5.56 Tumbling/Keyholing??

[BigRedDog]

Just pulled a range target from May when I shot my AR 5.56 pistol at 25 yards and dang if they weren't all tumbling! I blamed it at the time on the ammo which was milsurp frangible. Barrel length is 8" and it's a 5.56 chamber???

are we helping with a 14" bbl or an 8" one here?

1/12 twist should stay at 55 or lighter - try the 40 grain to 50 grain range

as pru stated, the velocity may not be what you think it is. get a chrono reading.

is there even the slightest possibility this is impacting a muzzle device or a suppressor?

 

[pruhdlr]

With a 12 twist bbl you could easily stabilize a 50gr Barnes however a 55gr would be questionable. Guessing .... I do believe that it would work depending on if you could keep the velocity up. There is a table somewhere that you input weight,AND length,caliber,and velocity,and it spits out good or bad.

I shoot a 55gr Barnes TSX out of a 1:9 Benelli MR1 and it kills both deer and hogs excellent. I believe that the Barnes site states the lengths of their bullets. --- pruhdlr

 

[Shooter]

Try my suggestion, and see how it goes. The lighter weight projectiles are going too fast to stabilize in a short barrel. the twist rate never quite makes it.

 

[Hogdaddy]

are we helping with a 14" bbl or an 8" one here?

1/12 twist should stay at 55 or lighter - try the 40 grain to 50 grain range

as pru stated, the velocity may not be what you think it is. get a chrono reading.

is there even the slightest possibility this is impacting a muzzle device or a suppressor?

14" barrel shooting 62 gr SS109, no muzzle device.
Shoots Hornady 55gr TAP fine

 

[Hogdaddy]

Try my suggestion, and see how it goes. The lighter weight projectiles are going too fast to stabilize in a short barrel. the twist rate never quite makes it.

77 gr Black Hills, I'll give it a try and let you know.

 

[Hogdaddy]

77 gr Black Hills, I'll give it a try and let you know.

You talking about 5.56 or .223 Shooter? Seems to be hard to find (and pricey)...

 

[Shooter]

they are pricey. Do not waste them, BH got a contract for ammo for some of the go fast groups in Afghanistan and he developed both 77 grain and 100 grain for the SBR. 100 grain is not available to the civilian market, that I know of. the 77 grain is but it is about 36 to 42 bucks a box-last i knew of. I really am not a ballistics guy but it sure does the job.

I am in the process of putting together a LWRC custom shop pistol, 10 inch barrel and a bunch of goodies. I am going to use it most of the time for hogs in the near future. It is light, easily controlled and I use a SF 60 round mag. It is suppressed, micro T-1, DBAL IR/Vis and IR light, and it works great with a PVS-14. It nails the 77 grain.

Hogdaddy-where are you located ?

 

[hdfireman]

14" barrel shooting 62 gr SS109, no muzzle device.
Shoots Hornady 55gr TAP fine

It shoots the 55gr ok and not the 62gr because of the twist rate. Heavier bullets need a faster twist to get them stabilized and a 1-12 is pretty slow for a 5.56. Most like the 8 twist because you can run 55gr up to the 77gr without a problem. With that twist you need to stay 55gr or below in bullet weight. Try the smaller 40-45 gr stuff and it will probably shoot best. Use .223 ammo!!!

From Barnes website-
45,50,53 and 55gr 1:12 or faster
62gr 1:9 or faster
70gr 1:8 or faster

 

[TEXASLAWMAN]

77 gr Black Hills, I'll give it a try and let you know.

In my opinion this is the wrong way you need to stick to 55Gr and below your twis rate is to slow for heavier bullets.

 

[pruhdlr]

Shooter, 100gr .224" huh ?? With what twist bbl,used at what range,with what magazine,with the COAL of ??? With what powder ?? --- pruhdlr

 

[Hogdaddy]

Just dug out the Greenhill Equation and plugged into my Excel:
Greenhill Equation: Twist equals 150 times diameter of bullet squared divided by bullet length
(180 vice 150 if velocity is between 1800 and 4000 fps)

5.56 SS109
Velocity 3100fps
Diameter .224
Length .904
Weight 61 gr
Greenhill twist from formula = 9.99
My TC 14" .223 barrel has 1:12 twist

I'm sorry I can't remember where I copied this quote, I've read so much over the last three days : "If an insufficient twist rate is used the bullet will begin to yaw and then tumble, this is usually seen as "keyholing"..."

