The first rubber on the invert guns are easy to load due to the pulleys, so tend to use thicker rubber to maximise spear load. The next 1 or 2 sets are thinner, as they are still a full stretch of the barrel without any mechanical advantage.
If a 90cm gun is at the maximum arm length, then a 100 would make sense for the invert roller (you lose a bit of length with the pulleys and rubber attachment points).
You are right on the money with the spear and double wrap. 8mm spear won't be overkill.
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