Moderators are in essence pressure vessels. They are similar to boilers, but on a smaller scale, dealing with the high pressure gases produced from a gunshot. In our experience, manufacturing moderators using both aluminium for reduced weight and stainless steel internal components to deal with the harsh exposure to combusting powder, creates multiple problems.
It is generally accepted that all oxide-protected metals that have contact with dissimilar metals, especially in moist environments, should be avoided.
1: Galvanic Corrosion: Using aluminium and stainless steel together creates a galvanic reaction, similar to what occurs to make a wet battery work. This is a chemical reaction between two dissimilar metals. Metals have a galvanic rating which is a guideline in the combining of materials in manufacturing. The more positive the numbers for the relative metal the more noble the metal. The more noble the metal the more resistant it is to Galvanic Corrosion. Therefore putting aluminium and stainless steel together creates this reaction which compromises the structural integrity of the less noble metal, in this case the aluminium. To further prevent this reaction the aluminium components must be anodised or similarly treated to create a boundary layer between the two metals.
2: Thermal Expansion: When metals are subjected to heat eg gun shots, thermal expansion of the two dissimilar metals leads to the components with a slower thermal expansion rate to become loose in the assembly. This allows the high pressure gases from the combustion to force residue into the gaps between the components created by this expansion. As the moderator cools, the exterior aluminium tube contracts and traps the residue which has lodged between the tube and the internals. For this reason, aluminium moderators must be able to be disassembled to allow access cleaning the internal components and removal of this residue. Either the front or rear caps, or both, must be unscrewed to gain access. The unscrewing of these caps and the removal of the internals, combined with the residue, eventually wears through the thin anodised layer on both the exterior tube thread and the end cap threads compromising the corrosive resistant properties of the aluminium and accelerating the galvanic reaction. With 100% 304L stainless steel, the expansion rate of the components is uniform thereby reducing the possibility of parts coming loose. As there are no dissimilar materials a galvanic reaction is obviated.
3: Flame-Cutting: In our experience we have found that unlike aluminium, 304L stainless steel has good flame-cutting resistance. Flame-cutting results from the high temperature and high pressure generated from the burning propellant. We have found that flame-cutting will erode the inside of the aluminium tube directly where the high pressure gases are exiting the muzzle of the rifle barrel. Such erosion greatly reduces the life of the aluminium.
4: Bullet Strike: Over the years we have refined our products to ensure safety to the firearm operator, as well as to the bystander, in the event of bullet-strike inside the moderator. This does occur, albeit infrequently, and it is a very real problem. We have been improving our moderator design to mitigate its consequences, in the event of a bullet strike. The machined internal components are designed to collapse and to project the flight of the bullet out the front of the moderator. The baffles forward of the blast baffle are made of lighter gauge stainless steel material than the exterior tubing therefore they present less resistance to the erratic flight of a bullet which has experienced a strike within the moderator. This is to ensure that the projectile does not compromise the exterior shell of the moderator and place the operator or bystander at risk. Refer to articles to read and view images relating to products which have encountered bullet strike.