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Thread: Hypothetical: What Still Needs Fixing?

  1. #31
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    Quote Originally Posted by MistWolf View Post
    I do acknowledge that fact. That's why I said "I don't know what the practical limits are"



    What about a WSM case?
    That's not a .308 case. The WSM has about 30% higher case capacity.



    Quote Originally Posted by MistWolf View Post
    Do you have empirical data showing that increasing the leade of the chamber will not allow enough of a velocity increase for a 75 gr bullet loaded to an OAL that allows feeding from a magazine to achieve at least 3100 fps?
    You can increase the free bore all the way to the muzzle and get about 5000 fps, but will the accuracy hold up? If people regularly do it, it is possible, if no one bothers to do it, there are probably reasons.

    The 5.56/.223 case is at, or nearly at, the limits of its performance at this pressure level. If there are requirements for increased performance, you'll need a different case or a different bolt design to handle the higher pressure.

  2. #32
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    It seems the brass case is the limiting factor on pressure.

    The saami max pressure found in most common cartridges is 62k or 65k.

    5.56 is already there.

    The bolt design primarily affects fatigue life and the standard design usually holds up to one time kBs.

    Quote Originally Posted by lysander View Post

    The 5.56/.223 case is at, or nearly at, the limits of its performance at this pressure level. If there are requirements for increased performance, you'll need a different case or a different bolt design to handle the higher pressure.
    Last edited by Clint; 04-30-15 at 19:05.
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  3. #33
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    Quote Originally Posted by lysander View Post
    That's not a .308 case. The WSM has about 30% higher case capacity.
    I didn't ask you about a 308 case. I asked you about a WSM case

    You can increase the free bore all the way to the muzzle and get about 5000 fps, but will the accuracy hold up? If people regularly do it, it is possible, if no one bothers to do it, there are probably reasons.
    One engineering problem at a time. Get the velocity first, solve for accuracy second.

    Yes there are reasons why no one bothers to do it, but the actually are using longer leades to increase velocity

    The 5.56/.223 case is at, or nearly at, the limits of its performance at this pressure level. If there are requirements for increased performance, you'll need a different case or a different bolt design to handle the higher pressure.
    So, you do not have data
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

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  4. #34
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    Quote Originally Posted by MistWolf View Post
    I'd like to see four particular improvements-
    - Easier way to lock the bolt back without a mag
    - The charging handle moved to a more convenient location
    - A lightweight adjustable buttstock with a cheek piece like that used on the UBR
    - The death of metal handguards
    Weird, because some if not all of those issues have been remedied with good aftermarket solutions or new lower/upper designs.

    And why the death of metal handguards?? Plastic melts you know...


    Quote Originally Posted by SilverCat View Post
    Aside from the gas port arguement...

    If the question is just asking about the AR platform in general, I like to believe that there are enough aftermarket accessories to change anything about it the end user would like. Bolt catch redesign? BAD lever. Charging handle location? Side charger uppers (I'm honestly not sure if they make non reciprocating side chargers, but I don't doubt it.)

    If we are talking about the M4 TDP, that's a whole other discussion.
    Agreed. I think most of the top-tier rifles from BCM, DD, LMT, etc are great designs; no need to really modify them from that. If you want a piston system, those are available too.

    If we are talking about limits of the 5.56 cartridge, is 6.8 or 6.5 not good enough?? I'm no ballistics expert, but those 2 calibers make a good alternative in an AR15 platform don't they?

  5. #35
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    Quote Originally Posted by MistWolf View Post
    I didn't ask you about a 308 case. I asked you about a WSM case
    Very pretty.

    But like I said, you cannot get Lapua performance out of a .308



    Quote Originally Posted by MistWolf View Post
    One engineering problem at a time. Get the velocity first, solve for accuracy second.

    So, you do not have data
    Do you have data that even indicates that a 15% increase in velocity can be achieved by just changing the freebore length? There is a lot of data out there that shows there is a measurable loss of accuracy by increasing the freebore.

    Although, some people think that too much free bore will start to decrease the velocity, similar to an eroded throat...

    https://thefiringline.com/forums/sho...d.php?t=503402

    http://forum.accurateshooter.com/ind...opic=3771999.0

    Most people are of the opinion that increasing the freebore gives better velocity not because to the drag-free jump, but the allowing the bullet to be seated further out giving more usable case volume.

    (EDIT Since we have a COL limited by the magazine, we cannot use this particular advantage.)
    Last edited by lysander; 04-30-15 at 22:52. Reason: spelling

  6. #36
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    Quote Originally Posted by JusticeM4 View Post
    If we are talking about limits of the 5.56 cartridge, is 6.8 or 6.5 not good enough?? I'm no ballistics expert, but those 2 calibers make a good alternative in an AR15 platform don't they?
    I'd be interested in how people would feel about the lethality of the AR platform if 6.5 was the same price, as widely available, and marketed by the likes of BCM, Colt, Noveske, etc.

