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Old January 13, 2022, 06:11 PM   #11
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 19,897
Late to the party, but thought some might find this useful.

The bigger the volume behind the bullet, the smaller the percent change in volume each additional inch of bullet travel expands the volume behind it (the volume the propellant gas and burning powder occupy). Any rule of thumb will be right at some particular barrel length, but it changes with barrel length, becoming smaller as the barrel you start with gets longer. If it changes 25 fps going from 18 to 19 inches, it might change 15 fps going from 31 to 32 inches.

In this approximating table, 308 will be in the left column, as it is technically a medium power cartridge and not an "overbore" cartridge. Just multiply the published 308 ammunition box velocity by the number in the table for your barrel length to learn what the velocity would be if you had a chamber and bore with the same exact dimensions as the manufacturer's SAAMI P&V (pressure and velocity) test barrel and fired it under the same conditions. 24" is where the multiplying number is 1.00 because 24" is the single most common SAAMI P&V rifle barrel length.

There are a few SAAMI rifle P&V barrels that are exceptions to the 24" standard. These are:

.277 SIG Fury 16"
7.62x39 20"
300 AAC Blackout 16"
.30 Carbine 20"
350 Legend 16"
350 Rem. Mag. 20"
351 Winchester 20"
44 Rem Mag 20" (rifle only)

For those, take the velocity multiplier at your barrel length and divide it by the velocity multiplier for the SAAMI actual test barrel length and use the resulting number as the multiplier for the velocity on the box to estimate your velocity.

In real life, you can figure your velocity will typically be for a looser chamber and will be slower, as others have already spelled out. SAAMI P&V barrels have minimum chambers. If you can't measure velocity, I would generally run the ballistic table for the velocity resulting from that calculation and again for a velocity 100 fps slower and figure your result is most likely to be somewhere between the two. I think you will be pleasantly surprised to find it makes less difference than you might expect. The reason is that when you zero your sights, you are already compensating for the velocity difference at your zeroing range. If, say, your barrel shoots slower than expected, the extra elevation from zeroing at that lower velocity will compensate the point of impact over quite a range.

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