The reason why you still got to the correct answer is more about co-incidence and "off-setting errors" than anything else.
The co-incidence is that whoever wrote the question chose to make the diameter 1 ft, so the radius = 1/2 ft.
When you tried to use the circumference....2r(pi) = 1....you ended up with 1/(2pi).
The offsetting errors take a bit more explanation....
1/(2pi) is close to 1/6, so you SHOULD have come up with V = (5)(pi)(1/2pi)^2 = 5/(4pi) = about 5/12.... which would have given you a final answer that should have been MUCH smaller (a little over 100 ft^3). But you ignored the pi in the denominator when you did the calculation, so you ended up with 5/4....which is what you get when you do the math correctly.
This is certainly a funny set of circumstances, but you have to 'weed out' these little mistakes from your 'process' - on Test Day, they can seriously hurt your performance.
GMAT assassins aren't born, they're made,