If your using Anything other than Madbull Hop up Rubbers with the Madbull teflon barrels, you will have the above fitment issue. The reason being that Madbull teflon inner barrels have a slightly smaller outter diameter than standard IBs. Best way is to use MB Rubbers with the Teflons and wet your finger with some water and coat the outside of the hop up rubber as you slide it in with a twisting motion. Pretend the Hop up is your Girlfriend, and the IB + Rubber is a dildo
IMO Razors, knives etc have no business being near hop up rubbers.