How can I get S rank?

So I have decided to go for the S rank now that I've finished the game on the different difficulty settings. So I jumped on to the Standard and set out to do it. I decided to make things easier by using the infinite ammo handgun, which was unlocked in the shop.

I read that using infinite ammo guns wont void an S rank as there are guides saying the easiest way to do it on inferno is use infinite ammo rocket launcher.

I completed the game with all the requirements, finished in 2:11 (under 2:30) and saved less than 5 times (1). However, I didnt get my S rank. Not even A. I got a B for some reason.

Does anyone know why this may have happened?

In order to get S Rank in Standard mode, you need to beat the game in under 2 hours, not 2:30. Beating the game in under two hours and thirty minutes is the time requirement for S Rank when playing Assisted mode - however, Standard's requirements are stricter. Each increased difficulty level through Hardcore cuts time from the maximum allowed to receive an S Rank.

For reference, the time requirements for every difficulty:

Assisted: 2:30

Standard: 2:00

Hardcore: 1:45

Nightmare: 2:00

Inferno: 2:00

(Source and Source.)