Minimum Length Barrel for a .338 Lapua Magnum
- Thread starter Thomas D'Arcy McGee
- Start date
174 in inches. - dan
- Location
- Prince Albert, Sk
It's true - but it's easy to understand why there are so many misconceptions. For example, manufacturers typically offer magnum ("overbore") chamberings in longer barrels than non-standard, but that's not because the magnum "needs" a longer barrel (to burn all the powder, perform well, etc.), it's because people typically buy magnum chamberings not to carry in the bush where a long barrel would be a nuisance, but for long, clear shots. When length and weight don't matter, the extra velocity that comes from a longer barrel is worth it, and for the long shots that magnum owners want to make, the extra velocity can be critical, especially at extreme ranges. It's also true that magnum chamberings gain and lose more velocity with each inch of barrel added or removed, but a 20" barelled 338 LM will still produce far more MV than a 338-06, 338 WM, etc. People think that the greater velocity gain per inch of barrel proves that the length is "needed", but it's simply that you're working with higher velocities to start with. Then there's the question of what does it mean when people say the powder "burns". If it means "is producing pressure", which is all that matters to the shooter, then it's all "burned" with the 6 inches I mentionned. You can see that point from the pressure graph associated with the load in question. The pressure will rise very quickly and within less than 0.001 seconds start to fall in a pattern that simply reflects the fall in pressure that occurs as the volume behind the bullet increases proportional to its distance travelled. Every load will have its own unique pressure curve, but thay all look like this "typical" curve.
After the peak, even though no additional pressure is being added because the powder is no longer "burning" (in fact the pressure is dropping), the pressure is still high enough to accelerate the bullet and could (depending on various factors) continue to accelerate the bullet for 30, 40, even 50+ inches, if the barrel was that long. Once the force from the high pressure gas is overcome by the friction of the bullet in the bore, the bullet will stop accelerating.
Finally, other factors add to the misconception, e.g. that no powder is ever 100% consumed (and can't be regardless of barrel length or pressure), and that each powder neeeds to operate in its intended pressure range, or even less wil be consumed. People see particles and fire exit the muzzle and that's often enough proof to them that they need a longer barrel to burn it, but that's just the by-products of the combustion being expelled, and the hot gasses reacting with oxygen - it was no longer producing pressure, hence no longer "burning".
Hope that helps.
To think all these guys testing different cartridges and cutting the barrel back and seeing actual velocity loss..what were they thinking. So a 7mm STW would get the same velocity out of a 20" barrel as a 26"? I wasn't going to build a 22" 7mm Rem mag because of velocity loss..guess she is a go..yah0000000000.
