16 vs 20 inch barrel 5.56 AR velocity

[TonyAngel]

Are you sure you set the chrony up right. Were the screens the appropriate distance from the muzzle? Were there any factors that would affect the accuracy of the chrony?

Still, I wouldn't worry too much about it. Use the chrony data to get your dope and confirm that the ammo shoots accurately.

Some barrels are faster than others. My 13.9 Criterion Core clocks the same velocity as my 16" Ballistic Advantage Hanson.

 

[MachIVshooter]

I think the average is about 25 - 30 FPS difference per inch of barrel.

I've read it before, you can Google it and check.

That's a good rule of thumb for many centerfire rifle rounds, assuming you're within 5-6" of an "ideal" length, like 22" +/- 5"

But intermediate rounds or rounds with greater bore area to case capacity ratios tend to be less affected. And of course the changes can become more dramatic at much shorter lengths, while conversely there will be little or sometimes no gain at exceptionally long lengths. A .308, for example, will drop much more than 150-180 FPS from 14" to 8", and will gain very little with most loads going from 24" to 30".

In my experience, 5.56 doesn't gain a whole lot from 16" to 20" with ball ammo, 70 to maybe 100 FPS.

 

[Necessary_Nutrient]

I’ve always gotten around the 80-100 fps difference obtained in that barrel cutting test between 16 and 20”

You need to maintain the 10’ distance as specified by the manufacturer from muzzle to chronograph- or whatever the instructions say or your readings are suspect.

As far as test barrel lengths, the only concrete rule is that 6.5 Grendel results from a 24” barrel must be compared to 6.8 SPC results from a 16” for forum arguments.