JLC has an interesting article in their archives regarding simulating rainfall under various conditions to check out windows and siding.

You don't have to be a fireman to simulate rainfall and I did that on one occasion to test some gutters.

This was a modest house with strange looking gutters that looked like they were designed to keep foliage out of the troughs by using a solid sheet of aluminum from drip edge to the outer lip with about one inch of opening above the outer rim/lip.

I was concerned that fast moving water sheeting down the 3 tab asphalt shingles would shoot over the outer rim/lip of the gutter.

Several years later I attended a presentation by a large gutter company.
It was then that I realized that I was testing the incredibly expensive "Rain Helmet" gutters.