For tube-feeding, I've always used rubber hospital grade urethral catheters & 60 cc. syringes, & actually, you can get them from
http://www.beanfarm.com/collections/...-feeding-tools
Some vets will take pity & supply them to you also, but most medical suppliers (for human stuff) look at you like you're nuts & demand a prescription...? (I never have figured out what nefarious thing they thought I wanted a urethral catheter for, & apparently they didn't believe- or care?- that I was tube-feeding a snake! LOL!)
The reason urethral catheters are perfect for this is that the tip is rounded & closed: there are 2 holes just above the end on either side of the tube, so you don't have the blunt-cut tubing causing any internal scrapes as you push it in. I always lubricate the outer tubing with a drop of olive oil anyway, as it slides better. And when you first put the tube in, expel just a bit of liquid in the upper throat, as that too will make insertion easier on the snake. Unless, of course, you got them to take a drink of water...right before doing this?