I would suggest building a motor first.
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To build stamina long slow distance jogging is best
The farthest distance a mousetrap car has gone is approximately 1,650 feet, achieved by a team of students from the University of Nevada, Reno in 2016. Mousetrap cars are powered by the potential energy stored in a wound-up mousetrap, which is converted into kinetic energy to propel the vehicle forward. Factors such as the design of the car, the efficiency of the wheels, and the surface it is traveling on all play a role in determining how far the car can travel.
long distance jogging, cycle, swimming
When the trap is fully closed, its best it the string is no longer touching axle to prevent back-wind.
Long distance telephone lines.
As well as great exercise,you can build stamina and endurance for both long and short events
Long distance running helps your body burn calories, build muscle, and increase overall fitness. It does cause additional strain to the legs and joins.
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An odometer for measuring long walkable distance. Long distances on the earth are probably best measured by geographical positioning systems (GPS).
With long-travel supension and the best air filtration you can get.
There is no one single best caliber, but to get an answer, you need to be a little more specific. Rifle or handgun? Define long distance. 50 yards? 100 yards? 1000 yards?