Should the gas block steel match the barrel steel?

[Berg]

How important it is to match the type of metal of the gas block to the metal of the barrel?

In other words because stainless and CM barrels will expand and contract differently as the temperature changes, is it important to have the gas block expand and contract at the same rate as the barrel?

If the above is important, how about an aluminum block?

[BufordTJustice]

How important it is to match the type of metal of the gas block to the metal of the barrel?

In other words because stainless and CM barrels will expand and contract differently as the temperature changes, is it important to have the gas block expand and contract at the same rate as the barrel?

If the above is important, how about an aluminum block?

Not very. Also, NEVER use an aluminum gas block. Titanium is okay to use.

[lysander]

Stainless steel 410 - 5.5 x 10^-6 in/(in °R)
Titanium - 4.8 x 10^-6 in/(in °R)
Alloy steel - 6.7 x 10 ^-6 in/(in °R)

The difference in diametrical expansion for .750" journal diameter at a 500°F temperature delta is just under 0.0005" between 410 stainless and alloy steel, with the alloy steel growing more than the stainless. If the gas block were alloy and the barrel stainless, it would get looser, assuming there was no mechanical clamping. If the barrel were alloy and gas block stainless, the block would get tighter.

The difference in diametrical expansion for .750" journal diameter at a 500°F temperature delta is just under 0.0007" between titanium and alloy steel, with the alloy steel growing more than the titanium. An alloy steel barrel with a titanium block will see the block get tighter.

The difference in diametrical expansion for .750" journal diameter at a 500°F temperature delta is just under 0.0002" inch between titanium and 410 stainless steel, with the stainless growing more than the titanium. Again the block gets tighter.

If your getting the barrel and gas block part of the barrel that hot, you are shooting around the maximum sustained rate of fire for and AR (12 to 15 rounds per minute) with a mid-length gas system, and have gone through about 150 rounds. At about this point the temperature levels off, if you keep on shooting at this rate, the barrel temperature will not increase. (EDIT: Government profile, ie about .6" aft of the gas block.)

The problem with aluminum is not the heating and expansion, the problem is the strength of aluminum at temperature. A temperature of 500°F is not that hard to get, but the strength of 7075 aluminum drops off around 200°F.


(EDIT #2: 6061 is slightly better as the strength starts to drop off at about 300°F, but it starts off at about 40,000 psi at room temp.)