These are just my thoughts and by no means the definitive answer. I think it's a bad idea to trim the contour. I'm assuming the tapered barrel is thicker towards the chamber? That is where material is needed the most. Trimming it down to get a straight contour will likely affect your accuracy and barrel harmonics; by how much? I do not know. The S&W M&P 10 has a tapered 18" barrel, and its accuracy is well documented in various online reviews. Given the -M's record for accuracy, I would not do anything to that barrel to make it even less accurate.