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Velocity, barrel length and the AR15

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  • #54122
    Profile photo of Anonymous
    Inactive

    If you have a 20 inch AR15 that is more accurate than a 16 inch with all rounds tried (55grain, 77 OTM and M885), but weighs one pound more (7.3 pounds versus 6.3 pounds; is the extra velocity and accuracy worth the extra weight assuming we are looking at 600 yards and under for realistic combat?

    The USMC was using 20 inch as standard the last time I checked; while the Army had moved to the M4 at 14.5 inches.

    Theoretically, the M855 travels about 2900 fps with a 20″ while only 2750 from 16″ and about 2700 from 14.5″

    #54123
    Profile photo of Mike QMike Q
    Participant

    Can you see out to 600 yards in your AO? Is your AO all open fields towards that range? Is the scope on your rifle good enough to see clearly out to 600 yards? If so what is the added weight for that scope to your rifle versus a light weight red dot or battle sight? Finally can you make that 600 yard shot on a moving target with your blood pumping and your chest heaving?

    All questions you have to answer. As 1st Sergeant and Max always say. “ it depends”

    There never seems to be enough time to do it right, but there is always enough time to do it twice.

    CRM Sept. 2014, CTT 1505, CTT July 2015, RC-Rifleman 1502, CP Nov. 2015, FoF March 2016, CCW May 2016, FoF Oct. 2016, FoF Nov. 2016, CLC April 2017, FoF Nov. 2017, Alumni weekend Aug. 2018, CQB Dec. 2018

    #54124
    Profile photo of Mike QMike Q
    Participant

    I’ve shot my 16” barrel with 62 gr rounds out to 600 yards but I was on a bench with all the time in the world… in real life I think 300 to 350 would be really pushing it. But that’s just me using an ACOG… lots of guys are better shots than me.

    There never seems to be enough time to do it right, but there is always enough time to do it twice.

    CRM Sept. 2014, CTT 1505, CTT July 2015, RC-Rifleman 1502, CP Nov. 2015, FoF March 2016, CCW May 2016, FoF Oct. 2016, FoF Nov. 2016, CLC April 2017, FoF Nov. 2017, Alumni weekend Aug. 2018, CQB Dec. 2018

    #54125
    Profile photo of Anonymous
    Inactive

    Valid questions. My AO is Utah where 600 yards is CQB:)

    The 600 yards is a number that Max was using as the greatest distance combat operations with rifles usually takes place; with the most events taking place in less that 300. He also mentioned that even with ranges of 600 yards there are micro terrain that can be taken advantage of. However; long distances are the norm here. (he talks about this in my 308 or 556 for Western Area thread)

    But the gist of my question was more related to velocity. The 5.56 is more effective with greater velocity. So, the question is really, is it worth a pound and 4 inches of extra barrel length to get better accuracy (with my rifles) and velocity.

    utah

    #54126
    Profile photo of Max VelocityMax
    Keymaster

    Hmmmm. There is a balance here that the standard 16″ AR barrel actually sits on pretty well. I would not be concerned by that minimal weight of a 20″ barrel. Weight is more a funtion of ammo carried etc, where it gets significant.

    With the 5.56 round it is definitely more effective on impact with higher velocity. There are tables that show the range, I forget the term, that the 5.56 fails to be as lethal, and that drops off pretty fast with SBR. So you shoot them again and again LOL!

    If you are concerned with being accurate, and hitting power, out to longer ranges, there is nothing wrong with 20″. It is hardly excessive. But it depends on if you are talking alone or with a team. If with a team, I urge the MSG concept with a small team equipped with weapons capable of SUSTAINED accurate suppressive fire at longer ranges. Replacing belt fed volume with greater accuracy. These still need to be battle rifles for all circumstances.

    If you want a good compromise between CQB and range, go for the 16″. I see no real need for SBR, in all honesty.

    But I suspect many of these questions come from the search for the perfect answer for lone operators, which is dangerous ground.

    As I mentioned before, just because you can see for miles, does not mean you need to try and fight it that range. If you have the skill for long range, then you could take advantage of standoff ranges. The other factor here is that many mong rnage shooters are kidding themselves in terms of combat application at these extreme ranges. And what about target discrimination?

    #54127
    Profile photo of Max VelocityMax
    Keymaster

    Also, your photo. Pretty picture where you can see for miles, right? Lack of ground covering vegetation is often a tactical mistake made. All I see in that photo is terrain features, folds in the ground and covered approaches. Infantryman’s eye for the ground.

    #54130
    Profile photo of Anonymous
    Inactive

    Thanks. I am mentally moving from a ‘308 solves all problems’ to ‘the 5.56 is the better all around choice’. Luckily I have a few to choose from so I am just picking the one to make my number #1

    A bit more cover here.

    Utah 2

    #54139
    Profile photo of TCTC
    Participant

    How are you planning on ranging and hitting your targets past 400 yards when wind and bullet drop start coming into play? If you have a BDC reticle then barrel length may factor into what muzzle velocity for a particular round works best with the reticle.

    For example, the TA31ACSS ACOG has ranging and BDC built in; from the Strelok ballistics app, the BDC in that ACOG is optimized for M855 leaving the muzzle at 3000fps, which by most data I’m finding online is typical of a 16″ barrel. So if I had that optic I would choose 16″ over 20″. I have the Primary Arms 4-14x for use with 77gr OTM and those two work great together on my 16″, for what it’s worth. The ranging and BDC on both make for fast shots at distance including wind holds and running speed lead holds.

    Otherwise a mil/moa scope will obviously be agnostic with respect to load or caliber but be way slower having to do ballistics calcs on the fly. Or I guess worst case can just walk up the rounds but I’ve never tried that at 400-600 yards.

