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Choosing the “right” barrel length.

There is also the tune up issue. A longer barrel is more flexible. At long range, it helps with a tune that is more forgiving.
 
Mikecr – balance and actually overall weight is one reason why I would not consider the really heavy long barrels. I think those would be OK bench guns but I would have to travel with this one a bit so as you say a balanced moderation would work better here.

As to a barrel's flexibility, do you guys think that a longer more flexible barrel will have a narrower accuracy node?
 
This is just what I think:
Longer barrels have wider nodes with higher amplitudes(more swinging/twisting, & time)
Short barrels have narrow nodes with lower amplitudes(less swinging/twisting, & time)
This is not reflected with OBT(physical position of muzzle release)(non-linear nature of powder burn in longer barrels).
And it's amazing to me that seemingly nobody considers muzzle pressures in choice.
 
Mikecr – interesting comments!

Could you elaborate a bit more in terms of “higher amplitudes”? I presume it relates to as you said about how the barrel “swinging/twisting, & time” that makes sense for a longer barrel, but how does that contribute to a wider node?

Also if possible expand on your comment on “muzzle pressure in choice”.
 
OBT is about dimensional change in bores -sweeping back & forth breech to muzzle to breech, etc.
But on firing barrel/bore straightness changes as well. The barrel straightens from rest condition like a newly pressurized hose. It untwists a bit from the counter energy of spinning up the bullet, this also applies through bedding and on down the fore arm to lift the gun in torque against the bag. The bore expands directly behind the bullet as it travels down the barrel. The powder burn rate changes with pressure and heat as the bullet travels down the bore. And there are pressure nodes, which trump many things, as timing is set way into diminished returns, and it's hard to change this even if you tried -by then. Pressure nodes are like an OCW, only at extreme peak pressures as is common with competitive underbores.
Given all this, there is a lot going on at the muzzle before, during, and after bullet release. And there is a lot going on as extended over more time with a longer barrel(like slower motion). So if we're to pick a place for the muzzle to be on release, a longer barrel will make the precise timing of this easier. But it's even more important that we do so here, because any off timing will have a larger affect, because some of the movements are way bigger with longer barrels.
With a shorter barrel, slight mis-timing is less damaging, because some of the movements are a lot less. This is a good thing because it's more difficult to nail timing with enough precision with shorter barrels, because there is less time to do so(like fast forward).

Muzzle pressure is that slapping the back of bullet bases on release. It's a disturbance to stability if nothing else. With big boomers and short barrels(like 26") muzzle pressures can reach extremes(12-15Kpsi) and this does nothing good for results. A competitive 6PPC, even with it's ~22" barrel and extreme pressure load, produces low muzzle pressures(~8Kpsi). This helps while shooting light boat tails. FB bullets are less susceptible to muzzle pressures either way.
More powder burning produces more gasses that if not extended inside the bore will release at the muzzle. And I believe there is a price to pay here.
I notice 6Dashers shooting really well in 30" barrels, and I wouldn't be surprised if some of that is due to low muzzle pressures. Most of it is the shooters, and it's a damn good cartridge, but you might think there would be some detriment(price to pay) for such long barrels with such a small cartridge.
It's interesting to me.
 
Thanks for the detailed response and sorry for the slow response but that is a lot to take in and I am not sure I got it all.

It would seem that you are saying at least in a limited sense that the longer the barrel, the longer it takes the dimensional change in the bore to reach the crown and so you might have more time between when this happens (something to avoid) thus a wider accuracy node. This is interesting as I have not heard of accuracy node explained in this manner. Not saying this is incorrect but it would seem that if accuracy is only lost when the dimensional change is happening at the crown, it would mean that you are safe and in the accuracy node most of the time. Unfortunately for me at least, what I have seen in real life is the node is usually narrow with the rest of the time being inaccurate node. Am I reading what you are writing wrong?

Also agree that a longer barrel will whip more and so larger effect if the bullet happens to exit at the wrong time.

I am also wondering how muzzle pressure is a slap in the back of the bullet, it would seem that this is a constantly building pressure that is always there as soon as the powder starts to burn.
 
mikecr said:
OBT is about dimensional change in bores -sweeping back & forth breech to muzzle to breech, etc.
But on firing barrel/bore straightness changes as well. The barrel straightens from rest condition like a newly pressurized hose. It untwists a bit from the counter energy of spinning up the bullet, this also applies through bedding and on down the fore arm to lift the gun in torque against the bag. The bore expands directly behind the bullet as it travels down the barrel. The powder burn rate changes with pressure and heat as the bullet travels down the bore. And there are pressure nodes, which trump many things, as timing is set way into diminished returns, and it's hard to change this even if you tried -by then. Pressure nodes are like an OCW, only at extreme peak pressures as is common with competitive underbores.
Given all this, there is a lot going on at the muzzle before, during, and after bullet release. And there is a lot going on as extended over more time with a longer barrel(like slower motion). So if we're to pick a place for the muzzle to be on release, a longer barrel will make the precise timing of this easier. But it's even more important that we do so here, because any off timing will have a larger affect, because some of the movements are way bigger with longer barrels.
With a shorter barrel, slight mis-timing is less damaging, because some of the movements are a lot less. This is a good thing because it's more difficult to nail timing with enough precision with shorter barrels, because there is less time to do so(like fast forward).

