16 vs 20 inch barrel 5.56 AR velocity

[silveroak]

I have some unexpected velocity measurement results that I hope someone can explain away.

I expected to see a significantly higher average in a 20 inch AR from a chronographed shot string, but ended up about the same.
The 20-inch is an Armalite rifle-length system, 1/9, the 16-inch is a mid-length PSA, 1/7, both are heavy/thick throughout barrels.

The 16-inch averaged to 2786 fps.
The 20-inch averaged to only 2790. It did see a couple of shots closer to or over 3000 fps and those were offset by some lower values.

Ammunition was same as this. I can be found at several sources:

IMI Ammo 5.56x45mm 69 Grain Razor Core (Sierra MatchKing Hollow Point)

FeaturesProvides improved accuracy and lethality over longer distancesOpen Tip Match projectile provides higher stopping power than the standard 5.56mm...

www.midwayusa.com

(IMI Razor Core - Sierra MatchKing HP 69 grain)

While at the advertised velocity (2740), I was hoping for quite a bit more from the 20.

My chronograph is just a basic Prochrono. I admit to shooting close to it, but the numbers made sense.

Appreciate any thoughts!

 

[1KPerDay]

Sounds like barrel to barrel variation combined with some really inconsistent ammo. If you’re getting speeds near 3000 that are offset by some below 2700 to average 2800 try some better ammo IMO. But it’s not uncommon to have one barrel from one maker shoot significantly faster/slower than a different barrel from a different maker, regardless of a 4” difference in length.

 

[silveroak]

Sounds like barrel to barrel variation combined with some really inconsistent ammo. If you’re getting speeds near 3000 that are offset by some below 2700 to average 2800 try some better ammo IMO. But it’s not uncommon to have one barrel from one maker shoot significantly faster/slower than a different barrel from a different maker, regardless of a 4” difference in length.

I see. Before measuring, I always took the additional 4 inches as guaranteed and clear-cut gain in 5.56. Maybe not so simple. May need to try a different cartridge to maximize the 20 inch.

 

[someguy2800]

Yeah some barrels are just faster than others so its not guaranteed that one 20" barrel will be faster than one 16" barrel. You may not ever find an ammo that is significantly faster in that specific barrel. And like 1KPerDay said, if you are getting over 200 fps in extreme spread in velocity, that ammo is junk.

 

[mavracer]

Plenty of variables that can make a particular barrel be faster or slower than "average".
My 12.5" 5.56 SBR chronographs faster than a 12.5" barrel should.

 

[someguy2800]

Plenty of variables that can make a particular barrel be faster or slower than "average".
My 12.5" 5.56 SBR chronographs faster than a 12.5" barrel should.

yeah, I have a 14" 300 blackout barrel that chrono's like 100 fps faster than most people report from their 16" barrels. I wish I knew what the magic was.

 

[N555]

If you want a 69gr that reliably does 2,900fps out of a 20 inch barrel you're going to have to make them your self. A max load of cfe223, w760, leverevolution possibly others will get you there.

 

[mavracer]

yeah, I have a 14" 300 blackout barrel that chrono's like 100 fps faster than most people report from their 16" barrels. I wish I knew what the magic was.

I think the magic is just a bunch of variables aligning.
I mean there are plenty of identifiable variables that theoretically make a difference, but in the words of Yogi Berra
"In theory, theory and practice are the same, in practice they ain't"

 

[silveroak]

If you want a 69gr that reliably does 2,900fps out of a 20 inch barrel you're going to have to make them your self. A max load of cfe223, w760, leverevolution possibly others will get you there.

That is very good information, thank you.
Prior to asking here, I've done some cursory searches and could not see any claims of over 2700-ish with 69-grain. I thought I am probably seeing 16-inch figures.
This may be the ticket for this 1/9 20-inch.

 

[N555]

That is very good information, thank you.
Prior to asking here, I've done some cursory searches and could not see any claims of over 2700-ish with 69-grain. I thought I am probably seeing 16-inch figures.
This may be the ticket for this 1/9 20-inch.

