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.
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.