Muzzle pressure is that slapping the back of bullet bases on release. It's a disturbance to stability if nothing else. With big boomers and short barrels(like 26") muzzle pressures can reach extremes(12-15Kpsi) and this does nothing good for results. A competitive 6PPC, even with it's ~22" barrel and extreme pressure load, produces low muzzle pressures(~8Kpsi). This helps while shooting light boat tails. FB bullets are less susceptible to muzzle pressures either way.
More powder burning produces more gasses that if not extended inside the bore will release at the muzzle. And I believe there is a price to pay here.
I notice 6Dashers shooting really well in 30" barrels, and I wouldn't be surprised if some of that is due to low muzzle pressures. Most of it is the shooters, and it's a damn good cartridge, but you might think there would be some detriment(price to pay) for such long barrels with such a small cartridge.
It's interesting to me.


Mike, your last few posts have been superb! This is very good information...some insight you don't see in print often.--Mike Ezell
 
jlow said:
it would seem that if accuracy is only lost when the dimensional change is happening at the crown, it would mean that you are safe and in the accuracy node most of the time.
I'm referring to all the muzzle movements that are beyond OBT. As in OBT is just one factor.

jlow said:
I am also wondering how muzzle pressure is a slap in the back of the bullet, it would seem that this is a constantly building pressure that is always there as soon as the powder starts to burn.
There is a thread around here somewhere about powder burn percentage(that got really ugly). It didn't conclude with consensus about the cause of muzzle fireballs & secondary pressure peaks -whether powder burn or secondary gas burn off. But I think most would agree that the main contributor is a relatively short barrel for capacity.
While that bullet is going fast with all the pressure behind it, that pressure wants to, and can go even faster. The very instant pressure can get around the bullet -it does.
Watch this slow-mo at the 25sec point: https://www.youtube.com/watch?v=lWn_NhYzD_Y
(keep pausing) You can see the gas expansion disturbing the projectile.
Of course, if you could disturb your projectiles exactly the same every time, then maybe it would never be an issue other than reducing effective BC.
But it's my contention that reducing muzzle pressures below detriment is a worthy goal.

Muzzle brakes do not relieve this pressure and likely aggravate the situation. I've yet to see an actual fix, which would be porting EDM cut into the muzzle(around the bore) to relieve pressure while the bullet is still captured. So other than this, we have only barrel length, load, and a pristine crown to control it.
 
Thanks! OK, so this dimensional change is on top of the wiping movement associated with OBT – that makes more sense.

Yes, I about the pressure at the point of release, I can imagine since the gas has lower mass than the bullet, it will immediately move faster than the bullet once it is released at the crown and at that point can and will disturb the bullet – that also makes sense to me. It is my understanding that this is one reason why a clean even crown is so important.
 
FWIW - Virgil King of the fabled "Houston Warehouse" stated emphatically that 21.75" was the magic length - period. No other length ever came close to matching it for .224, .243, or even .308 caliber benchrest accuracy in that indoor range, and he shot consistently in the "ones", and was not satisfied with groups over .070".

(Don't shoot me - I'm only the messenger.)
 
Steve Blair said:
Did Virgil compete at 1000 yards?

I don't know, but I don't think so.

Are you saying a 21.75" barrel is too short to be effective at 1000 yards (with any cartridge/bullet combination)?

Suppose you had a rifle that could shoot groups consistently in the 0.1s at 100 yards with a 21.75" barrel, but could only muster groups in the .3s with a 26" barrel, would you accept 3x larger 100-yard groups in order to gain, say, 250 fps muzzle velocity? (Assume the 21.75" barrel can launch a VLD bullet from some cartridge just fast enough to stay supersonic out to 1000 yards, if that's possible.)

These are honest questions - I don't shoot 1000-yd matches, so I don't know the answers.

PS The OP said "600 yards plus", not 1000 yards specifically. Significant in this discussion.
 
It is not impossible, but a barrel that short would be a significant disadvantage. Longer barrels allow large capacity cartridges, slower powders, higher B.C. bullets and higher muzzle velocities while keeping muzzle pressures reasonable with prudent component selection. Long barrels can also shoot quite well. My 34" .300 WSM will agg 5-shot, 100 yard groups ~.22" and I am a mediocre benchrest shooter shooting from a flexible, wooden bench. [br]
I have no doubt when the demands of long range shooting are removed, a short, very stiff barrel will produce more reliable results. When all of the problems between muzzle and 1000 are considered, it is not the best choice.
 
