Zeak,any basic 'machinist manual (library or maybe even online now) will tell how to use properly use a hacksaw; the problem is that we grow up thinking that you just have at it and 'hack' something off. Just like any machining operation, the stock (barrel in this case) must be nicely marked (at least use masking tape and put a strong line on that all around), jigged up straight and square, in a solid bench vise at the right level to your elbows, use a new fine cut blade (properly mounted and tensioned in the, hopefully, heavy duty, not cheapie $1.99 special bin, frame) and you only cut on the forward stroke, letting the blade teeth do the work, not your speed or pressure. If you do this in a 'workman' manner you can make a surprising straight cut and have little to have to square up with a file.
BTW, whenever we mention using a rotary stone to crown with, I sure wish we would make the point that it is used by hand only, NEVER with powertools.
Once I got my HD frame I was amazed at what a difference it made, but even with a lesser tool, if you always strive to make the best cut possible, you will get better at it.