As an engineer, I liked the use of the spring with a smaller "K" to demonstrate the way the inertia of the bolt compress the spring as the lighter components move faster. The need for the bolt to remain still for the gun to run explains why a good mount of the Benelli is so important. If the whole gun moves, the bolt mass compresses the spring much less.
As far as additional topics, it would be cool to cover the Browning 'long recoil" from the Browning Auto-5 and Remington Model 11, especially the usage of the friction pieces, as well as the gas operation of the Remington 1100. The 1100 operation would be a good place to transition to the AR-15 direct impingement gas operation.