Ballistics

[k2gvp]

When I look at my reloading manual, the velocity is giving using a certain length test barrel.  If for example the test barrel is 8 inches with a listed powder and amount the velocity should be around 1350 FPS , is there a rule of thumb that one can use to get a ball park reduced velocity with a 6 inch or a 4 inch barrel with the same load?  I realize that powder burn rate and other factors come into play.

[Badgersmilk]

Not what you want to hear.  But, without testing your particular gun / ammo combination, your just throwing numbers around that don't mean anything at all.  

Example.  A bolt gun will shoot the same ammo at higher FPS than a semi-auto.  What type of threading is in the barrel?  How far off proper dimension is the bore of your particular gun (no, it's not perfect)?  How tight is the chamber of your gun?  How worn is it?  How was it "broke in"?  How clean is it?  Temperature?  How much of a gap is there between the particular bullet type your using and the threads of the barrel?  Even the type of primer used makes a notable difference.  These and about two hundred more factors effect velocities.  Someone may say.  "Oh, that stuff only effects it a little bit.  Maybe 50fps or so".  OK, multiply that 50fps times those 200 factors.  Guessing at velocities is pointless.  Reading velocities from a book will usually get you within about 300fps or so if you have the EXACT same ammunition and barrel length if your lucky.

Ballistics charts ARE super useful as a "this bullet is faster than that one" because they are (presumably) using the same gun and shooting conditions each time they fire a different round.  If you change both those two things, and just use the same ammo the charts numbers are nearly meaningless. 

My guess is most people just make up a number they're happy with in that situation rather than actually testing.  Magic math, calculators, and slide rules just make it "feel" better.   

[tombogan03884]

 One word, Chronograph.

[Badgersmilk]

One word, Chronograph.

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