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7.5" Barrel AR-15s

Discussion in 'Rifle Country' started by Olympus, Aug 30, 2015.

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  1. Olympus

    Olympus Member

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    Why do these barrels seem to be so popular? Everything I read about them is that they are similar to a 22 mag in ballistics, everyone says they'd never own one, and they are glorified noise maker and range toy.

    If they hold no practical purpose, why do they seem to prevalent?
     
  2. MachIVshooter

    MachIVshooter Member

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    I don't know where you read that, but it's horse hockey. 22 mag can barely muster 1,700 FPS with a 50 grain bullet in a 22" rifle, while 5.56mm will spit a 55 gr. pill out of a 7.5" tube at about 2,400 FPS. That's more than double the kinetic energy and, more importantly, .22 WMR is far enough below the ~2,000 FPS threshhold that it behaves as a pistol bullet, while even the short barrel velocity of the 5.56 is squarely in rifle wound territory.
     
    Last edited: Aug 30, 2015
  3. Sunray

    Sunray Member

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    Guys pretending they have an SMG.
    "...they are glorified noise maker and range toy..." Don't see anything wrong with a guy having expensive, noisy, toys myself. A guy's toys don't have to be practical. Not really different than a heavy barreled, scoped, .338 Lapua or .50 BMG bolt action or the really daft 12" and 14" shotguns.
    They're all the rage because the assorted marketing departments are doing their job so well. Callin' 'em 'Tactical' sells.
     
  4. Zerodefect

    Zerodefect Member

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    Plenty of entry teams use them. But the 7.5" that I'm building now, will be .300blk, not 5.56. So it should have plenty of punch, and less noise. Will be using a 9" keymod rail and a flash can. .300blk has really revitalized the whole pistol/sbr thing.

    Similar to dis:
    75-1_zps300989b4.jpg
     
  5. Redlg155

    Redlg155 Member

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    A lot of folks would not have a second thought except for the popularity of the Sig Brace and other assorted braces. They have them for everything from ARS to .308 pistol FALs.
     
  6. silicosys4

    silicosys4 Member

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    From what I can find, a 7" barrel will put a 55 grain pill out at under 2200 fps.
    At that velocity, you'd better have boutique bullets to get any kind of expansion. At 2200 fps you won't have any fragmentation from any of the common fmj loadings.
    What rifle bullets offer expansion down to 2200 fps, without blowing up at higher velocities?

    "square in rifle wound territory"...Not really, imo. At 2200 fps you are praying you have enough velocity to give enough expansion that you aren't just poking icepick sized holes through something.
    Rifles playing in that territory tend to rely on big, heavy bullets with wide meplats for penetration, not expanding bullets.

    Even people who hunt deer size game with a .223 admit they have a range limitation
    If a .223 at 55gr. leaves the muzzle at 3200fps, it arrives at 275 yards at just over 2200fps.

    I have never yet, after reading multiple threads on the subject, seen it suggested by even proponents of using the .223 for hunting deer sized game, that 275 yards is within the reasonable range of the cartridge for deer.

    For a cartridge that NEEDS expansion or fragmentation to be effective, that's a pretty poor place to start....at the very bottom edge of its ability to perform.
     
    Last edited: Aug 30, 2015
  7. henschman

    henschman Member

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    I see that length mostly on rifles chambered in .300 Blackout.
     
  8. Olympus

    Olympus Member

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    Aside from a range toy, does a 7.5" 5.56 have any practical value?

    Don't have the time to reload 300blk in the same quantities that I can by bulk .223.
     
  9. blue32

    blue32 Member

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    If I had the money I'd get one. A 7" carbine in my front passenger seat gear bag beats trying to unholster a pistol while wearing a seatbelt any day.
     
  10. silicosys4

    silicosys4 Member

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    How does that make sense? If you can't unholster your sidearm while belted in, find a different holster.
    Because you aren't going to CCW an AR,
    You now have to leave your AR in the car every time you leave your vehicle.
    Why wouldn't you just find a holster that works for CCW AND while you are driving?

    Besides,
    A pistol in an accessible spot beats an unwieldy carbine inside a bag on your seat any day.
    A 7.5" barrel doesn't turn an AR, buffer tube, 30rd mag and all, into a gun that you can just pick up and flick around the inside of a vehicle.
     
    Last edited: Aug 30, 2015
  11. Hookeye

    Hookeye Member

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    From reg rifle length bbl.....55gr varmint bullets upset on chucks to 300.

    7.5" is short........but also allows for a can, or a longer flash hider if wanted.
     
    Last edited: Aug 31, 2015
  12. Hookeye

    Hookeye Member

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    16" AR rifle is still a clunker inside of a vehicle due to OAL (I use one sometimes to yote hunt).
    Shorter would be better for some other applications I reckon.
    But I find a 16" AR to be noisy enough when hunting.
     
  13. MachIVshooter

    MachIVshooter Member

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    You don't really know what I'm talking about when I mention the 2,000 FPS threshold, do you?

    There's a really big difference in the wounding characteristics of a 1,700 FPS projectile and a 2,400 FPS projectile. And yes, I have chronographed M193 out of my 7.5" SBR with an average velocity of 2,380. M855 is closer to your figure.

    Lastly, we citizens are not a military force voluntarily complying with Hague convention; why on God's green earth would I rely on ball ammo when I can run my 50 gr. V-max hand loads that clock 2,625 FPS and most definitely open up at that speed? I'm not trying to engage insurgents at 300 meters; my 7.5 is an HD weapon.
     
