Here's a great app note on Nyquist Theory and Oscilloscope sample rates
Basically, to get an accurate depiction (and measurement) of your signal you have to have a high enough oscilloscope bandwidth and a high enough oscilloscope sample rate. I put together a quick 3m video a while back explaining the difference between oscilloscope bandwidth and sample rate.
In the case of your question, the oscilloscope bandwidth is fine, but the sample rate is so low the resolution wouldn't be accurate enough to give you a good idea of the signal's characteristics.
Like other comments mentioned 50 MSa/s = 1/50,000,000 = 20 ns/sample. To accurately sample a 1 ns rise-time signal, you'd theoretically need at least 500 ps resolution. In the real world, though, you'd still want faster than that.