Service Rifle Barrel Length

Beaver

Beaver

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At our last Service Rifle match, we had an interesting debate about the preferred choice of barrel for Service Rifle.

I'll share my findings here...

I mistakenly thought that increasing my barrel length from 16.75" to 20" would increase accuracy -- accuracy was not increased.

1. Elevation chart for a 16.75" and 20" barrel were identical (chronograph showed only a 200 ft./sec increase in muzzle velocity when the 20" barrel was used and the bullet remains supersonic beyond 500 yards in both cases).

2. The factory action spring had to be replaced with an extra power action spring when the 20" barrel was used, to increase action reliability. This also increased the amount of recoil when a shot was taken.

3. The 20" barrel had noticeable more weight on the front end of the rifle.

4. The optimum 5.56 barrel in terms of accuracy is said to be 16" as shorter barrels flex less for the same barrel thickness (the impact of barrel heating on accuracy). In other words, a 24" barrel would flex more than a 16" barrel, but a barrel short than 14" would likely not have enough velocity to stabilize the bullet as much as it would in 16" or more (assuming 1:9 twist rate to stabilize 55 grain up to 69+ grain bullets). Once the bullet has been stabilized in the barrel, it cannot be made more stabilized ;)

So why have a 20" barrel on a C7 then? Because the 5.56 ammo relies on fragmentation and the 20" barrel will give bullet terminal fragmentation out to about 200 yards, whereas the 16" barrel will have a bullet terminal fragmentation distance about 50 yards less than that...

M193 with 20" = approx. 200 yards
M193 with 16" = approx. 150 yards

M855 with 20" = approx. 150 yards
M855 with 16" = approx. 90 yards

Soft point and hollow point ammunition will likely have further terminal impact fragmentation distances.

See also
http://www.angelfire.com/art/enchanter/terminal.html
 
Just speaking to the cometitive aspect,

Dwell time.
Most 16 inchers are carbine length with a few being Middys. The difference in shooting the two side by each is very noticeable. The mid length is smoother and more reliable. With a brake attached, the mid-length is more manageable over the still sharp carbine.
The rifle length is a pussy cat compared to even the mid-length system, and still more manageable.
Add to that 100 to 150 extra fps that may matter in wind.
The whole whip issue (IMHO) makes sense if we are discussing an H- bar, Varmint, or Palma style contour to the barrel. If that is the case, that extra four inches won't matter much in weight.
 
1. Ran the 69gr SMK through the Sierra ballistics program at 2600 and 2800 (200fps difference). Intuitively, it does not make sense that a bullet travelling 200 fps slower would have the same elevations as a faster one.
According to Sierra, it doesn't.
With a 100m zero, in minutes, 200 comeups were 1.8 and 2.3; 300m, 5.0 and 6.0; 400m, 8.9 and 10.7; 500m, 13.8 and 16.5; 600m, 19.7 and 23.6, respectively.
4mph wind at 90 degrees produces 2.7 and 3.0 minutes of wind.
Obviously, the increased velocity is benficial.
2. There may be other things going on; gas port size, etc. In my experience, a 20" barrel with a good brake will result in less recoil, less flip than a 14.5" or 16" barrel with a comparable brake.
3. That is more a matter of barrel contour than barrel length.
4. A short heavy barrel is stiffer than a longer barrel of the same weight. In practice, particularly with an auto rifle, there are a lot of other factors.
As you point out, once a bullet is stabilized, it is stabilized. A 14" barrel will stabilize any bullet if the rifling twist is appropriate.
The 20" C7 barrel was selected to promote fragmentation at longer ranges? I wouldn't know. Seems that you do.
There are many factors affecting accuracy. Barrel quality will be more of a factor than barrel length.
 
Beaver said:
Soft point and hollow point ammunition will likely have further terminal impact fragmentation distances.

See also
http://www.angelfire.com/art/enchanter/terminal.html
Click to expand...

Ballistics have 3 categories:
1) internal
2)external
3)terminal ( what the bullet does to a target)

Please explain "further terminal impact fragmentation distances"
Are you talking about the bullet seperating? the jacket from the core?
The "impact" would be considered part of the "terminal" ballistics... I dont get what you are saying.
 
When Armalite put a 20" barrel on the AR back in the 50's or 60's, no one knew JACK ALL about fragmentation. The effect was a fluke.

I do not know how they picked a 20" - but most likely it was based on a study of incremental increase of velocity over barrel length. If you look at M14, 20" seems to be quite short.

BTW: Brakes are for cheaters!
 
tiriaq said:
1. Ran the 69gr SMK through the Sierra ballistics program at 2600 and 2800 (200fps difference). Intuitively, it does not make sense that a bullet travelling 200 fps slower would have the same elevations as a faster one.
According to Sierra, it doesn't.
Click to expand...

Intuitively, I agree with you.

However, having actually shot a 16.75" barrelled upper and a 20" barrelled upper, my identical elevations for the 69gr Sierra are:

100 yards = 0.0 MOA
200 yards = 0.5 MOA
300 yards = 5.0 MOA
400 yards = 9.0 MOA
500 yards = 13.0 MOA

20" 1x9 CMV H-Bar barrel with 69 gr Sierra & 24 gr H335 = 2805 ft/sec
16.75" 1x9 CMV barrel with 69 gr Sierra & 24 gr H335 = 2688 ft/sec

Actual difference 129 ft/sec (difference was closer to 200 ft/sec when a 55 grain bullet with 23.5 gr WC735 was used and I don't use 55 grain bullets for distances greater than 100 yards).

