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7.5in AR15 SBR max effective range?

Discussion in 'NFA Firearms and Accessories' started by Lindy7443, Oct 29, 2012.

  1. Lindy7443

    Lindy7443 Active Member

    I cant seem to find a straight answer...

    Im building up a 7.5 in SBR in 5.56/223...I simply love the look.

    In terms of shooting a standard 55FMJ that are right at about 2750fps...

    What can I expect in terms of hitting the target at 50-150 meters?

    What is the max effective range of this round and is there any power left for some knock-down?

    Ive got to believe at 50-100 meters, it has plenty to get the job done...

    Experiences? Chronographs? Target pics of groups/proof?

    Thank you, Dave
  2. klyph

    klyph Well-Known Member

    Knock down power is a myth, especially in 5.56, there is no knock down, only penetration. You say you are building it for the look, so look at it, it won't perform very well in relation to a full length rifle. You're using a very small caliber and stripping it of it's only saving grace: speed. Enjoy the way it looks, and shoot it at night so you'll be able to enjoy all that extra powder the bullet isn't using.

    If you want an AR in sbr form that performs well, consider other chamberings.
  3. p2000sk

    p2000sk Well-Known Member

    Accuracy over a distance can vary depending on not only the quality of barrel, but also the throat between chamber and rifling. A more expensive barrel might be worth the extra money, however plenty of folks have been satisfied with barrels on the lower side of the price spectrum.

    These links I am providing are not about accuracy with a short barrel. These links are about chamber throats being important to accuracy. My point being that a more expensive barrel should be more accurate, regardless of length.


  4. MachIVshooter

    MachIVshooter Well-Known Member

    Under 14.5", ballistics suffer enough that the guns become CQ or entry weapons. I have a 7.5" pistol that is being SBR'd, and an 11" upper on the way for it. I would consider the 7.5" effective to 100 yards, 200 for the 11". 5.56 isn't a good performer once velocity drops below ~2,400-2,500 FPS, especially with FMJ rounds.

    If you need to shoot further, I'd suggest 16"-20" of tube.
  5. Girodin

    Girodin Well-Known Member

    As stated above, "knock down" is not really a quantifiable measure of terminal ballistics. In terms of accuracy you can take a quality 7.5" gun out to 100 yards easily. Accuracy is not an issue. The issue is the reduced muzzle velocity. This is going to be much more of an issue in terms of how it affects terminal ballistics of certain rounds. Many 5.56 rounds depend on fragmentation. The muzzle velocity of an XM193 round out of a 7.5" gun tends to be around 2400 FPS. You can run your muzzle velocity and other load data in a ballistic calculator to see what velocity you have at a given distance. The low velocity of the mentioned load means it is already below the velocity at which it reliably fragments well. Most FMJ rounds are unlikley to exhibit great terminal ballistics from a 7.5" gun. They likely will not fragment. They more likely will penetrate and perhaps yaw. That said you are not stuck with using those loads. There are other loads that may offer better terminal ballistics with the lower velocity.

    If terminal ballistics was my chief concern I would probably opt for a different cartridge. Can a 7.5" gun be effective out to 100 yards? While it will not be as effective as longer barreled guns, I believe it can still be viable to that distance with the right loads. However, if terminal ballistics is something that actually matters, and not just a thought exercise, I would give some thought to whether 3-4" is worth it vis-a-vis the better performance of a 10.5 or even 11.5" gun. And again if terminal ballistics is a big factor I would consider a 6.8 or the like even with the longer barrel.
  6. jmorris

    jmorris Well-Known Member

    I went as short as 10.5" on one of my .223 uppers but have only shot it out to 300 yes. If you really want one even shorter use a pistol caliber or a round that uses pistol powders like 458 socom. The smaller charges of faster powders won't take the same speed hit as you make the barrel shorter.
  7. JustinJ

    JustinJ Well-Known Member

    Check out the link below:


    Out of a 7.5" barrel a M855 will be going only about 2,200 FPS. I'm not sure what that would equate to at 100' but not much. 5.56 weapons for real world use, even when intended for CQB, have at least 10.5" of barrel.

    Outside of living room distances a 7.5" barrel will give you little more than a wound channel similar to that from an ice pick.
  8. Sam Cade

    Sam Cade Member

    That would depend on the bullet.

    You don't have to shoot surplus M855. There are many better choices,especially out of shorties.

    My HD gun is a 12.5" and it is currently loaded with 64gr JSP.
  9. JustinJ

    JustinJ Well-Known Member

    I realize that but was using it as an example. However, even expanding bullets need sufficient velocity to expand and are still more effective at higher velocities. 12.5" is also significantly longer than 7.5". Regardless, barrel length is always a trade off for velocity, range and performance and i believe 7.5" gives up way too much for way too little gain. Not to mention flash and noise.
  10. KC&97TA

    KC&97TA Well-Known Member

    The Wifes 7" JSE does 50m clover leafs 4 rounds shooting WWB 5.56 so accuracy hasn't suffered in the shoulder of a good shooter, how ever at 100m we've seen many holes that look like the bullet tumbled through.

    Biggest issue with a 7" SBR 5.56, At night it produces a 15 foot fire ball, it's quite intersting to see the difference between a 20", 16" and the 7" in the ammount of unburned powder leaveing the barrel... it totally screws up your night vision in very low light.
  11. Gordon

    Gordon Well-Known Member

    I have a Noveske Diplomat pistol with that barrel length. I chronographed a few loads out of it and the 45 grain Winchester Varmint load was 2880 fps FWIW. Any way since the Aimpoint T-1 micro has a 4 MOA dot that covers 6" at 150 yards that would be about it where that load hits over 2000 fps and would likely mushroom and penetrate a ways, certainly by 200 yards the sight it self limits it to a com general body hit which is not adequate for me. So I would say the diplomat 7.5 is GTG to 150 yards using the tube as a shoulder reference . :) From 15 yards out tho it beats any pistol IMHO.
  12. C-grunt

    C-grunt Well-Known Member

    Out of that short of a barrel you need to run a soft or hollow point bullet to get good terminal ballistics. You just lose way to much velocity.

    The shortest I would go with a 5.56 is 10.3 inches. That length barrel still produces enough velocity to get fragmentation at some range with FMJs. My old squad leader has a confirmed kill at 400 meters with his Mk18 (10.3 inch) shooting the 77grn M262.

    I believe M262 will fragment down to about 2200 FPS and Mk318 with show some sort up fragmenting down to around 1900 FPS.
  13. rjrivero

    rjrivero Well-Known Member

    Last edited: Nov 1, 2012
  14. wally

    wally Well-Known Member

    I've got a 6.5" CMMG .22lr AR pistol that I've made into an SBR. At 10 yards shooting steel plates it blows away any .22lr pistol in terms of speed and accuracy

    Proves the old saying "you handgun is so you can fight your way back to your rifle!"

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