    SE Florida ☆ CQBC 2017 ☆ CTT/DA 2017 ☆ CLC 2018

    #54144
    Profile photo of Anonymous
    Inactive

    Used the M16A2 for years with the 20″ Bbl. No trouble carrying it. Maneuverability is fine. Remember that our Grandfathers cleared houses in Italy, France and Germany with M-1 Garands…

    #54153
    Profile photo of Hello Kitty (Craig)hellokitty
    Participant

    Actually barrel length has nothing to do with accuracy. It is merely an effect on velocity. Which gives you more distance. I have a 10.5 inch SBR that is extremely accurate but obviously suffers at distance due to drop off of velocity.

    Its a fine point that you probably know but some get confused on this.

    CTT 1502, NODF 1502, CP 1503, RC 002- Rifleman, FoF x 2, Run and Gun, RS/CTT, CLC, CQBC, Heat 1

    Craig S.

    #54156
    Profile photo of Virgil KaneVirgil Kane
    Participant

    Used the M16A2 for years with the 20″ Bbl. No trouble carrying it. Maneuverability is fine. Remember that our Grandfathers cleared houses in Italy, France and Germany with M-1 Garands…

    A 20″ AR is still a carbine.

    Masters II
    Georgia
    June 2017 Intro CQB, FOF-Team Deplorables
    October 2017 MVT South - CTT/DA
    March 2018 DCH

    #54163
    Profile photo of Support Sideffhounddog
    Participant

    I’ve been thinking of getting a 12.5 SBR to go with my 3 10.5s and my 8 inch SBR. I Have set up a few 16 inch carbines andnI still have one A2 and a MK12 sitting there. I shoot suppressed a lot now. I’ve cleared houses with 14.5, 11.5, and 20 inchers. I like The 11.5 for house work but I’m issued I was just a M9 in my current job, Battalion commander, so in my current command I kept 20 M16a2s for all the pistol toters. Said t was for ceremony…

    Support Side. Not flattering but better than weak side.

    #54165
    Profile photo of Anonymous
    Inactive

    Used the M16A2 for years with the 20″ Bbl. No trouble carrying it. Maneuverability is fine. Remember that our Grandfathers cleared houses in Italy, France and Germany with M-1 Garands…

    A 20″ AR is still a carbine.

    No, it’s a rifle, using a rifle-length gas system, and a rifle buffer tube with a rifle buffer.

    #54216
    Profile photo of BrigandActualBrigandActual
    Participant

    IMO, the most practical use of the extra velocity from a 20″ vs shorter barrels is not range. Most of us are not employing ARs at ranges where that difference is important. The real benefit is the flatter trajectory.

    All other things being equal, which they usually aren’t, you an get a better BZO from longer barrels.

    These graphics present a notional idea of what I’m talking about. Pick a range of choice for a zero, and compare how the impacts fall for each boost in velocity.




    I don’t have one for 20″, but I think you get the point. Higher velocity helps keep everything a little tighter. That said, the difference between 16″ and 20″ is not much in a practical sense here.

    Keep in mind, these examples are for impact only. They say nothing of terminal effect. But, as Max said, shoot them more often.

    This chart is an illustration of initial velocity as a barrel was cut down one inch at a time. The red line indicates the point where M855 will no longer reliably produce maximum terminal effect (basically…you’re poking .22 in things).

    What does this mean in a practical sense? Starting faster means the bullet stays more lethal for longer. If I plug the velocity numbers from the graphics above into a ballistic calculator and use the 2500 fps number for M855’s terminal performance envelope, I end up up with the following ‘Maximum Terminal Range’ estimates

    10.5″ SBR: 25 yards
    14.5″ M4: 105 yards
    16″ standard civvy: 130 yards
    20″ gov’t: 200 yards

    "Man is still the first weapon of war" - Field Marshal Montgomery

    Matt B - VA

    #56717
    Profile photo of HitsCountHitsCount
    Participant

    Some calculations I ran through a ballistics calc awhile ago are below. All numbers are for M193. Frag velocity is calculated at 2700 FPS. 600 ft-lbs is around what a pistol delivers. If you run MK262 you can likely add a bit to the below ranges (at least 50 yards).

    Barrel Length: 20″
    Fragmentation Range (2700 FPS): 150 yards
    600 Ft Lbs Range (2250 FPS): 275 yards
    Trans-sonic Range (1116 FPS): 725 yards

    Barrel Length: 18″
    Fragmentation Range (2700 FPS): 125 yards
    600 Ft Lbs Range (2250 FPS): 250+ yards
    Trans-sonic Range (1116 FPS): 700+ yards

    Barrel Length: 16″
    Fragmentation Range (2700 FPS): 100 yards
    600 Ft Lbs Range (2250 FPS): 225+ yards
    Trans-sonic Range (1116 FPS): 675+ yards

    Barrel Length: 14.5″
    Fragmentation Range (2700 FPS): 90 yards
    600 Ft Lbs Range (2250 FPS): 225 yards
    Trans-sonic Range (1116 FPS): 650+ yards

    Barrel Length: 10.5″
    Fragmentation Range (2700 FPS): 10 yards (45 yards)*
    600 Ft Lbs Range (2250 FPS): 150 yards (180 yards)*
    Trans-sonic Range (1116 FPS): 570+ yards (625 yards)*

    If you want some sort of “designated marksman” type AR I personally prefer the 18″ over the 20″. The 2″ shorter makes a noticeable difference in handling. That said 16″ or even 14.5″ with a pin & weld FH works just fine for most foreseeable applications. I’m not a fan of SBRs under 11.5″ or 12″ in length — as you can see the 10.5″ SBR pretty much mandates MK262 to get useful effects at any reasonable distance. Going from 10.5″ to 12″ or 14.5″ makes a big difference for velocity.

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