Any Tips For Props In Rugby?

Question

I'Ve Been Playing For A WHile Now And While I Have My Place On The Team, I Dont Seem To Be Able To Force Myself Physically On Apposing Looseheads In The Scrums.Im Kinda Short (5foot 10inches) and weigh around 14-15 stone.I Reguarly Come Up Against Some Guys Up To 18 Stone, I Just Want To FInd A WAy To Even The Odds.