Barrel length matters more in .223?

S

Stats Shooter

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A physics question.

So I have 2 semi auto.223/5.56's. one has a 16.125" barrel and the other a 20" bull barrel.

I have 3, .308's. one is a 16.125" Ruger SR 7.62, one a Saiga.308, and a Remmy 700 24" barrel.

My question is: Why does it seem that the velocity loss in my 16" .223 barrel to my 20" barrel is so much more than my loss from my 24" .308 to my 16" .308?

Put another way, it seems barrel length beyond 16" makes a larger difference in the .223 cartridge than .308.
 
You're looking at a small sample. For barrel length and velocity comparisons to have any validity you have to start with a long barrel. Measure the velocity, cut the barrel shorter and measure velocity again with the same ammo. It would be rare for 2 different barrels, even of the same length to shoot ammo to the same speed. A difference of 25-50 fps is the norm, but it isn't unusual to see differences of over 100 fps. Remember, this is with the same length barrel.

If the "faster" barrel happens to be the longer one then it appears to be much faster than the shorter barrel.

If the "faster" barrel happens to be the shorter one, then it appears that there is very little if any speed lost over the longer barrel. I've seen several occasions where a 20" barrel shot the same ammo faster than a 22" barrel and was virtually tied with a 24" barrel.

If you spend a little time looking around you can find test results where people have taken velocity measurements with a variety of cartridges as barrels were cut. You'll find that 223 and 308 both are pretty efficient at 20" and neither gain a huge amount of speed at 24". Both are very similar as to velocity change in relation to barrel length.

What you are seeing is the difference in your individual rifles more than between the 2 cartridges.

Just a SWAG, but my guess is that your Remington 308 is shooting slower than normal, and the 16" 308 barrel may well be faster than normal. This could combine to made the difference between a 16" and 24" barrel appear closer than it really is.

Or you could have a "fast" 20" 223 barrel combined with a "slow" 16" 223 barrel.
 
Maybe the size and location of the gas ports?? Plus the 700 doesnt have a gas port. The 308 gas port would be small, because the bullet is in the barrel longer then the 223? Less loss of pressure, velocity?

In the M16A1, the 14.5" carbine, compared to the 20" rifle, have very different gas port size and locations.
 
I realize it is a small sample. But find it unlikely that there will be enough variability among SAMMI spec chambers to account for that much difference. For instance: my AR10 has a tight chamber and short throat, in fact I have to seat bullets slightly short of the recommended lenght in it. But it has the same length barrel as my SAIGA .308 which has a loose chamber and the difference between the two is 35-45 fps in most of my loads. So, ....while the sample is smaller, I doubt chamber makes such a huge difference.....
What I really wanted to know was, if someone has cut down barrels on .223 and .308 to chart a velocity change or if anyone knew of such data available.
 
http://rifleshooter.com/2014/04/223...gth-a-man-his-chop-box-and-his-friends-rifle/

Sorry for my laziness. Should have looked around more. But the series of articles posted above confirmed what I suspected...which is that beyond 16" length of barrel makes a bigger velocity difference in .223 than .308 and once you get beyond 20" in either of them, each inch of barrel has diminished marginal velocity increases.

Also, barrel length helps less with larger pills.
 
Interesting

243win:
What I find most interesting about the link you posted was how the groups changed. They would get better or worse....and then better or worse again.

For instance: the vvn 26.4 grain load shot a 1/3" group with a 22" barrel. Then it got worse as the barrel got shorter. But then at a 15" barrel it was back to 1/3".
 
Length is only one variable. Bore area, rifling, and land/groove ratio can make a difference too. I've seen 5R and 3R Polygonal barrels shoot more than 100fps faster than a 6 groove Ballard rifled barrel, same length and with the same ammo.
 
From what i read.

better or worse. The barrels groove diameter may change a few times in the length of the barrel. Make a cut at a tight area, better accuracy? A match grade, air gauged barrel may not change as much?

I read where 22LR barrels can be air gauged, then cut and crowned at the tight spot for better accuracy. May or may not be true?

What if the new crown is not perfect?
 
All else being equal, the bore with the larger diameter will provide less of a velocity return for increasing the length.

That's because as the bullet moves down the bore, there's more volume behind it for the expanding gases to fill due to the larger bore diameter. The pressure will fall faster than it would if there were a smaller volume for the gases to fill.

