The only reason stainless inserts were made, was because they are stronger that aluminum. The aluminum ones were subject to damage more easily.
It was originally thought that changing weather conditions altered the inside bore of the aluminum barrels, so other materials were used that did not suffer near as much change. Brass was one of those other materials used. It was easy to machine and didn't expand and contract like aluminum. It was softer than anodized aluminum and could be scratched if you were not careful. The weight was significantly more (~3.5 times) than aluminum. A move towards stainless was made for the same reason. Stainless offered less tolerance issues due to temperature fluctuations than aluminum, and withstood wear and tear much better than brass. The downside to stainless, was that it was hard to machine and heavy (~3 time more than alum), but not as heavy as brass.
It was originally thought that changing weather conditions altered the inside bore of the aluminum barrels, so other materials were used that did not suffer near as much change. Brass was one of those other materials used. It was easy to machine and didn't expand and contract like aluminum. It was softer than anodized aluminum and could be scratched if you were not careful. The weight was significantly more (~3.5 times) than aluminum. A move towards stainless was made for the same reason. Stainless offered less tolerance issues due to temperature fluctuations than aluminum, and withstood wear and tear much better than brass. The downside to stainless, was that it was hard to machine and heavy (~3 time more than alum), but not as heavy as brass.

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