Thread: Short barrels and full powder burn?

Hi everyone.
Have heard talk about longer cartridges not getting a full powder burn in shorter barrels? Take for example a 280 rem in 20" barrel.
How true is this?
Can it be overcome some what with powder choice?
And is there a rule of thumb if you like to cartridge and barrel length?
Keen to hear people's thoughts on this.
Cheers steff.

It can definitely be overcome with powder choice.

I had a 16" 7mm08. Putting the loads into GRT, I saw I wasn't getting full powder burn with AR2209. I got some BM2, which was supposed to give me 100% powder burn.

Did I feel it was more efficient? Yes.
Did it increase the velocity? Not really

I found that loads that suposedly achieved "full powder burn" in GRT still produced a visually large muzzle flash. The GRT calculated velocities were within a few fps to the measured ones.

Generally heavier bullets and faster powders should increase the amount of powder burn, maximum pressures need to be kept in kind though!

When it comes to standard centrefire rounds, all the powder *that is going to be burnt* is consumed within the first few inches of the barrel. No powder ever burns 100% of the kernels inside the barrel.

Muzzle flash is the gas created during conflagration reigniting on contact with the atmosphere.

The most efficient range of powders for a given cartridge/bullet combination is a result of the case capacity to bore size ratio, moderated by projectile weight (heavier bullets equals slower burn rate for best velocity/pressure). The notion of using faster burn rate powders in shorter barrels is an incorrect and dangerous myth that refuses to die.

^

Depends what you are trying to accomplish. If you want to reduce the muzzle pressure to go a little bit easier on your suppressor and your ears, then a faster powder can make a small difference, sure. But if you are trying to attain the maximum possible velocity, then the slower powders will still give you that. They may be less efficient (in terms of chemical energy of the powder converted to kinetic energy imparted to the bullet) but it is more than offset by the larger charge weights used. The bullet gets driven harder for longer, giving more speed.

The best way to get speed safely, regardless of barrel length, is to jam the case full with the slowest stuff reasonably possible and blow a healthy portion of it out the end of the barrel "unburned".

Originally Posted by Steelisreal

When it comes to standard centrefire rounds, all the powder *that is going to be burnt* is consumed within the first few inches of the barrel. No powder ever burns 100% of the kernels inside the barrel.

Muzzle flash is the gas created during conflagration reigniting on contact with the atmosphere.

The most efficient range of powders for a given cartridge/bullet combination is a result of the case capacity to bore size ratio, moderated by projectile weight (heavier bullets equals slower burn rate for best velocity/pressure). The notion of using faster burn rate powders in shorter barrels is an incorrect and dangerous myth that refuses to die.

Brain fart moment - I meant deflagration not conflagration in regard to powder burning...

The whole thing with GRT and Quickload before it is that they're mathematical models of a complex chemical process. I've read elsewhere from someone who works in the ballistics industry that the 'powder burnt' part of Quickload was a feature that he discussed with the programmer and disagreed that it should be presented in that way - funnily enough because it gave people the wrong impression about what was actually going on internally.

One outcome you can change with powder choice is muzzle pressure though. That part is true and has relevance to the pressure curve and getting gas operated semi-automatics to function correctly. It can also be played with when brewing up subsonic loads. A friend has played with this in various sub loads with noticeable effects.

What you definitely do get with faster burning powders is a (relatively!) quicker pressure spike. All things being kept equal (and safe!!!) the correct range of burn will give best velocity regardless of barrel length (within reason - we're not discussing pistol calibres/lengths).

As long as you remember that the projectile is being pushed out of the barrel by gas pressure then it makes sense. The powder burns quickly (first few inches regardless of burn rate) to create that pressure. If the faster powder in short barrels notion were true you could shoot this same mythical load in a longer barreled rifle and get even moar speeeed! Sadly it doesn't work this way however so we go back to using the appropriate powder that suits the case volume and projectile weight. The rules of physics are boringly rigid so stick to following published load data and stay safe!

