SpudFiles

planning a large coaxial combustion type . first I don't know if combustion and coaxial will work as I cant find much about them. second I'm planning to mount it like a howitzer cannon, using track wheels to move and recoil system, maybe model it after the M777. I was wondering if anyone had any ideas for aiming, especially when lobbing spuds as opposed to directly shooting at a target.

First off, in order to have any chance of hitting a target smaller than a barn, you need consistency. This means projectiles that are of similar shape, size and weight, and putting the same amount of fuel in your cannon every time.

Once you have that sorted out, you need a way of changing the angle of fire in a way that can be measured, precise and locked in place, same goes for traverse.

As to the actual aiming, have a quick read through this: http://en.wikipedia.org/wiki/Gun_laying

djsnowman06 wrote:first I don't know if combustion and coaxial will work as I cant find much about them.

Sure. It's not uncommon. Many breech loading combustions are coaxial, with the barrel extending back through the chamber and being loaded via a screw cap on the end of the chamber (also serving as a way to supply fresh air to the chamber).

I was wondering if anyone had any ideas for aiming

It's going to depend on your projectiles, but there's not much of a chance on that one. The consistency and repeatability of most spudguns is pretty poor to start with, something will only be exacerbated by a high launch angle.

Unless you're talking about a combustion with fuel metering, chamber fans, good ignition and very consistent (stable) projectiles, getting any degree of repeatability out of it is highly unlikely.
Even then, a high launch angle is probably going to put the range at at least a few hundred metres - and aside from that being further than it's actually possible to see a small, high-speed projectile in mid-air (or, likely, your target) that's far enough that you shouldn't expect a grouping smaller than tens of metres across even without the complications of wind and its unpredictability.

Basically, if you're "lobbing" projectiles high in the air, don't expect to hit any point targets. In fact, according to Murphy's law, you should only expect to hit things you don't want to.

my only advice is lots of practice to get it dialed in and maybe one of those range finders for golf and set up some 6 foot tall targets down range.

Right on. I have a cabin on large lake for a range. in thewinter I can setup some targets and try hitting em so thats why I ask ... I think I can dead reckon it but I was looking for something more "sophisticated" lol. as for coaxial combustion, Will ABS hold? About the only thing I can get my hands on. That or sewer grade pvc

djsnowman06 wrote:Will ABS hold? About the only thing I can get my hands on. That or sewer grade pvc

Better ABS than PVC in this case.