Sounds like my problem. It will be interesting now to try some inputs to this formula for the Hornady ammo I used when shooting with the SS109. I certainly will not be shooting any milsurp however in my SAAMI,

Too tired tonight plus the woman wants me out of my "man cave". Thanks guys and have a great Labor Day!

 

[FrankT]

100 gr
http://www.ar15.com/forums/t_3_16/145122_.html

From the Firing Line:
Black Hills 100gr Very little is known about this round, except that it is supposed to go at about 2500fps out of a 16" barrel or so, and retains a LOT of its velocity, enough for good performance out to 300 yards or so. However, it is a "military only" round, still in trials.
The bullet is made of tungsten. Tungsten if much denser than lead and therefore can reach up to 100 grains without being much larger than your normal 69 grain lead bullet... it might even be shorter. I would be worried stabilization with these bullets... although then again if it's not longer, I guess the stabilization would remain the same? Is gyroscopic stability based on length, or weight? I heard somewhere it was only based on length.

A 100 grain bullet made of tunsten would not only have great ballistics, but would also pierce armor like crazy. Especially since it's made of tunsten. Cap it off with a thick copper jacket so you don't trash your barrel, and you should be set.

Evidently they are trying this with typical issued Govt weapons and no special twist rates.

 

[Shooter]

I have no idea what the barrel twist was. The barrel was 10.5. The range was from about 5 yards to max of 200.

The mags were all standard mags. I do not know anything else about the ammo. Never a malfunction and no keyholing, Good stuff.

 

[Shooter]

Nice look up Frank. I never knew much about it, just used it a lot. All of it makes sence. Just got a note from a friend of mine. His unit is the same as mine is. 1/7 twist in a 10.5" barrel. Just FYI my 62 grain Black Hills works very well also

 

[FrankT]

Yeah Shooter, used my googlefoo. If it works I be the 100gr is a game changer, I have a 1-9, 556 so it probably would not do well in that but the 62-77gr should.

 

[histopicker]

A 1/12 twist is perfect for 55gr and lighter bullets. The green tips are I believe 62gr and needs a 1/9 twist to stabilize. The M16A1 was at first produced with a 1/14 twist and used a 55gr fmj bullet. In extreme cold it went south on accuracy in a hurry so specs were changed to a 1/12 twist. When we needed to use 62gr bullets and heavier the twist rate was tightened.

 

[pruhdlr]

Leave it up to the military to try and use a bullet made out of TUNGSTEN. This covered with a copper jacket. Wondering if the coper is stuck to the tungsten with some sort of super glue ..... **** ...... probably. These bullets,when finalized,should cost about the same as one of the toilet seats or hammers. Whoever came up with this probably had a hand in developeing the depleated uranium rd for the 50 cal.

Sorry .......... but ........ I just do not know what could be/would be accomplished with the 2500fps "toilet seat" over the M855's. At the "normal" ranges that a 5.56 weapon is used to engage the enemy(even in the sand box)I really cannot see the advantage ...... ok,maybe a extremely small advantage. After all does anyone in their right mind use the military issued 5.56 (M4 or any other)as a sniper rifle ??

I have fired tens of thousands of M855 rds out of several different platforms. As with any damned bullet,you gotta hit the BG. A close miss even with the super dooper 100gr,is still a miss. My thoughts --> forget the 100gr and the costs associated with it. Stick with the M855 and use the potential waste of money for more training. --- pruhdlr

 

[HeadHunter]

Length of bullet is important. Try some of the Razorbacks at 64 grain with the beveled round. I am shooting those out of 1 in 8 twist and getting good accuracy, barrel is 22" long. No key holing. Tried several weights/bullets to arrive at this...

 

[hdfireman]

You have a 1:8 twist he has a 1:12 so what shoots good in yours will prob not shoot well in his. To the OP- don't go around trying all the ammo people suggest in their rifles. The prob is twist rate which I think you agreed with using the Greenhill Equation.

 

[DaveABQ]

The most important difference between .223 and 5.56 chambers is the length of the throat (or leade) for each chamber. More specifically, the leade is located at the mouth of the barrel before the rifling occurs. Comparing the NATO and SAAMI regulations, the leade for 5.56 chambers is nearly twice as long as that of a .223 chamber (.162" to .085", respectively). If a 5.56 round contacts the barrel rifling too early, it can cause pressure spikes (leading to malfunction, and potentially damage) in the chamber. This explains why it is safe to fire .223 through a 5.56 chamber, but not recommended to fire 5.56 through a .223 chamber.