Steve Blair said:
It is not impossible, but a barrel that short would be a significant disadvantage. Longer barrels allow large capacity cartridges, slower powders, higher B.C. bullets and higher muzzle velocities while keeping muzzle pressures reasonable with prudent component selection. Long barrels can also shoot quite well. My 34" .300 WSM will agg 5-shot, 100 yard groups ~.22" and I am a mediocre benchrest shooter shooting from a flexible, wooden bench. [br]
I have no doubt when the demands of long range shooting are removed, a short, very stiff barrel will produce more reliable results. When all of the problems between muzzle and 1000 are considered, it is not the best choice.

Thanks, your commentary is in line with what I had gleaned, no surprises. My original comment was food for thought. But as it were, King stated that it couldn't just be any short stubby barrel, he stipulated 21.75". So 21.5" and 22.0", while certainly short and stubby by today's standards, didn't make the cut for absolute accuracy.

Question (again, from ignorance): How long, typically, are the barrels on the AR-15s doing well out beyond 600 yards in High Power Rifle competition?
 
You can always shorten a barrel that is too long.

That said, if you're putting this barrel in an AI AT, and it's more for steel/practical shooting than F-Class, I would recommend 22" (or if you must, 24"), especially if you intend to run a suppressor.

Of course all barrels are different, and this is pure anecdote/single data point, but I have a gun built on a Stiller action, chambered in 6.5x47L with a 22" barrel that shoots around ~100fps slower than my old 600yd F-Class gun in the same caliber with a 30" barrel. The relatively minor loss in MV (especially if you're planning on mostly 600yds or so) is well worth the handling benefits of a shorter rifle.
 
I did some simulation using QL to see if it can help me figure out the optimal barrel length. The caliber as mention in the OP is 308 and I am using Lapua 308 brass and 175 SMK. So the study here was done with 4 barrel lengths – 18, 20, 24, and 26. I put in 18 because I have an 18” 308 and so that is put in as a validation of the QL simulation. The other 3 barrel lengths are what is offered for the AT.

So as you can see, with an 18” barrel, you really only have two nodes (6 & 7) that you can reach and the powder weights are either 42.3 grains or 45 grains. The 42.3 only gives you an MV of 2356, so not that useful, but if you want to reach 2512 fps, you need to go up to 45 grains. This is exactly my experience with the 18” 308 rifle.

Now if you look at the other three barrel lengths, the higher node can be reach as expected with less powder i.e. 43.8, 43.7, and 42.75 grains respectively for 20”, 24”, and 26”. So the idea that a longer barrel will give you the optimal MV with less powder is correct.

However, what is interesting is although there is an approximate 100 fps jump in MV when you go from 20” to 24/26”, you don’t really get significantly more MV going from 24” to 26”. There is certainly a one grain reduction in powder to get to the same speed but that is all you get for that extra 2” in barrel length. Since neither 43.7 or 42.75 grain of Varget is really that hot and so may not significantly affect primer hole life, one may come to the conclusion that for a lighter and more maneuverable gun, the 24” is the better choice.
 

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jlow said:
I did some simulation using QL. ... So as you can see, with an 18” barrel, you really only have two nodes (6 & 7) that you can reach and the powder weights are either 42.3 grains or 45 grains.

In the interest of maximum clarity, could you define "node" WRT QuickLoad?
 
brians356 said:
jlow said:
I did some simulation using QL. ... So as you can see, with an 18” barrel, you really only have two nodes (6 & 7) that you can reach and the powder weights are either 42.3 grains or 45 grains.

In the interest of maximum clarity, could you define "node" WRT QuickLoad?
I suggest you read this review:

http://www.accurateshooter.com/gear-reviews/test-quickload-review/
 
jlow said:
brians356 said:
jlow said:
I did some simulation using QL. ... So as you can see, with an 18” barrel, you really only have two nodes (6 & 7) that you can reach and the powder weights are either 42.3 grains or 45 grains.

In the interest of maximum clarity, could you define "node" WRT QuickLoad?
I suggest you read this review:

http://www.accurateshooter.com/gear-reviews/test-quickload-review/

Thanks. But there is not one occurrence of the word "node" in that review - unless my Firefox page search has a gaping hole in it.
 
brians356 said:
jlow said:
brians356 said:
jlow said:
I did some simulation using QL. ... So as you can see, with an 18” barrel, you really only have two nodes (6 & 7) that you can reach and the powder weights are either 42.3 grains or 45 grains.

In the interest of maximum clarity, could you define "node" WRT QuickLoad?
I suggest you read this review:

http://www.accurateshooter.com/gear-reviews/test-quickload-review/

Thanks. But there is not one occurrence of the word "node" in that review - unless my Firefox page search has a gaping hole in it.
A node is a powder weight that produce a tight group. With any caliber/bullet/powder/case/temperature/seating configuration, there are discrete powder weights that gives a tight group. We call them accuracy nodes.
 

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