There is some weirdities in this though - some barrels just don't follow the rules or write their own despite the laws of physics and ballistics.

I have one here that is giving me an issue - it's a bog standard 1-10 Rem .308, but it just doesn't follow the velocity rules. With reloads I can hit the full case and start compressing the powder - with from what I can see no pressure signs at all and the main issue I have is with factory loads it just doesn't hit the ballistics charts by a long way off. If it was a sticky barrel that required more push I could understand it, but... As another example subsonic loads that work fine in another .308 are well supersonic in this one - it requires an entire grain less powder to get to a reliable subsonic velocity. Must just be a very slippery barrel, which from what I have in my head is the opposite of what should happen as 1-12" twist being the norm this barrel at 1-10" should be slower not slicker. For a factory barrel it's a bit of a head scratcher, I've never run into this before. It's not lacking for accuracy, just horsepower to get to a decent distance without falling off a cliff.

As far as barrel length and powder burn - the .22LR is a good example of this. Max speed is usually in the 14-16" barrel range, some 24" or 26" barrels actually run slower velocities than the shorter ones.

If your caliber is a 280 in a 20 inch barrel I don't think you need to worry about powder choice as it's not a huge volume to burn down a medium length barrel.
If it was a 16 inch barrel it might be a different conversation

Originally Posted by No.3

There is some weirdities in this though - some barrels just don't follow the rules or write their own despite the laws of physics and ballistics.

I have one here that is giving me an issue - it's a bog standard 1-10 Rem .308, but it just doesn't follow the velocity rules. With reloads I can hit the full case and start compressing the powder - with from what I can see no pressure signs at all and the main issue I have is with factory loads it just doesn't hit the ballistics charts by a long way off. If it was a sticky barrel that required more push I could understand it, but... As another example subsonic loads that work fine in another .308 are well supersonic in this one - it requires an entire grain less powder to get to a reliable subsonic velocity. Must just be a very slippery barrel, which from what I have in my head is the opposite of what should happen as 1-12" twist being the norm this barrel at 1-10" should be slower not slicker. For a factory barrel it's a bit of a head scratcher, I've never run into this before. It's not lacking for accuracy, just horsepower to get to a decent distance without falling off a cliff.

As far as barrel length and powder burn - the .22LR is a good example of this. Max speed is usually in the 14-16" barrel range, some 24" or 26" barrels actually run slower velocities than the shorter ones.

Long throat? Large chamber dimensions? 5r barrel?

Cheers guys, thanks heaps!
More than answers my question.

Originally Posted by veitnamcam

Long throat? Large chamber dimensions? 5r barrel?

Rem factory chamber, so it's not unusual to get a factory round that feels 'tight' on closing. Not a 5R barrel, they are 1 in 11-1/4" twist in .308. Mine is a standard 1-10" twist off the .30-06 tooling but in .308 - so there may be something there as the usual Rem factory .308 offerings are 1-12". I got this new as a multi-use rifle subs and supers - does this well as happily stabilises everything I've been able to get my hands on for sub loads and am currently using 175gr Sub-X pills. It's just the supers that are odd.

Don't know about long throat, but it doesn't look visibly different to my other .308 Rem barrels and I'd assume the same chamber... I have another that has "only just come back from having the barrel swapped about 3 years ago" haha - need to get that up to the range and do a side by side comparison with it to try and work out what the - is going on with this one.

As far as the original question, remember the case length difference between an -06 parent case in the long action and the -08 in the medium or short action is actually not that much - everything else will cause more variation than the difference in case length caliber for caliber.

It's more to do with how overbore the cartridge is. That is all else being equal a 3006 would lose less efficiency per inch of barrel removed versus a 280 rem. Likewise for a 6.506 and 270.

Rule of thumb as far as I've seen, is no shorter than 21 inches for a magnum. That's the sensible one anyway. From what I have seen people do what suits them and that's the name of the game