With the application you will be submitting some form of age/address proof document. Your name should be same as mentioned in the proof document that you are submitting. If the document has initials, expand them in the PAN application form.
You can have the first name as SACHIN, the middle name as B (the initial or expand it if you wish to) and the last name/surname as R..... (enter the whole name here; no initials allowed).
They have specifically mentioned this under Item No. 2:
Name, if abbreviated, should necessarily contain the last name.