Almost ALL the powder that's going to burn, will burn within a couple inches in front of the chamber. The same powders that result in the highest velocity in a 24-26" barrel will result in the highest muzzle velocity in a 20-22" barrel.
The "flash" at the muzzle isn't powder that's still burning, but the hot gas reigniting when it hits the atmosphere's oxygen. Rifle powder contains it's own oxygen source, but by the time it leaves the muzzle is oxygen-starved. It's still plenty hot, however, and with the "new" source of oxygen reignites--but isn't likely to cause a forest fire, unless you insert the muzzle into a bunch of dry tinder.
.................+2.............................Shamus....Here is what you do......You reload and treat your 20" 338 just as though it had either a 22" barrel, a 24" or a 26" er. So called flame throwers from shorter tubes are imo over exaggerated.
I own a 300 WSM Ruger Frontier w/ a 16.5" barrel and have yet to see all this so-called flame from the muzzle. Now I suppose if I were firing in pitch black darkness or during extremely low light then I might see some of these flames.
Yes. Pursue your velocity or performance. Use a chrony. But also pursue accuracy. Let the so called flame throwing chips fall where they may.
