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Thread: Rounds based on the 5.56x45 cartridge

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    Supporting Member AlaskanSD's Avatar
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    Default Rounds based on the 5.56x45 cartridge

    You can probably see where this is going. I'm wanting to have an AR upper with performance close to a 6.8 SPC that I can use normal in standard PMags.

    I'm not 100% sold on the .300 Blackout as I think that's a little too heavy of a bullet to be pushing with the volume of a 556 brass.

    There HAS to be a round in between the 223 and the 300 based on the NATO cartridge diameter! Right?

    I'm not worried about ammo availability as I would just reload. But of course I would need bullets...

    I wish they would have used a standard diameter case when they made the 6.8 or 6.5.

  2. #2

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    There are hundreds of rounds based on the 222,222mag, 223 case. The entire line of Thompson Center Ugalde cartridges (from 6mm-308, standard dies are available), the 6 x45 is currently produced w/dies available, the Blackout is actually based on a 221 Fireball case. Consider a 6.5 mm based on the 223 case (6.5x223), they are currently available so is reloading data.
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    Supporting Member AlaskanSD's Avatar
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    Ooh - I like the sound of that 6.5mm!! I guess the second issue is to find a barrel with that chamber!

    Thanks Brav!

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    Here's some info on the 6.5 TCU a standardized round on the 223 case. Just somethin I found !... The 6.5 PCC might work as well.





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    Member Scorpion8's Avatar
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    Remington also put out the .30AR.

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    Based on the 223/5.56x45.......Well there is more to it than that. As has been pointed out the 300 Whisper/300 AAC Blackout is based on the 221 Fireball, which is a shortened version of the 223 case. That case holds less powder than the 223 but the shorter case was chosen because with the .308" 150 + grain bullet (remember sub-sonic) gives an overall length equal to the 223/5.56 loading and will fit in the standard AR/M4/M-16 magazine. The 6.8 SPC also fits in the AR magazine, but is slightly fatter and the mag will take fewer rounds. The 6.5 Grendel also fits and is well established. Both these use cases of a diameter greater than the 5.56 but they do fit the magazine of the AR platform. The 6 x 45, 6.5 x 45 and 7mm of the TCU family though very good rounds derived form necking up the 223 case, they likely cannot be fit into the AR magazine due to bullet length and seating depth limitations. So......... when you say based on the 5.56 cartridge, there needs to be further study and considerations because even though its the same case it may not fid the magazine. And don't sell the 300 AAC out too quickly it is capable of duplicating the ballistics of the most used cartridge in the world, the 7.62x39 used in the AKM/SKS family of weapons. You statement about the 300 AAC is not valid as is proven by the ballistics of the round. The factory ammo in the 110- 130 grain range are very effective stoppers, and the case is smaller in capacity than the 223/5.56 case.
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    Two issues with the TCU, as Murhpy mentions the longer bullets will preclude the round fitting in the mag, the other issue is the TCU is an improved .223 case, i.e. the case taper is reduced and you may have feeding problems.

    So given the constraints of the AR-15 when you're creating a wildcat to function with a COL of ~2.25" and you increase bullet dia, you have to decrease the case length. That's why the 300 whisper/blackout uses the 221 f-ball case. While you could make a 6.5mm based on the 221, when you drop bullet dia you loose velocity as there is less surface area on the bullet for the force of the expanding powder to push on. Hence whatever the 300 bo can do with a 120 gr bullet, a 6.5 would give up 100-200 fps. You could drop to 100 gr bullets but not alot of selections.


    The real key to maximizing the AR-15 isn't necking up the .223, it's using a fast twist barrel and heavy bullets. While the fast twist is needed to stabalize the long bullets, it also aids bullets in expanding when they hit the target due to the forces involved. Just run a good 75 gr bullet and place it where it belongs.
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    Quote Originally Posted by Paul H View Post

    The real key to maximizing the AR-15 isn't necking up the .223, it's using a fast twist barrel and heavy bullets. While the fast twist is needed to stabalize the long bullets, it also aids bullets in expanding when they hit the target due to the forces involved. Just run a good 75 gr bullet and place it where it belongs.

    After Pizzin a lot of money.......that is the direction I am now going. Question will a 1 in 8 stabalize the 77 gr. bullet.

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    Quote Originally Posted by AGL4now View Post
    After Pizzin a lot of money.......that is the direction I am now going. Question will a 1 in 8 stabalize the 77 gr. bullet.

    I have an Armalite target/varminter barrel with Wildey chamber and 24" 1 in 8 twist barrel.. I shoot Mk 262 (Black Hills) and shot a 4.45" group two weeks ago at the 800 yard line. NO it won't!
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    Quote Originally Posted by Murphy View Post
    I have an Armalite target/varminter barrel with Wildey chamber and 24" 1 in 8 twist barrel.. I shoot Mk 262 (Black Hills) and shot a 4.45" group two weeks ago at the 800 yard line. NO it won't!

    Truly AWESOME.................What is Mk 262 ?

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    Moderator Paul H's Avatar
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    Quote Originally Posted by AGL4now View Post
    After Pizzin a lot of money.......that is the direction I am now going. Question will a 1 in 8 stabalize the 77 gr. bullet.
    Barrel length 16", 20" or 24" and hence what's your muzzle velocity? From the research I've done, 1:7 will, 1:9 won't and 1:8 might stabalize a 77. Only one way to find out!
    Those that are successful in Alaska are those who are flexible, and allow the reality of life in Alaska to shape their dreams, vs. trying to force their dreams on the reality of Alaska.