That sounds like a 223 load for a 223 chamber a least according to my old Hornady manual which has different sections for 223 and 5.56 guns.
The 5.56 loads are quite a bit hotter than the 223 loads.
If my full on 5.56 loads were shot in a 223 chambered gun I'm sure they would produce significant signs of over pressure.

 

[Mk-211]

I think the average is about 25 - 30 FPS difference per inch of barrel.

I've read it before, you can Google it and check.

 

[someguy2800]

223 Remington/ 5.56mm NATO barrel length and velocity: 26 inches to 6 inches

Have you ever wondered how much barrel length effects muzzle velocity on your rifle? Well I did, so I cut up rifles chambered in 223 Remington, 308 Winchester, 7 mm Remington Magnum and 300 Winche…

rifleshooter.com

this is a real world test where they cut the end off the barrel and shot a 5 shot average. Among the 4 different ammo types tested there was a little over 100 fps difference going from 20 to 16.5". As we've discussed there can easily be 100 fps difference from one barrel to another so there is no guarantee that this is actually realized when comparing 2 different barrels.

 

[WestKentucky]

There are variables that affect velocity of a projectile. Resistance (friction) inside the barrel is a big chunk of that but it’s kinda counterintuitive. It seems like a tighter barrel would produce crazy friction and slow a bullet down, but it actually seems to go a different direction altogether. Resistance inside the barrel is indicative of how tight the fit is of the bullet to the barrel. A looser fit will allow more pressure leakage whereas a tighter fit will be a better seal. More resistance also causes pressure to increase which typically results in a more complete and clean burn of powder so in that case pressure builds pressure. It’s kind of wonky to think about internal ballistics, because there are plenty of factors that seem to work opposite how they should at first glance.

 

[jmr40]

That's a little on the high side but isn't unusual. When you see numbers stating that a cartridge loses 25 fps for every one inch shorter barrel that is only relevant if you use the same barrel and cut it shorter. I have 2 rifles in 30-06. Both have 22" barrels, but the Winchester is right at 90 fps faster than the Remington with every load I've compared. My 20" barreled 308 is faster than any of my 22" 308's.

 

[silveroak]

223 Remington/ 5.56mm NATO barrel length and velocity: 26 inches to 6 inches

Have you ever wondered how much barrel length effects muzzle velocity on your rifle? Well I did, so I cut up rifles chambered in 223 Remington, 308 Winchester, 7 mm Remington Magnum and 300 Winche…

rifleshooter.com

View attachment 1184322

this is a real world test where they cut the end off the barrel and shot a 5 shot average. Among the 4 different ammo types tested there was a little over 100 fps difference going from 20 to 16.5". As we've discussed there can easily be 100 fps difference from one barrel to another so there is no guarantee that this is actually realized when comparing 2 different barrels.

Yes, this and other replies add to the idea that my 16 inch may be just a faster barrel. Thank you and everyone.

 

[ms6852]

While at the advertised velocity (2740), I was hoping for quite a bit more from the 20.

Do you know the barrel length the manufacturer used to reach the stated velocity? It could well have been a 20" barrel too as Hornady tests their ammo with 20" barrels. Also at the velocity stated of 2740 with a 69 gr Sierra bullet you are close to maximum load and with maximum loads , depending on powder used you may be able to reach 2900 fps. Sierra also uses a 20" barrel to test their rounds.

The fact that you stated that a couple of times you chronographed velocities of 3000 leads me to believe that the ammo is not good ammo and might be even dangerous , just to many inconsistencies and the standard deviation seems to be all over the place.

If you reload you will find that all rifles will shoot more accurately at certain velocities. I have found that in my rifles the best groups each rifle shoots is always a mid-range velocity, never the higher ones. If you are looking to get 3000 fps or more from your AR, you are going to have to shoot the lighter bullets in the 60 grain and under using a slower twist rate.

