CRm Sam,
While I agree with you on your statement regarding the velocity difference between like guns, I humbly disagree with your statement regarding barrel length.
Here is an example of a 9MM Luger load that was calculated by QuickLOAD Internal ballistics software
http://www.neconos.com/. The speed difference for the same powder charge and bullet weight is 238 FPS. The only variable I changed was the barrel lengths for the first one I used 2 inches for the second I used 4 inches.
Knowledge is power
2 Inch Barrel
Input data: Date: 15-Feb-2002 Time: 19:01:16
Cartridge................. = 9 mm Luger (Para)
Projectile type........... = 9 mm, 124 gr, Berrys TMJ with flatbase
Max.avg.pressure(Piezo CIP)= 34084 psi Shot start / init pressure = 2175 psi
Projectile weight (gr)= 124 Groove caliber (in)= 0.355
Length of cartridge (in)= 1.061 Length of case (in)= 0.745
Case capacity(Grains Water)= 13.31 Length of projectile (in)= 0.535
Length of barrel (in)= 2.000
**** Values calculated:
Seating depth (in)= 0.219 Volume displaced (gn H2O)= 5.48
Comb.chamber volume(gn H2O)= 7.82 Projectile travel (in)= 1.474
**** Input data:
Propellant type........... = Vihtavuori N320
Charge weight..........(gr)= 3.9 Load density......(g/cu.cm)= 0.498
Heat of Explosion (cal/lb)= 448530 Ratio of spec. heats cp/cv = 1.229
Solid density (gr/cu.in)= 349.0 Weighting factor.......... = 0.8
Burning rate factor Ba(1/s)= 3.585 Pro-/degressivity factor a = 1.029
Burning limit ......... z1 = 0.5 Combust.Chamber Vb(ft³)= 1.793748E-5
Factor.................. b = 1.662 X-S.Area of Bore A(ft²)= 6.739505E-4
Bulk density (gr/cu.in)= 141.6 Projectile mass mp (lb)= 1.771384E-2
Loading ratio (%)= 88.9 Projectile travel x(ft)= 1.228346E-1
**** Results calculated:
Maximum pressure ........ = 30834 psi Way of projectile at Pmax = 0.20 in
Muzzle velocity ......Ve = 883 fps Muzzle pressure ......Pe = 9127 psi
Project. energy at muzzle = 215 ft.lbs Fraction of powder burnt = 100 %
Projectile travel time from 10% Pmax to muzzle = .29 ms
6 Inch Barrel
**** Input data: Date: 15-Feb-2002 Time: 19:02:54
Cartridge................. = 9 mm Luger (Para)
Projectile type........... = 9 mm, 124 gr, Berrys TMJ with flatbase
Max.avg.pressure(Piezo CIP)= 34084 psi Shot start / init pressure = 2175 psi
Projectile weight (gr)= 124 Groove caliber (in)= 0.355
Length of cartridge (in)= 1.061 Length of case (in)= 0.745
Case capacity(Grains Water)= 13.31 Length of projectile (in)= 0.535
Length of barrel (in)= 6.000
**** Values calculated:
Seating depth (in)= 0.219 Volume displaced (gn H2O)= 5.48
Comb.chamber volume(gn H2O)= 7.82 Projectile travel (in)= 5.474
**** Input data:
Propellant type........... = Vihtavuori N320
Charge weight..........(gr)= 3.9 Load density......(g/cu.cm)= 0.498
Heat of Explosion (cal/lb)= 448530 Ratio of spec. heats cp/cv = 1.229
Solid density (gr/cu.in)= 349.0 Weighting factor.......... = 0.8
Burning rate factor Ba(1/s)= 3.585 Pro-/degressivity factor a = 1.029
Burning limit ......... z1 = 0.5 Combust.Chamber Vb(ft³)= 1.793748E-5
Factor.................. b = 1.662 X-S.Area of Bore A(ft²)= 6.739505E-4
Bulk density (gr/cu.in)= 141.6 Projectile mass mp (lb)= 1.771384E-2
Loading ratio (%)= 88.9 Projectile travel x(ft)= 4.56168E-1
**** Results calculated:
Maximum pressure ........ = 30834 psi Way of projectile at Pmax = 0.20 in
Muzzle velocity ......Ve = 1121 fps Muzzle pressure ......Pe = 2040 psi
Project. energy at muzzle = 346 ft.lbs Fraction of powder burnt = 100 %
Projectile travel time from 10% Pmax to muzzle = .61 ms
WARNING: Near Maximum Average Pressure - tolerances can cause dangerous pressures !
Real maximum of pressure inside barrel.
End of combustion before the projectile passes muzzle.