Note: Scope elevations are in 1/2 MOA increaments ;) So there would need to be at least a 1/2 MOA difference before an adjustment is needed. The 69 gr Sierra with 24 gr H335 had muzzle velocities between 2779 and 2842 ft/sec when shot through the 20" barrel and velocities between 2644 and 2714 ft/sec when shot through the 16.75" barrel.

No elevations changes were made for SR 3 (16.76" barrel) & SR 5 (20" barrel), but I fell approximately 50 points short of an acceptable 700 point score, so I am by no means an expert :(

My goal is to achieve 700+ next year ;)

tiriaq said:
Barrel quality will be more of a factor than barrel length.
Click to expand...

Yes, I agree with that statement, however, all things equal in the quality of the barrel, the 16" would appear to perform better once terminal fragmentation is removed from consideration (300 to 500 yards away from the target).
 
Duplicate your testing, firing carefully from rest at the different distances and observe the location of the groups. It may be that the scoring ring sizes on the target you were using were allowing a bit of leeway; a Fig. 11 target, for example, has scoring zones longer than they are wide. As far as the points you lost are concerned, were they lost because of zeroing, wind or the shooter? At which stages? Given a decently accurate rifle, and worthwhile sight, in any service conditions type shooting, the shooter is the main ingredient in success. If a shooter is superior, then an optimized rifle is going to affect the outcome.
 
^ something is off already. There is no way you only need 1/2 MOA come up from 100 to 200. At least 1.5 MOA for a slower bullet like 69SMK.

Anyways, don't waste time and ammo to figure that 2" at 500m. Your rifle is not sub MOA, and you are not a sub MOA shooter either. In practicality it does not mean anything at all. Looks like the main effort is to catch those kneeling shots.

BTW- your load is wimp. We used to shoot 25.5gr H335 at 2850fps out of 16".
 
greentips said:
^ something is off already. There is no way you only need 1/2 MOA come up from 100 to 200. At least 1.5 MOA for a slower bullet like 69SMK.
Click to expand...

Yes, that proplexed me too :confused:

If I used more than 1 MOA, it was too high (the ballistic calculator reads 1.3 MOA, with the scope 3" above the barrel) -- we have 5 sighters at the 200 yard mound.

Maybe hanging out with the Target Rifle shooters can help, so focus can be placed completely on the accuracy of the shot (no snaps, movers, etc. to confuse what is happening with the grouping on the target).

greentips said:
Looks like the main effort is to catch those kneeling shots.
Click to expand...

Absolutely!

greentips said:
BTW- your load is wimp. We used to shoot 25.5gr H335 at 2850fps out of 16".
Click to expand...

Okay, I'll revist the load too.
 
hoochie said:
Ballistics have 3 categories:
1) internal
2)external
3)terminal ( what the bullet does to a target)

Please explain "further terminal impact fragmentation distances"
Are you talking about the bullet seperating? the jacket from the core?
The "impact" would be considered part of the "terminal" ballistics... I dont get what you are saying.
Click to expand...

Hi Hoochie...

M193 & M855 is full metal jacket ammo which needs a certain amount of energy to fragment upon impact.

Soft point and hollow point ammo is design to fragment with less energy upon impact.

They could have made the NATO 5.56 round easier to fragment at further distances, but...

"During the 1970s, NATO members signed an agreement to select a second, smaller caliber cartridge to replace the 7.62mm NATO cartridge. Of the cartridges tendered, the 5.56x45mm was successful, but not the 55gr M193 round used by the U.S. at that time. The wounds produced by the M193 round were so devastating that many[5] consider it to be inhumane[6][7]. Instead, the Belgian 62gr SS109 round was chosen for standardization. The SS109 used a heavier bullet with a steel core and had a lower muzzle velocity for better long-range performance, specifically to meet a requirement that the bullet be able to penetrate through one side of a steel helmet at 600 meters. This requirement made the SS109 (M855) round less capable of fragmentation than the M193 and was considered more humane[8]"
Click to expand...

http://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO

NOTE: Impact fragmentation has nothing to do with target shooting, but just came up with associated barrel length info.
 
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Bolivar said:
I'm not familiar with the concept of transitional ballistics. Can you give a coles notes description? Thanks.
Click to expand...

some quick CF military definitions:

External Ballistics: “External ballistics is the study of the motion of the projectile inflight after it leaves the bore”

Intermediate Ballistics: “Intermediate ballistics is the study of the transition from internal to external ballistics, which occurs in the vicinity of the muzzle

Internal Ballistics: “Internal ballistics is the study of motion of a projectile within the bore of the weapon and the conditions that affect this motion from the moment that burning of the propellant is initiated”

Terminal Ballistics: Terminal ballistics is the study of the effects of the projectile when it strikes a medium other than air”

I would just add that wound ballistics is slightly different than terminal ballistics though many use terminal to describe both.
 
Transitional ballistics is why target rifles tend to have an 11 degree crown, and why it's a bad thing to scrape inside your flash suppressor with a Gerber to get the crown nice and shiny looking for inspection...

If you damage the crown, it will affect the bullet's transition from internal to external ballistic effects, and that is generally not a good thing.

NS
 
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