 

[silveroak]

Do you know the barrel length the manufacturer used to reach the stated velocity? It could well have been a 20" barrel too as Hornady tests their ammo with 20" barrels. Also at the velocity stated of 2740 with a 69 gr Sierra bullet you are close to maximum load and with maximum loads , depending on powder used you may be able to reach 2900 fps. Sierra also uses a 20" barrel to test their rounds.

The fact that you stated that a couple of times you chronographed velocities of 3000 leads me to believe that the ammo is not good ammo and might be even dangerous , just to many inconsistencies and the standard deviation seems to be all over the place.

If you reload you will find that all rifles will shoot more accurately at certain velocities. I have found that in my rifles the best groups each rifle shoots is always a mid-range velocity, never the higher ones. If you are looking to get 3000 fps or more from your AR, you are going to have to shoot the lighter bullets in the 60 grain and under using a slower twist rate.

No, and is that ever made public without writing to the manufacturer? I've only repeatedly read that manufacturers choose lengths that maximize the velocity of a load, it could be 24 for all I know. How is that normally investigated?
Yes, the 2600s to 3000s range is unnerving. At the same time, the 16, within just a few days between measurements, was much more consistent. Ammo from same 300-box.

 

[mavracer]

No, and is that ever made public without writing to the manufacturer? I've only repeatedly read that manufacturers choose lengths that maximize the velocity of a load, it could be 24 for all I know. How is that normally investigated?
Yes, the 2600s to 3000s range is unnerving. At the same time, the 16, within just a few days between measurements, was much more consistent. Ammo from same 300-box.

From midway

 

[someguy2800]

I've only repeatedly read that manufacturers choose lengths that maximize the velocity of a load, it could be 24 for all I know.

For most small bore centerfire calibers that would be 30+ inches. A lot of ammo's will have it written on the box or manufacturers website. 24 is most common for small bore high velocity cartridges. Some rounds like 7.62x39 or 300 blackout will sometimes have 16" test lengths or it might even have a 10" length and 16" length. There is no standard, its just whatever the manufacturer feels like.

I think you are way too concerned with what the chronograph says rather than how it actually shoots. The bullet has already slowed down by 100 fps by the time the bullet is 40 yards from the muzzle. So at best going from a 16 to 20 inch barrel buys you 40 more yards of effective range. A few fps of muzzle velocity is just really not that big of a thing to care about.

 

[someguy2800]

Assuming they both shoot as well as each other, your 16” 1:7 twist is actually the more capable long range platform of the two because you can shoot 77 grain in it with better ballistic coefficient.

 

[bfoosh006]

How far away was the chrono from the muzzle of both barrel lengths ?

And FWIW, I didn't get a chance to measure the IMI 69gr, but there is a ton of factory velocities to ponder.
https://www.ar15.com/forums/AR-15/V...s-16-Middie-and-20-Rifle--223-5-56/16-676005/

And , this link shows how "odd" barrels can be...
https://www.leverguns.com/articles/ballisticians.htm

 

[ms6852]

Yes, the 2600s to 3000s range is unnerving. At the same time, the 16, within just a few days between measurements, was much more consistent. Ammo from same 300-box.

The fact that measurements were taken days apart is another factor that adds to the disparities and differences in velocities. Depending on the temperature that day, depending on the type of powder used will effect pressures causing the spikes and drops in velocities .

I did not intend to scare you by stating that it could be dangerous to shoot that ammo, I made that statement before you posted that the ammo was shot several days apart, personally I would feel safe shooting it but I would never buy it as it is not consistent with the velocities as you have stated, which means that depending on the distances you are shooting or hunting you could miss your target or wound an animal because the bullet could be hitting low or high.

 

[Triggernosis]

Maybe the faster twist of the longer barrel slows the bullet down a bit? I'll bet if you compared the same twist rate in different length barrels you would see more of a difference.

 

[someguy2800]

Maybe the faster twist of the longer barrel slows the bullet down a bit? I'll bet if you compared the same twist rate in different length barrels you would see more of a difference.

the longer barrel has a slower twist

 

[Triggernosis]

the longer barrel has a slower twist

Gotcha. Bad reading comprehension skills today, I guess.