[Suicide Junkie]

Two shots at 50% should break down to 75% for one hit, 25% for both hits, and 25% both miss.

I believe you meant to type 50/25/25, so it adds up to 100%.

(25% x 2) + (50% x 1) + (25% x 0) = 100%

So you can expect an average of one hit per volley of double-shots @ 50%

If you fire N shot volleys, at 1/N accuracy, you'll get 1 hit per volley on average.