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    Quote Originally Posted by Paul H View Post
    Barrel length 16", 20" or 24" and hence what's your muzzle velocity? From the research I've done, 1:7 will, 1:9 won't and 1:8 might stabalize a 77. Only one way to find out!

    I have a NOVESKE with a 1:7 that is 16" and I have a Rock River Arms upper on order that is 1:8 with 18" Barrel. Question: Does the number of full revolutions in the barrel effect stability.....Would a 1:7 barrel 14" long be as stable as a 1:7 barrel 28" long....???

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    Quote Originally Posted by AGL4now View Post
    Truly AWESOME.................What is Mk 262 ?
    The latest and greatest ammo for our Warfighters. It uses a 77 grain HPBT. (Labeled OPE) with very slight cannelured. The bullet is available without cann.
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    Before we go down this stabilization trail again. Answer these questions.
    What does a bullet need to be stabile?

    what are the indicators of an unstable bullet?

    why do some guns stabilize a long bullet and others with same twist rate not?
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    Quote Originally Posted by AGL4now View Post
    I have a NOVESKE with a 1:7 that is 16" and I have a Rock River Arms upper on order that is 1:8 with 18" Barrel. Question: Does the number of full revolutions in the barrel effect stability.....Would a 1:7 barrel 14" long be as stable as a 1:7 barrel 28" long....???
    Bullet stability is based on how many rpm the bullet is spinning. Longer bullets have to be spun faster to be stable. How fast the barrel spins is based on the twist rate of the barrel and the muzzle velocity. For a given twist rate, in this case 1:7 the longer barrel will be able to spin the bullet faster because the longer barrel produces a higher velocity. It has nothing to do with the barrel length perse, just that longer barrels allow the burning powder to accelerate the bullet to a higher velocity.

    You can calculate rpm by MV X 720 / twist rate. Let's say for discussion purposes a 14" barrel can generate 2600 fps with a 77gr, a 20" barrel 2800 fps, and a 28" barrel 3000 fps.

    14" 267,428 rpm
    20" 288,000 rpm
    28" 308,571 rpm

    Conversley lets say your 18" barrel is capable of 2700 fps with the 77 gr. Calculating for that barrel we get

    18" 243,000 rpm

    Or another way to look at it is if we assume 267,000 rpm is sufficient to stabalize a 77gr bullet, what velocity do we need in a 1-8 barrle to achieve that rpm. Crunching the numbers we get 3000 fps.

    So that is why I said it depends on your barrel length, it's a matter of rpm and hence what muzzle velocity is required for that rpm.
    Those that are successful in Alaska are those who are flexible, and allow the reality of life in Alaska to shape their dreams, vs. trying to force their dreams on the reality of Alaska.

    If you have a tenuous grasp of reality, Alaska is not for you.

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    Quote Originally Posted by Paul H View Post
    Bullet stability is based on how many rpm the bullet is spinning. Longer bullets have to be spun faster to be stable. How fast the barrel spins is based on the twist rate of the barrel and the muzzle velocity. For a given twist rate, in this case 1:7 the longer barrel will be able to spin the bullet faster because the longer barrel produces a higher velocity. It has nothing to do with the barrel length perse, just that longer barrels allow the burning powder to accelerate the bullet to a higher velocity.

    You can calculate rpm by MV X 720 / twist rate. Let's say for discussion purposes a 14" barrel can generate 2600 fps with a 77gr, a 20" barrel 2800 fps, and a 28" barrel 3000 fps.

    14" 267,428 rpm
    20" 288,000 rpm
    28" 308,571 rpm

    Conversley lets say your 18" barrel is capable of 2700 fps with the 77 gr. Calculating for that barrel we get

    18" 243,000 rpm

    Or another way to look at it is if we assume 267,000 rpm is sufficient to stabalize a 77gr bullet, what velocity do we need in a 1-8 barrle to achieve that rpm. Crunching the numbers we get 3000 fps.

    So that is why I said it depends on your barrel length, it's a matter of rpm and hence what muzzle velocity is required for that rpm.

    Very good explanation there Paul. And I'm sure you didn't offend anyone. Very definitely velocity matters. That's why my 24" will stabilize the 77 grain with 1 in 8" twist.

    The real uncertainty here is at what rpm is the 77 grain bullet considered stable for the extent of its range capability?
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    Is there a table available that shows the best rate of twist and barrel length for each bullet weight.......???? 45....55.....62 gr. etc.

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    Supporting Member AlaskanSD's Avatar
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    I think my thread has been officially hijacked.

    I guess I need to get a box of 6.8spc and 6.5 grendel and try them out in a PMag myself.

    I was my understanding those cartridges would not fit at all in a polymer magazine because of them larger diameter, but I have not tried it out myself.

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    Quote Originally Posted by AlaskanSD View Post
    I think my thread has been officially hijacked.

    I guess I need to get a box of 6.8spc and 6.5 grendel and try them out in a PMag myself.

    I was my understanding those cartridges would not fit at all in a polymer magazine because of them larger diameter, but I have not tried it out myself.

    So.......you are making the decision on the upper.......based on P-Mag, is that correct.....?

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    Supporting Member AlaskanSD's Avatar
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    10-4. Laugh all you want, but I hold that plastic magazine in high esteem.

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