cameronfield wrote:And what exactly does metered fuel mean? And what is MAPP?
"Metering" refers to any method that accurately measures the amount of fuel that is put in the gun. For maximum (and consistent) performance the amount of fuel needs to be fairly precisely controlled. With either too much or too little fuel in the chamber the gun won't fire. "Squirt and screw" (aka "spray and pray") fueling means the fuel canister is just pointed in the general direction of the opened chamber and the fuel is sprayed for a second or two. The actual amount of fuel that ends up in the chamber is highly variable and many people spend days trying to figure out the correct number of "one-potato, two-potato, ..." counting required to get the gun to fire at all. A gun fueled via "squirt and screw" will be inconsitent at best. The only advantage of "squirt and screw" is that it is cheap.
Meters can either be fairly complicated devices consisting of various plumbing parts, regulators, valves, gas cylinder etc., or it can be as simple as a $1 syringe. See the SpudWiki on
fuel meters, or
here or
here for info on using a syringe. Both methods will reproducably put the correct amount of fuel in the chamber. This maximizes performance of the gun and, perhaps more importantly, makes the gun much more consistent and less prone to mis-firing.
MAPP is a variant on a welding gas, it has a small advantage over propane in terms of performance. See the SpudWiki on
fuels or
MAPP.
Also I've upped the CB ratio to .95:1. Is this a good balance between efficiency and power?
In a combustion spudgun there is really no tradeoff ("balance") between power and efficiency. The most efficent barrel for a given chamber will basically be the most powerful. The actual CB ratio does not have to be all that precise. Any CB between about 0.5 and 1.0 generally behaves about the same. See the SpudWiki on
CB Ratio for some typical data. Outside the CB range of 0.5~1.0 the performance, measured as the muzzle velocity, starts to drop off pretty quickly. It is not unusual for a 1.5 CB gun to underperform an 0.8 gun by a factor of two in the muzzle velocity.
Lastly, will potatoes go further from a 1.5" barrel or from a 2" barrel? Using the same chamber both times with the same CB ratio. I would guess 2" cause it has more mass but I dunno.
Good question. This involves both the internal ballistics (behavior of the spud in the gun's barrel) and external ballistics (behavior of the spud after it has left the barrel).
A 2" barrel has 2<sup>2</sup>/1.5<sup>2</sup> = 4 times the area of a 1.5" barrel, for the combustion gases to push against. The mass of the ammo also scales roughly as the square of the barrel diameter. So a spud from a 4" barrel has 4x the area and 4x the mass of a spud from a 1.5" barrel. Since acceleration = Force/mass then the two barrels will give about the same acceleration to the spud. But a 1.5" barrel will be 2/1.5 = 1.33 times longer than the 2" barrel for a fixed CB ratio. The longer barrel should give higher muzzle velocities.
How far the spud carries depends on the muzzle velocity and the mass, frontal area and drag coeficient of the ammo.
I wouldn't worry about the range the gun'l shoot. There are more practical things to consider. In particular, spuds that'll fit tightly in a 2" barrel are expensive and not always easy to find. It is much easier and cheaper to get spuds that fit a 1.5" barrel.