me. However, as an AR builder with literally hundreds of AR’s under my belt, between carbon and stainless steel, aluminum, and titanium blocks, I’ve never witnessed gas leaking as the block heated faster than the barrel. Why? Dunno. Doesn’t really matter - hundreds of data points prove the case.

 

I just stay away from aluminum gas blocks.
I'm not sure if the finish matters much

 

I'm pretty sure that you would have to do a whole lot of full auto mag dumps before you see a problem with an aluminum gas block on a steel barrel. And you are just as likely to see other heat related problems pop up first.

 

Let's look at this and see if we can learn something . . . .

At 1 rounds every 6 seconds (60 rounds per minute, or RPM), for 15 minutes the barrel under the gas block gets to 350 F, and 15 minutes is long enough for the block to be in equilibrium with the barrel. So, with the following block materials you will have the following fits (assuming the standard 0.7495" diam barrel and 0.7500" diameter block):

- 7075 aluminum: + 0.0019" (4x room temperature clearance)
- Ti6Al4V titanium: + 0.0001" (1/5 TR clearance)
- 304 stainless steel: + 0.0010" (2x RT gap)
- 1137 mild steel: + 0.0001" (same as Ti, this is the material the FSB is made from)
- 41XX steel: + 0.0005 (original clearance)

At 1 round every second (60 RPM) for two magazines, the barrel gets to 800 F, the gas block will not get to equilibrium with the barrel so will be slightly cooler. These are the predicted clearances for the same block material:

- 7075 aluminum: + 0.0036" (7x RT gap)
- Ti6Al4V titanium: - 0.0010" (you now have 0.001" crush on the barrel, that is a significant constriction)
- 304 stainless steel: + 0.0015" (3x RT gap)
- 1137 mild steel: + 0.0005" (original clearance)
- 41XX steel: +0.0005 (original clearance)

Throwing out aluminum, because it has erosion issues, you can see the best choices are 1137 (the original material chosen) and 41XX alloy steel as these maintain some clearance, while staying within the original 0.0005" gap.

 

lysanderxiii well done and many thanks.
I no longer have access to many of my detailed ME design material.

Working over wide temperature ranges at tight tolerances has always been a bit of a nightmare.

 

I sure wonder why nobody ever has any such imagined issues as promoted in this thread?

I wonder why third degree burns don’t happen at every 3 gun or PRS match? 350F surface temperature is pretty damned hard to achieve.

 

Lol. This is why I don't help out as much anymore.

It doesn't have to make sense. LGS momo can't answer your question, because he doesn't know why. Nobody does.

If you use a smooth mirror finished melonite barrel, with a melonite gas block, it's a major pita keeping the bolts and gas block tight. Use for a SPR only if both are visible outside the rail and you want a consistent black appearance. If the rail covers the gas block, use a phosphate block. (Could just be cheap aftermarket parts and tolerance stacking. But I've had plenty of good luck with cheapo builds.)

The rough finish is like a lock washer on a screw bolt, maybe. ???

 

If a gas block is leaking, you’ll see the evidence as crud is expelled around the perimeter of the block and onto the barrel. Go check now...nothing? Hmm...

 

Round count is not high enough at most 3 gun matches. I think the most I have ever shot in a single stage was just over 70 rds the handguard was unpleasantly hot.

Data point of one so..., using a thermal camera I measured gas block/muzzle temps while firing 30rd magazines of 556 fired as fast as you can easily do with a semi-auto. It was a fairly low res thermal camera (first gen Seek Thermal that plugs into you phone) Gas block to ~165 F after the first magazine, ~230 F after the second, ~340 F after the third, ~460 F after the fourth, 550 F after the fifth and peg the thermal camera at its hottest measurable temp of 626 F before then end of the sixth. ~63 F ambient temperature.

I bet a 308 gas block would climb even faster.

 

This guy claims to have had over 11,000 AR's on his bench over the years, he points out the problems that might crop up with aluminum gas blocks.

 

So...gas port erosion is caused by expansion/contraction? He’s never found an eroded steel gas block in 11,000 samples?

I’d like to see the corresponding barrel that gas block was fitted to. Expendable parts are expendable.

 

Yup. Ditch the gas block with the barrel when it’s burned out, and that kind of thing is never a concern. Which, in itself, means barrels shouldn’t be employed past their useful service life if you expect peak performance from your rifle either. If a guy puts 10,000 rounds through a barrel at 40¢ per, eating up $4,000 in ammo cost, you really can’t complain when even a $300 barrel and a $100 gas block is also consumed...