.223 AR Velocities - Longer Barrel, lower Velocity?

[markm87]

A buddy got a Garmin Chrono and we took it out yesterday. We were trying a .223 load with 69 gr RMR bullets using N135, Wolf SRMs, and the same lot of Win Brass.

We tried two ARs, a 14.5 Minnies Machine Govt contract overrun, and a Colt 6920 16.0. Both carbine Gas systems. The Colt has maybe 200 rounds, and the SOCOM has at least 1000-1500 rounds.

I patched both to feel for obvious anomalies, and nothing felt odd. Both have jumps from 2.250 of about .135 as best I can tell with that crappy hornady tool.

Velocities were as follows:

14.5 BCM SOCOM 2677 fps

16.0 Colt 6920 2638 fps (39 fps slower!)

If they shot close to the same I'd be surprised, but the shorter one shot faster? Any ideas?

 

[markm87]

The evil weapons in question.

 

[1raggedhole]

That’s very close, not surprising

 

[whatsupdoc]

The internal barrel/chamber dimension differences between the two is what you are experiencing.
Normal.

 

[karrarararararar]

Some barrels are faster than others

 

[Herzo]

You might check the water capacity of a fired case from each of the rifles. One grain of water difference can make a noticeable difference in the pressure realized.

 

[markm87]

I'm guessing that there's a slight bore diameter difference here. No doubt that each barrel is its own animal and some shoot faster, but I've never had a shorter barrel shoot faster.

 

[David Milisock]

A buddy got a Garmin Chrono and we took it out yesterday. We were trying a .223 load with 69 gr RMR bullets using N135, Wolf SRMs, and the same lot of Win Brass.

We tried two ARs, a 14.5 Minnies Machine Govt contract overrun, and a Colt 6920 16.0. Both carbine Gas systems. The Colt has maybe 200 rounds, and the SOCOM has at least 1000-1500 rounds.

I patched both to feel for obvious anomalies, and nothing felt odd. Both have jumps from 2.250 of about .135 as best I can tell with that crappy hornady tool.

Velocities were as follows:

14.5 BCM SOCOM 2677 fps

16.0 Colt 6920 2638 fps (39 fps slower!)

If they shot close to the same I'd be surprised, but the shorter one shot faster? Any ideas?

Are they both chambered for the same cartridge, 223, 223 Wylde or 5.56 x 45?

 

[762willdo]

Chrome lined barrel on the M4…….chrome is slippery.

 

[VA_XTC_Shooter]

The barrels are chambered differently, at least according to the stamping on the barrels.

 

[BillC79]

Velocities were as follows:
14.5 BCM SOCOM 2677 fps
16.0 Colt 6920 2638 fps (39 fps slower!)

Based on how many replicates shots per rifle?

 

[markm87]

Are they both chambered for the same cartridge, 223, 223 Wylde or 5.56 x 45?

They're both 5.56 despite the 223 designation on the Euro 6920. Apparently they can't have a military caliber in civilian hands in Europe. So, as I understand it, Colt marks these 223.

And wouldn't the shorter leade on a 223 chamber yield higher pressure and thus more velocity?

I'm assuming the 6920 is also Chrome lined. I'll have to check later today.

 

[David Milisock]

They're both 5.56 despite the 223 designation on the Euro 6920. Apparently they can't have a military caliber in civilian hands in Europe. So, as I understand it, Colt marks these 223.

And wouldn't the shorter leade on a 223 chamber yield higher pressure and thus more velocity?

I'm assuming the 6920 is also Chrome lined. I'll have to check later today.

Barrel length is just one of a multitude of factors in velocity. You may even find that a different lot of ammunition provides different results.

One would think that the shorter lead of a 223 chamber (if it was truly cut that way) would provide more pressure but the barrel may have properties that negate that.

The only things that matter is that you know how the rifle reacts. It's producing a bit slower velocities, so what? Is it accurate and dependable? Those mean a hell of a lot more than a few feet per second.

 

[markm87]

The only things that matter is that you know how the rifle reacts. It's producing a bit slower velocities, so what? Is it accurate and dependable? Those mean a hell of a lot more than a few feet per second.

I agree. I'd just never seen anything like this. We normally chrono EVERY .223 round on the same 20" bolt gun for comparability. But with the Garmin, we have the freedom to capture everything all the time if we want.

 

[fkimble@charter.net]

Only 39 fps difference between 2 different barrels. Two identical barrels can have that much difference. Your two have lots of differences, chambers, throats, wear, lengths, materials. Shoot'em, don't worry about 'em!

Frank

 

[markm87]

Chrome lined barrel on the M4…….chrome is slippery.

Confirmed the Euro Colt is Chrome lined. So both are Chrome lined.

 

[Ned Ludd]

What were the ES and SD values for each of the two velocity sets for the different rifles?

I wouldn't be too concerned with the velocity of the shorter barrel being slightly higher than the longer one. I'd guess it could be some subtle difference the bore/groove configuration of the two barrels, or possibly between the gas systems. However, there are so many posible variables that could go into the difference in velocity that trying to chase down what might cause the difference is likely to be an exercise in futility.

 

[David Milisock]

I agree. I'd just never seen anything like this. We normally chrono EVERY .223 round on the same 20" bolt gun for comparability. But with the Garmin, we have the freedom to capture everything all the time if we want.

Give it time you'll see plenty of stuff that makes no sense.

 

[markm87]

Give it time you'll see plenty of stuff that makes no sense.

We're going to take all 6 of my 14.5 AR barrels and shoot the same ammo through them to see what kind of variances there are.

This pain free Garmin Chrono is going to put me in the looney house!

 

[David Milisock]

We're going to take all 6 of my 14.5 AR barrels and shoot the same ammo through them to see what kind of variances there are.

This pain free Garmin Chrono is going to put me in the looney house!

I tell you this stuff will make you crazy.