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I guess we're all pretty much aware that up to a point, a longer rifle barrel generally produces higher velocities. To my surprise, I just discovered, "Tain't necessarily so!"
We know that the velocities spec'd on factory ammo is based on a SAAMI specified 24" barrel. That usually means we aren't going to see those box velocities from our 16" ARs.
Recently I decided to go with an 18" barrel just to get a bit more velocity (and energy) from an AR. I chose a Daniel Defense S2W 18" barrel with a 1:7 twist. Imagine how surprised I was when two BCM 16" barrels shot 5% higher velocities with 5 different brands of ammo than the longer DD 18"!
Here's a pic of the three rifles, not that it means much, but...the 18" is in the middle - - - and I had to make the pic in low light so it's grainy:

Here is the chrono results. I emboldened the fastest for each brand of bullet. Notice in every case the 16" barrels were faster for all brands of bullets than the 18":
............................Dan Def 18"....BCM 16" SS410....BCM 16" LWE
HPR 55 gr VMAX........2658..................2759.................2935
IMI 77 gr..................2654..................2624..................2728
IMI 69 gr..................2817..................2895..................2863
Aus Outback 69 gr......2458.................2590..................2671
Aus Outback 55 gr......2764.................2786..................2894
The devil is in the details:
- All are mid length gas systems
- All three had adjustable gas blocks that were tuned to the ammo being shot
- The DD 18" has a salt bath nitride internal with a 1:7 twist, DI
- The BCM SS410 16" is ion bonded with a 1:8 twist with Adams Arms piston drive
- The BCM 16" Light Weight Enhanced is chrome lined with a 1:7 twist with Adams Arms piston drive
At first, I thought the difference was in the twist rate, but then I realized the BCM LWE 16" with the tighter 1:7 was faster than both the DD with 1:7 and the BCM SS410 with the 1:8???
So, what did DD have to say about this?
"The differences you are seeing are in relation to the gas port hole size and the length of the barrel. The dwell time on your 18" Mid-Length S2W and the dwell time on the BCM 16" will be different due to the difference in length and also the differences in the manufacturing process of each barrel. The gas port hole diameter being the biggest contributing factor.
Our 18" 5.56 Mid-Length S2W barrel has a gas port hole diameter of 0.07. I am unsure as to what exact dimensions the BCM 16" barrel is made to.
Another factor in this will be the gas block used and ensuring both gas blocks are properly installed. If the gas isn't cycling as it should, you would see a difference in velocity (over pressure or under pressure). "
I presume there is some factual basis for their claims, but starting with the last first, the gas blocks, all three had adjustable gas blocks and were adjusted to the ammo. Not buying the dwell time claim at all. Gas port hole diameter could play a part I suppose.
And I suspect manufacturing tolerances and barrel treatments, i.e. chrome lined etc. could make a difference too.
But the lesson here is, just because you get a longer barrel doesn't necessarily mean you'll gain any velocity. In fact, my results show the 18" barrel is about 4-5% slower than the two 16" barrels.
So the question is, why would I want to tote around 2 extra inches of barrel to get less velocity???
We know that the velocities spec'd on factory ammo is based on a SAAMI specified 24" barrel. That usually means we aren't going to see those box velocities from our 16" ARs.
Recently I decided to go with an 18" barrel just to get a bit more velocity (and energy) from an AR. I chose a Daniel Defense S2W 18" barrel with a 1:7 twist. Imagine how surprised I was when two BCM 16" barrels shot 5% higher velocities with 5 different brands of ammo than the longer DD 18"!
Here's a pic of the three rifles, not that it means much, but...the 18" is in the middle - - - and I had to make the pic in low light so it's grainy:

Here is the chrono results. I emboldened the fastest for each brand of bullet. Notice in every case the 16" barrels were faster for all brands of bullets than the 18":
............................Dan Def 18"....BCM 16" SS410....BCM 16" LWE
HPR 55 gr VMAX........2658..................2759.................2935
IMI 77 gr..................2654..................2624..................2728
IMI 69 gr..................2817..................2895..................2863
Aus Outback 69 gr......2458.................2590..................2671
Aus Outback 55 gr......2764.................2786..................2894
The devil is in the details:
- All are mid length gas systems
- All three had adjustable gas blocks that were tuned to the ammo being shot
- The DD 18" has a salt bath nitride internal with a 1:7 twist, DI
- The BCM SS410 16" is ion bonded with a 1:8 twist with Adams Arms piston drive
- The BCM 16" Light Weight Enhanced is chrome lined with a 1:7 twist with Adams Arms piston drive
At first, I thought the difference was in the twist rate, but then I realized the BCM LWE 16" with the tighter 1:7 was faster than both the DD with 1:7 and the BCM SS410 with the 1:8???
So, what did DD have to say about this?
"The differences you are seeing are in relation to the gas port hole size and the length of the barrel. The dwell time on your 18" Mid-Length S2W and the dwell time on the BCM 16" will be different due to the difference in length and also the differences in the manufacturing process of each barrel. The gas port hole diameter being the biggest contributing factor.
Our 18" 5.56 Mid-Length S2W barrel has a gas port hole diameter of 0.07. I am unsure as to what exact dimensions the BCM 16" barrel is made to.
Another factor in this will be the gas block used and ensuring both gas blocks are properly installed. If the gas isn't cycling as it should, you would see a difference in velocity (over pressure or under pressure). "
I presume there is some factual basis for their claims, but starting with the last first, the gas blocks, all three had adjustable gas blocks and were adjusted to the ammo. Not buying the dwell time claim at all. Gas port hole diameter could play a part I suppose.
And I suspect manufacturing tolerances and barrel treatments, i.e. chrome lined etc. could make a difference too.
But the lesson here is, just because you get a longer barrel doesn't necessarily mean you'll gain any velocity. In fact, my results show the 18" barrel is about 4-5% slower than the two 16" barrels.
So the question is, why would I want to tote around 2 extra inches of barrel to get less velocity???