Thats easy, i burn the oil/carbon out with the oxy/acetylene torch. Make sure you do it outside and away from dry grass. Once i get the insides on fire i then use compressed air trigger gun to blow the fire along till it get to the other end. Yes it makes tonnes of smoke
The end result is a spottlessly clean inner of pipe and heaps lighter too! make sure you wear safety glasses as sometimes some sparks can shoot back out at you and also do it with a few meters clearance around the end of the pipe because you can get explosions out the other end of the pipe. Its quite easy and fun to do actually
I also wear thick welding gloves and have some sort of stick or rod handy so i can move the pipe around a bit. Every so often stop blowing the air in it and turn the pipe over to make sure you dont burn a hole through it because the metal can glow red hot if your not careful.