Of course, not much is equal between the .308 and the .223, but the general principle applies.
 
Interesting John.

Your comment leads to the argument that the diameter of the bullet increases linearly while the surface of the bullet increases by the square also. Therefore the is relatively more drag as the bullet travels down the barrel requiring more power to overcome. That and the weight disparity.

Which, in the end, implies that smaller projectiles benefit more from longer barrels than bigger/longer/ heavier projectiles.
 
Yes, too bad the BBI folks started at 18 inches.

For a discussion of what is really going on with barrel length, study Expansion Ratio.
https://www.shootersforum.com/balli...-bore-capacity-expansion-ratio-math-help.html

Rifle magazine once reprinted a chart drawn up by Phil Sharpe. He took a 31" .30-06 barrel and chronographed several loads in it. Sawed it off an inch or two at a time and chronographed the same loads in the same barrel. This was a major project, chronographs were expensive instruments in his era.
Velocities were almost unpredictable, they varied up, down, or not at all for increments of length.
 
It's the bullet terminal ballistics most people forget about.
Mass (weight) X Velocity Squared (X2).

The smaller bullets .223 Rem started with used hyper velocity (multiples of the speed of sound) to inflict damage,
While larger, heavier (5.56mm NATO) used velocity to cause the lighter nose of the bullet to cause yaw during terminal ballistics to increase damage.

This yaw in a target also caused the bullet to break into two or more pieces, creating more than one wound channel...

If you are punching paper, no big deal.
If you are using it for hunting, a velocity decrease below about 2,800 FPS decrease effectiveness considerably.

Hyper velocity also induces a shock wave in the wound channel, again somewhere around 2,800 FPS that shock wave decreases by an order of magnitude.

I find that with even 'Hot' loads it's difficult to hit 3,000 FPS with a 16" barrel.
It's also much more difficult to tune an AR rifle's gas system to not 'Slam' with a 16" barrel.
Hitting 3,000 FPS or more isn't difficult with a 20" barrel, and even with 'Hot' loads the rifle can be tuned to give a reasonable feed & even bolt lock-up.
 
Expansion ratio is the main factor if the powder and bullet sectional density are the same and both rounds are loaded to the same peak pressure. Otherwise you are comparing apples to oranges. The expansion ratios for the .308 and .223 are not terribly different, but it takes a 77 grain bullet in the .223 to have the same sectional density as a 150 grain .308 bullet, so those are what you have to compare and do it with the same powder. If you do, you will find the velocities and velocity loss with barrel length between the two guns is about the same.
 
Which, in the end, implies that smaller projectiles benefit more from longer barrels than bigger/longer/ heavier projectiles.
Click to expand...

IT would seem so, when you are only looking at one, or two of the factors involved. There is a lot more to it, and only looking at a couple of the factors gives a false impression.

That's because as the bullet moves down the bore, there's more volume behind it for the expanding gases to fill due to the larger bore diameter.
Click to expand...

This is called the expansion ratio, and it is true. HOWEVER, while the bigger bore size means more volume over the smaller bore size, bigger bore sizes most frequently burn MORE POWDER, meaning more gas to fill that volume, so the net result is not an absolute given.

Rounding off the numbers for convenience, a .308 size round shoots a bullet 3 times heavier, and burns twice as much powder as a .223. IF you go to a larger .30 round (one of the magnums), you see three times the amount of powder burned (more or less) than the .223, and approximately equal velocities from "full length" barrels.

The velocity obtained, from a given barrel length is not a constant, it varies, considerably due to a large number of factors, SOME of which are the amount and TYPE of powder burned. "Identical" barrels, shooting "identical" ammunition can vary in velocity, quite a bit. Even 100fps difference is not unheard of. The comments about "fast" and "slow" barrels are correct. Minute individual differences between all factors and tolerance stacking result in this, quite often.

When you compare different rifle barrels, and different cartridges being fired through them, the factors multiply radically, and its NEVER an "apples to apples" comparison.

The smaller bullets .223 Rem started with used hyper velocity (multiples of the speed of sound) to inflict damage,
While larger, heavier (5.56mm NATO) used velocity to cause the lighter nose of the bullet to cause yaw during terminal ballistics to increase damage.
Click to expand...

