Simple answer.... no two barrels are alike. One may have a shorter throat, which has the bullet just kissing the lands, which most milspec barrels have a more generous leade ( 5.56 v. .223 )...
to many possible differences to list.... ( rifling type, actual bore diameter, bullet construction... solid copper )
And now for your reading pleasure....
http://www.leverguns.com/articles/ballisticians.htm
"Dear Ballistician: Your reloading manual is all wrong! You say on page 713 that 11.2 grains of Super-Duper powder will push the 125 grain bullet at 1468 fps. My barrel length is the same as yours, but when I tried this load and had my friend chronograph it the velocity was only 1411 fps. Why are you so far off?"
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Letters like this imaginary one are all too common. In an effort to pinpoint one reason for such velocity differences, the Speer Ballistic Laboratory selected three lots of .357 Magnum ammunition in different bullet weights. These particular lots of ammunition were selected because of their uniformity, not because of high velocity. The ammunition was fired in all of the .357 Magnum guns available to the lab at the time.
The different handguns were all tested in same manner with the gun muzzle elevated and then gently lowered to the horizontal for each shot. Every effort was made to make the results as accurate as possible.
The table shows the average velocities of the three different bullet weights in each of the guns tested. Note that in the standard 10" test barrel, made to tight ammunition industry specifications, the extreme variation (EV) in the velocities ranged from 48 fps for the 125 grain hollow point bullet, to 38 fps for the 158 grain soft point bullet. Using the 6" barreled revolvers as an example, the EV between all 125 grain bullets fired in all the 6" barrels was 376 fps, almost 8 times the EV in the test barrel. The EV for all 6" barreled revolvers with the 140 grain hollow point ammunition as 275 fps, over 10 times the EV in the test barrel. The 158 grain soft point ammunition showed an EV of 282 fps, almost seven times the test barrel EV of 38 fps.
These large variations are due partly to the relatively very small differences in chamber, bore, forcing cone rifling, and barrel-cylinder gap dimensions and in the finish or smoothness of these interior surfaces. Chambers will vary minutely even though cut with the same reamer, as will all other machined surfaces. It is virtually impossible to manufacture two of some machined metal item, even as simple as a revolver, with all dimensions and finishes exactly the same. When hundreds of thousands of .357 Magnum revolvers have been made by thousands of different people, in different factories, with different materials and tooling, it cannot be surprising that there are differences between guns.
These minor differences between guns cause some of the differences in ballistic measurements. Additional variations, due to differences between different makes or lots of bullets, powder, primers and cases, powder charges, loading dies, loading techniques and chronographs complicate the problem. Many times these small differences tend to cancel each other, but when everything goes one way, the resulting variation may be relatively large.
These velocity tests are not presented with any idea of claiming that one particular brand or model of revolver is superior to another. A repeat of the test with different ammunition might well reverse the relative standings shown here. The point we want to make is that even with the very best quality ammunition available, there will always be velocity variations when the ammunition is fired in a different gun.
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Gun Description Barrel
Length
-Ammunition Used in Test-
125 grain
140 grain
158 grain
S&W M19#1 2.50" 1190 1132 1034
Colt Python 2.50" 1205 118 989
S&W M19#2 2.50" 1209 1118 1018
Ruger Security Six 2.75" 1233 1154 1075
Colt Trooper Mark III 4.00" 1317 1175 1101
S&W M66 4.00" 1385 1225 1117
S&W M19#1 4.00" 1368 1227 1153
S&W M19#2 4.00" 1374 1242 1146
Ruger Security Six #1 4.00" 1370 1242 1130
Ruger Security Six #2 4.00" 1380 1267 1151
Dan Wesson 4.00" 1358 1280 1160
Ruger Blackhawk #1 4.625" 1361 1266 1159
Ruger Blackhawk #2 4.625" 1480 1336 1196
Ruger Security Six #3 6.00" 1436 1311 1210
S&W M19 #1 6.00" 1400 1282 1179
S&W M19 #2 6.00" 1372 1281 1154
S&W M19 #3 6.00" 1603 1417 1284
S&W M28 #1 6.00" 1307 1246 1080
S&W M28 #2 6.00" 1499 1364 1207
S&W M27 6.00" 1547 1358 1248
Colt Python #1 6.00" 1227 1142 1002
Colt Python #2 6.00" 1477 1373 1251
Colt Python #3 6.00" 1468 1364 1207
Ruger Blackhawk (new) 6.50" 1471 1375 1262
S&W M27 8.375" 1547 1358 1248
Ruger Blackhawk 10.00" 1738 1544 1365
T/C Contender 10.00" 1944 1726 1587
Martini Rifle 17.375" 2121 1906 1678
Winchester 92 Rifle 20.00 2153 1964 1824
Marlin 1894 Rifle 24.00" 2212 1994 1835
Velocity Test Barrel 10.00" 1866 1732 1591
Velocity Test Barrel Extreme
Variation
48 26 38
Note the difference in velocity for some of the identical makes and models..... In particular the 3 S&W M19s