Quoted: >SNIP How important is the chrome lining? and any other opinions on these two choices would be welcomed.
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Aside from benefits of chrome-lined barrels that have already been mentioned, here are a couple more considerations:
1. Less barrel heat - The greatest amount of heat is generated from the friction of the projectile traveling through the bore. Chrome is harder and more lubricious than steel and therefore provides less resistance to the traveling projectile. Less resistance equals less friction which in turn equals less heat generation.
2. Reduced wear and impact loads for bolt carrier, cam-pin, bolt, extractor and extractor pivot pin - This is especially important with regards to short carbines over rifles because of the proximity of the gas port to the chamber. When fired, the cartridge case obturates against the chamber wall where it seals the high pressure. A carbine's gas port is located inside of the "all-burnt-range". This means that the pressure curve is near peak when the projectile passes the gas port. As the carrier begins its recoil movement, it biases the bolt to rotate and extract the case. But at peak pressure, the case is held firmly against the chamber wall, which increases the impact loads on all of the moving parts. Chrome releases the cartridge case easier than steel. And the shorter the distance between the chamber and gas port, the larger the gas port, the hotter the ammunition, the lighter the bolt carrier and buffer group, the hotter the ambient temperature, the more fouled the chamber, etc. the more important it is that one have a chrome-lined chamber.
Rifles such as Garand M1 and full length AR-15's are more tolerant of non-chrome barrels because the port is on the down-side of the pressure curve. Once the peak pressure has passed, the cartridge case can contract and lessen the load on the above-mentioned moving parts. -Hope this helps.
Geoffrey