5.56-223 from 10.5 inch Barel

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A 1:7 will stablize any bullet to 80 grains. If you are only shooting to 100y, its not going to make much difference which bullet weight you use.
 
Ar180shooter said:
55grs won't be stabilized in a 1 in 7, so you'd need 69grs.
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where do you get that from? The issue with the 10.5 is 55s wont fragment (generally) due to decreased velocity. This turns the fragmenting .223 round into an expensive .22

I shoot 55s all the time in my 1:7 and they fly nice and straight.
 
The reason I asked is that I'm shooting 55gr PMC Bronze stuff at the range and not getting very good accuracy. Almost 4 moa from a Noveske 10.5inch on a lead sled with a 3X IOR. I'm assuming that either my gun does not like this PMC ammo or 55 grainners in general. Thus the queston to go heavier or lighter.

I'm probably never going to use this gun in any defensive situations so fragmenting has no value. But for the sake of argument would a slower heavier hunting round (Non FMJ)from a 10.5 inch barrel offset the lack of frag.
 
could be feed issues with soft or hollow points. Try another cheap brand like American Eagle or Winchester White box in 55s. Hows the mount on your scope?
 
What target-material are you using to test the round's fragmentation?

1:7 is good for anything 55gr and up. Using a median grain would be the best, like 62gr or 68gr, but can handle quite heavier loads with effectiveness. I wouldn't shoot anything under 55gr, though.
 
carlchevalier said:
The reason I asked is that I'm shooting 55gr PMC Bronze stuff at the range and not getting very good accuracy. Almost 4 moa from a Noveske 10.5inch on a lead sled with a 3X IOR. I'm assuming that either my gun does not like this PMC ammo or 55 grainners in general. Thus the queston to go heavier or lighter.

I'm probably never going to use this gun in any defensive situations so fragmenting has no value. But for the sake of argument would a slower heavier hunting round (Non FMJ)from a 10.5 inch barrel offset the lack of frag.
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From the reading I did on the interweeb, 55 grain FMJ out of a 10.5 inch barrel should have enough velocity to break apart at the cannelure out to about 50 meters or so.

If you've got a 1 in 7 twist, I'd start looking at heavier bullets instead of lighter bullets. I'd wager that any of the heavier bullets you want to use will be nice and stable.

Now here's the other question...what makes you think that the culprit is the bullets not being stable? Are the 55 grainers keyholing? I obviously have nothing here to suggest that it's anything particular thing, but there are lots of things that affect grouping.
 
The gun is a brand new complete Noveske upper that I put on a billet lower. I'm sure I should be getting much better accuracy from it (at least 1moa).

I'm going to buy a bunch of different ammo and see what gets the best results. I'm shooting tommorow so I'll let you guys know what happens
 
Ar180shooter said:
55grs won't be stabilized in a 1 in 7, so you'd need 69grs.
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The twist rate bullet weight formulas are predicated on a full length barrel.

In a 10 inch barrel with 1 in 12, a 55 grainer doesnt even get one full spin, in a 1 in 9 it gets one full spin, and in a 1 in 7 it gets even more spin - (albeit not much more).

In a shortie barrel wouldn't a bullet with more spin on it be more accurate ?

A 7 inch barrel with a 1 in 7 twist should perform about the same as a 10 incher with a 1 in 10 twist using 55 grainers in both.

In my experience my 10 inch AR with 1 in 9 twist shoots 55 grainers more accurately than 69 grainers because the 69 grainers need more spin than the barrel provides.

blowup.jpg
 
In a 10 inch 1:12 barrel, the bullet is spun at the rate of 1 turn in 12 inches. In a 24 inch barrel with a 1:12 twist, the bullet is also spun at the rate of 1 turn in 12 inches. The rifling pitch doesn't change just because the barrel length changes.

A 10 1/2" AR based carbine consistantly producing minute of angle or less groups is going to be a pretty special firearm.
 
In my 11.5 - 1:9 twist, my best accuracy has been with 69gr Sierra's. I don't know if theres any commercial ammo in Canada that uses that bullet.
 
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The twist rate bullet weight formulas are predicated on a full length barrel.
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Where did you get this nugget of wisdom? Barrel length has ZIP to do with twist rate and bullet flight stability.

If barrel length was a factor, then how do those uber shorty revolvers stabilize bullets with only a 1.5" long barrel?

In a 10 inch barrel with 1 in 12, a 55 grainer doesnt even get one full spin, in a 1 in 9 it gets one full spin, and in a 1 in 7 it gets even more spin - (albeit not much more).

In a shortie barrel wouldn't a bullet with more spin on it be more accurate ?
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What do you mean by "more spin"? The bullet is at full rotational velocity almost as soon as it is fully engraved into the rifling. Otherwise the bullet jacket would get torn to bits at the bullet was spinning slower than the rifling.

The number of rotations a bullet makes before it leaves the barrel is irrelevant. The factor that determines flight stability is rotational velocity.

In my experience my 10 inch AR with 1 in 9 twist shoots 55 grainers more accurately than 69 grainers because the 69 grainers need more spin than the barrel provides.
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That is because 1:9 is marginal for the heavy bullets. Has nothing to do with the length of the barrel.
 
I'll try to put this delicately, it's not the platform (assuming it's properly assembled) and it's not the ammo (unless it's a defective batch - doubtful) but you could try some other brands to confirm. All things being equal, the weakest link in any rifle's performance is neither the rifle nor the ammunition. It sucks, but it's true.
 
Suputin's comments made me curious...


I based my chart off Ammo Oracle values for M193 @ 3200 FPS muzle velocity from an assumed 20" barrel.

Altitude 1480 feet 70 degrees 50% humidity. 55gr FMJBT Average cD over velocities: .267

http://ammo.ar15.com/ammo/project/perf_ballmil.html


This is the chart I created of resultant twist rates by range from barrel based off the formula R = (12/R) * V.
Twist%20Rates.jpg


Suputin, am I correct in understanding that you are saying that the rotational velocity is the key element to understanding the effects of barrel twist rates on gyroscopic stabilization of a round?

I believe it is. But please correct me if I'm wrong on that. I am quite aware that I am making the presumption that RPM rate decreases in parallel with deceleration of the bullet's velocity. I am not sure if this is accurate. If not, what caveats need to be applied?


My current believe then is that a given bullet weight and length will have a rotational velocity threshold below which it will no longer remain gyroscopically stable if it falls below that value anywhere within the total duration of bullet flight.


If so, it really makes anecdotal displays of 'accuracy' at out to 100m pointless.


Thoughts?
 
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enefgee, Did I mention that I get less than MOA with my STI AR with the same bullets on the bench without a rest (it has a 1 in 8 twist). So the shooter's the weak link theory is unfortunately CRAP! from what I've read in different publications, bullet weight and barrel twist rates are intrinsicaly linked to a rifles accuracy. As I've said i will bring to the range different makes and weights of ammo and find what the best bullet weight combination for this perticular rifle. Thank to all who posted intelligent replies
 
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