  14. Hookeye

    Hookeye Member

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    2K I thought an understood threshold for hydrostatic shock.
    Barely get that from 14" TC in 7mmTCU running 120's (at 100 yds) so why I never got that bbl. (handgun of deer).
     
  15. rcmodel

    rcmodel Member in memoriam

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    Shoot a 7.5" .223 inside a car once and get back to me on that!!

    rc
     
  16. Hookeye

    Hookeye Member

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    Any .223 shot inside of a car is gonna suck.
    Hell they suck on an indoor range.
    I thought the idea was that the shorter offerings........might help with smoother exit of vehicle.
     
  17. blue32

    blue32 Member

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    A Glock 22 with a weapon light under a seatbelt next to console and all other gear around can be done. I can do it. All I'm saying is that when you're deep in thought on a report and some goober comes walking up at zero dark 30, or better yet, somebody drives right up to me, it'd be nice to have an sbr ready to go. Switching pistol holster isn't an option for me, neither is different mounting options for the 16" I already have in the car.
     
  18. silicosys4

    silicosys4 Member

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    Sure I do, hydrostatic shock and all that
    It's not an end all be all, and just doesn't come into play here.
    The .223 or 5.56 fmj's are not effective at speeds under the velocity needed to fragment, which is about 2700+ fps. Repeating that term doesn't change physics, millions of dollars worth of govt. number crunching and field studies, or make a death ray out of an otherwise underperforming round.


    Is there? If a .223 bullet fails to expand or fragment at 2400fps, is it going to be a significantly different wound pattern than an identical bullet that doesn't expand or fragment at 1700fps?
    I don't think you yourself understand what you are referring to.
    The "2000fps threshold" isn't the point when animals get blown inside out from the sheer velocity of a projectile.
    Its when a projectile designed to expand or fragment at rifle velocities starts behaving differently upon impact, by expansion or fragmentation. If the projectile fails to expand or fragment at 2400fps, the "2000fps threshold" won't make it more effective than one going 1700fps.

    From the book
    Wound ballistics; basics and application...chapter 4.3, page 213.
    https://books.google.com/books?id=q...#v=onepage&q=rifle velocity threshold&f=false

    "There is currently no evidence for the existence of a velocity or energy threshold for conventional designs of handgun or rifle bullets above which different wound ballistics phenomenon would occur.
    However, there is a velocity threshold at which the behavior of a bullet changes. Full metal jacketed bullets remained shape stable on impacting water or gelatin at velocities up to 500 to 600m/s. At higher velocities they begin to deform or fragment."

    Ok, so you got an M193 to go 320fps UNDER the effective velocity of the round.

    I go by what Ballistics by the Inch gives me in real world numbers.
    http://www.ballisticsbytheinch.com/223rifle.html


    50 grain Vmax handloads?

    Sounds pretty boutique to me
    Regardless, they are completely irrelevant to the discussion as they cannot be obtained by anyone other than you.
    "50 grain vmax handloads (that only I have)" is pretty vague and unverifiable.
    On a lighter note however,
    A Vmax might just be the ticket though, at that lower speed it might retain enough weight to get through into vitals.
    I know after seeing what it did to a coyote at closer to 3100fps, I wouldn't trust my life to it stopping a large, enraged bad guy. Basically made a really nasty flesh wound and left me with one wounded 40lb coyote.

    Glad you like it, I'm sure it will serve you well in that regard, as long as you don't need to hear or see after the fact.
    Do I feel the need to pick a 7.5" AR15 for home defense over other weapons I have? No.
     
    Last edited: Aug 31, 2015
  19. jon86

    jon86 Member

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    I believe you asked me a question, but I couldn't hear you. Can you repeat that? :D
     
  20. husbandofaromanian

    husbandofaromanian Member

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    Most suppressor manufacturers would void your warranty for using a barrel so short.
     
  21. alsaqr

    alsaqr Member

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    Exactly!!!

    i've killed dozens of wild hogs using the M193 5.56mm round. Also killed some hogs with the M855 round: Both bullets perform in the same manner. At distances to 150-175 yards, when fired from a 20" or longer barrel, the bullet penetrates 4-6", yaws and fragments. The spleen and liver of hogs shot in the heart lung area are often lacerated by bullet fragments. The lungs are destroyed.

    At ranges in excess of 150-175 yards the magic is gone. The bullet will not fragment and the hog may suffer and die a lingering death.
     
  22. dogmush

    dogmush Member

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    Mk 262 fragments much slower. You might get frag velocity out of a 7.5" barrel out to 50-60 yds or so. Not sure if that counts as a "boutique" bullet since it's .mil issue, albeit not standard.

    It's what I use in my 10.5" 5.56. It's not all THAT loud either.

    Mk318 also fragments quite nicely from short barrels. I haven't seen any in the wild, civi side, but it might be out there. One could duplicate the load if you could get the projectile.
     
  23. jmorris

    jmorris Member

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    That is not what drives many buying decisions.
     
  24. IndianaBoy

    IndianaBoy Member

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    10.3 or 10.5 is about as short as I would be willing to go, and I would prefer 11.5.


    My intended use would be as a suppressor host. I've seen plenty of pictures online but I have never seen a 7.5 AR at any range I have been at.
     
  25. Olympus

    Olympus Member

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    Are we saying that a Hornady Vmax is a "boutique" bullet? I'm confused as I've never heard that term before.

    I'm only interested in 7.5" ballistics in .223.
     
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