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How I made the Panther gun 
The biggest challenge in making the tank gun barrel was cutting a small, thin taper in soft aluminium. The aluminium that I used was the kind that is easily available at the local home-improvement stores and I discovered  that it is very SOFT!!! 
Obviously, the maximum travel of the Sherline compound slide is too short to cut the taper in one step. The "classical" way described by experienced  machinists is to rotate the headstock and to turn the metal between centers with a face plate and dog, using them as a universal joint that can accept a small angle. Unfortunately, it did not work for me! Well,  maybe I did something wrong, but right in the middle part of the taper, the barrel was being bent by the toolbit instead of being cut. In the best case the result was a terrible surface finish and in the worst case a bent barrel!. This happened twice, each time ruining an almost finished barrel. I now have a set of gun barrels that can fire around corners, if anyone is interested! ;-) 
So, I returned to the obvious but tedious idea I had in the first place, to cut the taper in multiple sections, repositioning the compound slide for each section. I was afraid that there was not going to be a smooth transition from one section to the next, but in fact it was really easy, thanks to the Sherline lathe's accuracy. 
Here's how I did it. 
The aluminium rod. 
The first step is to trim the ends so that they are flat. 
At the same time, make a perfectly centered hole on each end using a center drill. 
Drill the gun's barrel with the appropriate drill bit. 
Now I have a hole on each side, one of them very deep. I than use the face plate and dog to support the rod, and make a very light cut, just to true the rod and make it perfectly round. The trued surface of the rod will ensure proper alignment when using the 3 jaw chuck. I use a carbide bit to cut the aluminium. I have had great results with a brazed carbide tip, but then I am easily amazed! If the rotation speed is high enough, the surface finish is like a mirror, and a very light cuts can be made. The cuts are much lighter than what I 'm able to do with a perfectly sharpened HSS bit (perfect because it is new from the box and not ruined yet by my limited grinding skills!). In fact, I'm able to remove a cut of half a 1/100 mm (about 0.0002") on the lathe. I use a very small amount of VARSOL, a paint thinner, as a lubricant. A small drop or two using a cotton Q-tip is enough. 
Now I have a rod that I can put in the 3 jaw chuck, the other end is supported by a live center. 
To set the proper angle of the compound slide, I cut small sections and measure the angle made between the beginning and the end of the taper. Is there a better solution? This method is very tedious. Anyways, when the proper angle is set, I start cutting the gun. 
I then move the barrel in the chuck to the next section. This is not the best method because the 3-jaw chuck is not accurate enough to position the rod exactly at the same center. Well, this results is a second tapered section that is a little off-center, but so little that it is almost invisible. A little sanding at the final stages and it is gone. But next time I will try not to move the rod from the chuck during the cut. I have to repeat this step a third time because of the very limited travel of the compound slide. The finished gun barrel was tapered in three sections. 
I cut the tapers until they are visually an almost perfect match. Then, with a big magnifying-glass, I smooth and blend the junctions until the section marks are invisible. Easily done, thanks to the lathe's accuracy. 
The last step is to use the cut-off tool at the proper position to separate the finished gun from the rod. 
And here is the result. This was my first gun and I choose a simple design. I have many others to try in the future, some way more complicated. Many hours of fun to come!