    I'm not asking for magic, but what if 25 rounds of 6.5 were the standard, and at the price of 5.56. Would it "move the platform forward", or would any advantage be lost by the average user?
    Last edited by opngrnd; 04-30-15 at 23:14.

  7. #37
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    Quote Originally Posted by lysander View Post
    Very pretty.

    But like I said, you cannot get Lapua performance out of a .308




    Do you have data that even indicates that a 15% increase in velocity can be achieved by just changing the freebore length? There is a lot of data out there that shows there is a measurable loss of accuracy by increasing the freebore.

    Although, some people think that too much free bore will start to decrease the velocity, similar to an eroded throat...

    https://thefiringline.com/forums/sho...d.php?t=503402

    http://forum.accurateshooter.com/ind...opic=3771999.0

    Most people are of the opinion that increasing the freebore gives better velocity not because to the drag-free jump, but the allowing the bullet to be seated further out giving more usable case volume.

    (EDIT Since we have a COL limited by the magazine, we cannot use this particular advantage.)
    I don't have any hard data it can be done but there is evidence. I betcha dollars to donuts somebody has tried it and I'd like to see what their set up is and what the results were.

    Too much free bore will result in lower velocities if it allows too much gas to leak past the bullet and loss of accuracy is a concern. However, the cannon of the M1 Abrams is a smooth bore- basically it's all free bore- in order to increase velocity. I don't know all the tricks they use to maintain accuracy.

    A long leade allows a bullet to be seated out further to increase case capacity but that is an increase of what I would call the static capacity. The 5.56 chamber uses a longer leade to up the velocity without using a longer OAL and without upping the pressure. The extra jump increases case volume before peak pressure is reached. In other words, it acts as if a larger case is being used. I think of this as the dynamic case capacity (as I don't know what the actual term is). This is also the reason why 223 ammo produces lower pressure in 5.56 chamber than it does in a SAAMI spec 223 chamber (and why 5.56 ammo can produce dangerous pressures in a SAAMI spec 223 chamber). To get the velocity gain, more powder or a different powder must be used.

    I think it's possible to increase the leade further to allow bullets of about 75-77 gr to achieve another 200 fps from a 20 inch barrel. If I had the resources to do so, I'd try it to see what happens
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

  8. #38
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    Quote Originally Posted by MistWolf View Post
    ... However, the cannon of the M1 Abrams is a smooth bore- basically it's all free bore- in order to increase velocity. I don't know all the tricks they use to maintain accuracy....
    A silicon rubber base seal to prevent leakage passed the sabot, PTFE ultra-low friction bore riders, carbon fiber sabot and a very low drag, high ballistic coefficient fin stabilized projectile. They also uses a muzzle reference system to the measure barrel droop and/or thermally induced bending, laser range finding that also measures if you are shooting uphill or down hill, wind sensor, the ammunition temperature is monitored, all of this data is fed to a ballistic computer which calculates the correct super-elevation and azimuth correction. The gunner and commander have 10X magnification in their sights. The ammunition also has some of the tightest specifications in terms of muzzle velocity around.

    All that together gives the total system about 2.0 MOA error.

    But, most of their super high muzzle velocity (>5,500 fps) is achieved by working at a chamber pressure of 83,000 to 96,000 psi and at the cost of a barrel life of 1500 full charge equivalent rounds. Even the "low pressure" training rounds have chamber pressures in the 70,000 psi range and has a muzzle velocity of 5600 fps with a 6-8 lb launch weight (projectile and sabot).

    The rifled 105mm M68 tank gun gets 5000 fps at 75,000 psi with a 16 lb launch weight, so the lack of rifling isn't getting you much.

    (EDIT: For comparison, the velocity difference between a 30-06 loaded with a 110 grain bullet and a 30-06 loaded with a 220 grain bullets is around 700 to 1000 fps with the same nominal chamber pressure.)
    Last edited by lysander; 05-01-15 at 08:48.

  9. #39
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    It should be noted that the reason tank guns use smooth bores is not because smooth bores give generally higher muzzle velocities, but because projectile terminal ballistics.

    In order the penetrate great thicknesses of armor, you want a long, thin, high density projectile, in order to spin stabilize a long, thin, high density projectile, you need a really fast twist. A fast twist puts high shear stress on the engraved rifling of the projectile, so high in the case of big guns, that it will fail the material of the rotating band. This could be solved by using gain twist rifling, that way during the initial high loading, the twist is very slow. But, it is cheaper to forgo the rifling and just stick fins on the projectile.

    Also, HEAT rounds do not like to be spun. It causes the flame jet to de-focus and reduces effectivity. The M456 HEAT round for the rifled 105mm M68 gun had teflon slip ring type obturating bands, so the rifling would not impart any spin to the projectile, but still seal in the rifling. And, the Shillelagh gun-launcher actually have a keyway broached down the length of the barrel to prevent the Shillelagh missile from rotating.
    Last edited by lysander; 05-01-15 at 09:10.

  10. #40
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