How is the 5.56mm "larger, heavier" than the .223? Both can shoot the same bullets.

EVERYTHING that shoots a bullet that is longer than it is wide will yaw when it passes through flesh, ballistic gel, or any other target medium. Essentially, the back end of the bullet is heavier than the front, and when any resistance is met, the bullet will try to swap ends.

What differs is how "soon" in terms of distance through the target that this happens. The old classic military rounds in .308,.30-06, 8mm, .303BRit, .30 Russian (54R), and all the others, ALL TUMBLE!!!!!

However, these larger rounds don't have a reputation for that, because the yaw/tumble generally happens AFTER the bullets have passed through the thickness of an average person.

The shorter, lighter, smaller bullets such as the .224" FMJs require less distance of travel in resistance than the larger .30s, and so most often show their yaw/tumble within the thickness of a body.

This yaw in a target also caused the bullet to break into two or more pieces, creating more than one wound channel...
Click to expand...

Yes, BUT ONLY if the bullet is designed and made to break up, and speed matters. Reports from actual combat in the middle East are showing that the standard 5.56mm rounds are losing that performance at about 200yds and beyond. Fired from the normal rifle length barrels (M16, 20") at 200yds they have enough velocity for the bullet to (usually) break apart at the cannelure, creating two or more pieces and increasing the wound channel, and thereby the effectiveness of the bullet.

That same ammo, fired from the much shorter carbine barrels is losing enough velocity that, at 200yds (and beyond) it often does NOT break apart, and so is less effective.

This is NOT a flaw, nor a defect, it is the result of the design. Bullets are designed, and built to do something in a certain speed range. Change that speed, and bullets will do something else, and usually not do what they were intended to do. A bullet designed to break up /expand is built to do that within a narrow range of impact speed. Higher or lower, and the performance becomes erratic or non existent.

Shoot a HP too slow, it doesn't expand. Shoot one too fast (such as a JHP pistol bullet at rifle speeds) and it expands too much, losing most of its penetrating ability.

The loss of performance with those 5.56mm rounds is not due to a failure of the design, it is due to the military's choice to use that ammo in a gun with a shorter barrel length then the ammo was built to perform best in.

It is the military's failure to realize that would change the performance that creates the seeming "failure". Alternate is that they did recognize that performance (breaking apart etc) would change, and decided it didn't matter, or matter enough to do anything about....

In time, if they have not already, they will get ammo optimized for the short barrel carbines, but knowing the way the world works, not everyone who needs it will have it, when they need it...
 
Rifling twist difference will do that.
"...if someone has cut down barrels on .223 and .308 to chart a velocity change..." Wasn't those cartridges, but it was done about 30 years ago in one of the gun rags. Done by Ballistics by the Inch".com too. Mostly handgun stuff there , but there is some data using .223.
5.56mm NATO and .223 Rem are the same thing.
 
The yawing is about length and entry yaw and velocity. These determine the overturning force on the bullet. This is proportional to the square of the sine of the angle at which the bullet enters the medium and the square of the impact velocity and the density of the particular medium, all multiplied by a shape-dependent constant. Near the muzzle you have the combination of highest velocity and fairly high initial yaw angle, so its most likely to initiate tumbling then. The bullet's angular velocity (spin) does not slow down as quickly as its forward velocity, so the gyroscopic stability factor increases with range. That causes the yaw of repose also to increase its angle with range, and in Litz's example it has reached the magnitude of the initial yaw extremes at about 600 yards. But by then the bullet has lost velocity, so the overturning forces are still not up to what they were near the muzzle.

Hatcher has a good pair of photos of 150 grain 30-06 ball ammo shot into blocks of oak. IIRC, one was fired at 50 feet and yawed and turned sideways, penetrating the oak less than 12". The other was fired at something like 150 yards and penetrated straight in for over 30".
 
243win:
What I find most interesting about the link you posted was how the groups changed. They would get better or worse....and then better or worse again.

For instance: the vvn 26.4 grain load shot a 1/3" group with a 22" barrel. Then it got worse as the barrel got shorter. But then at a 15" barrel it was back to 1/3".
Click to expand...

Barrel harmonics. It got taken out of its sweet spot then found another one.

One guy said the magic number was 20 3/4, doesn't seem right, its the combo I think. Maybe a caliber thing but.....
 
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