The passive participle refers to the person or thing affected by the action. From رَمَى comes مَرْمِيٌّ, “thrown”:
FormPassive participle
m. singularمَرْمِيٌّ
m. dualمَرْمِيَّانِ
m. pluralمَرْمِيُّونَ
f. singularمَرْمِيَّةٌ
f. dualمَرْمِيَّتَانِ
f. pluralمَرْمِيَّاتٌ
رَامٍ is a thrower; مَرْمِيٌّ is something thrown. The doubled yāʾ in مَرْمِيٌّ results from the passive-participle pattern and assimilation, not from the same process that reduces رَامِيٌ to رَامٍ.