10.5" ar huge groups? RESOLVED

Why are so many people buying 1:7 twist barrels and then running 55gr ammo and expecting tight groups? From my experience with a 1:7 you can get decent groups using 55gr but the twist rate really performs with 62gr and heavier projectiles. I had great results from my last 1:8 barrel and if I was ordering a new one that's the twist rate I would go with. It stabilized everything from 50-77 grain very well.

I hope you get this sorted out but I wouldn't shoot that rifle till you check the headspace is correct, KevinB knows what he's talking about and you may have created a dangerous headspace condition when you tightened it back up.

Good luck
 
Ok so the barrel extension problem may not be the issue here but it is going back to S&J to get rectified none the less....

Thank you Simon and James for your great customer service!
 
cr5 said:
Why are so many people buying 1:7 twist barrels and then running 55gr ammo and expecting tight groups? From my experience with a 1:7 you can get decent groups using 55gr but the twist rate really performs with 62gr and heavier projectiles. I had great results from my last 1:8 barrel and if I was ordering a new one that's the twist rate I would go with. It stabilized everything from 50-77 grain very well.

I hope you get this sorted out but I wouldn't shoot that rifle till you check the headspace is correct, KevinB knows what he's talking about and you may have created a dangerous headspace condition when you tightened it back up.

Good luck
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The barrel is going back to S&J at their request, i am not and will not shoot this rifle untill a proper fix is done...

When i was sourcing out a short barrel i had a very hard time finding one in anything but 1:7 twist
 
When this happened to me, I tightened up the barrel as best I could and shot it again. Did that for 2 sessions. Machine ran perfectly fine. However, it was loose by the end, causing my irons zero to go out.
 
cr5 said:
Why are so many people buying 1:7 twist barrels and then running 55gr ammo and expecting tight groups? From my experience with a 1:7 you can get decent groups using 55gr but the twist rate really performs with 62gr and heavier projectiles. I had great results from my last 1:8 barrel and if I was ordering a new one that's the twist rate I would go with. It stabilized everything from 50-77 grain very well.
Good luck
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Shorter barrels generate less velocity, requiring a faster twist to stabilize whatever bullet the same way a longer barrel would with a lesser twist. That's why 1-7 is a hot ticket these days anyway, it's more an SBR feature that has spilled over to all barrels as a default. I don't expect an 11" 1-7 to stabilize any better then a 20" with a 1-9 or 1-12.

Mind you, I don't expect groups out of an 11" inch either way.
 
Conte said:
Shorter barrels generate less velocity, requiring a faster twist to stabilize whatever bullet the same way a longer barrel would with a lesser twist. That's why 1-7 is a hot ticket these days anyway, it's more an SBR feature that has spilled over to all barrels as a default. I don't expect an 11" 1-7 to stabilize any better then a 20" with a 1-9 or 1-12.

Mind you, I don't expect groups out of an 11" inch either way.
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You do realize twist rate is a matter of bullet lenght, not barrel lenght.
 
Dietz bullet lenght is dependant on twist rate yes, but accordingly to velocity as well. As you increase a typical bullets weight, we know the length of the bullet increases and we need a faster twist to produce a faster rotation. We need to do either one of two things to get acceptable rotation, increase twist rate or increase velocity. A 180gr bullet not spinning fast enough leaving a .308Winchesters 1/12" twist at a MV of 2600fps for optimum rotational stability, may be fully stabilized fired in a .300Winchester Magnum with 1/12" twist leaving the barrel at 3000fps. The increased velocity with same twist rate, increases rotational speed which is what we need to stabilize the under stable bullet.


If a 55 gr 556 is adequately stabilized in a 1/12" twist 20" M-16 barrel, cut that barrel to 10.5", and now the 1/8 or 1/9 is necessary to get the same rotational speed because of the much lower muzzle velocity on the 10.5" barrel. It's spinning around 194,000 rpm in the 20" barrel at 3240fps. If we cut the barrel to 10.5" we now only get 162,000 rpm. If we increase the twist rate in the 10.5 inch barrel to 1/9", now the bullet is spinning at 216,000rpm even a little faster than the 20" barrel rotational speed. I'd say a 1/8" or even 1/7" would be required maybe in a 7.5" PDW with 55grs, maybe. These numbers are merely to show different rotational speeds at different muzzle velocities in the same twist rate barrel, not to determine best twist rate for a given barrel length.
 
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RememberTheSomme said:
As you increase a typical bullets length, we know the weight of the bullet increases and we need a faster twist to produce a faster rotation.
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There, I fixed it for you. :p

But yeah, me sentiments exactly, just with more math.

It has nothing to do with length, and everything to do with weight and velocity. Just so happens that heavier bullets are longer.
 
cr5 said:
Why are so many people buying 1:7 twist barrels and then running 55gr ammo and expecting tight groups? From my experience with a 1:7 you can get decent groups using 55gr but the twist rate really performs with 62gr and heavier projectiles. I had great results from my last 1:8 barrel and if I was ordering a new one that's the twist rate I would go with. It stabilized everything from 50-77 grain very well.

I hope you get this sorted out but I wouldn't shoot that rifle till you check the headspace is correct, KevinB knows what he's talking about and you may have created a dangerous headspace condition when you tightened it back up.

Good luck
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A 1:7 barrel has the ability to shoot all types of bullets without any major disadvantage. Even if you are mostly shooting 55 grain FMJs, you still have the capability to use longer bullets if you ever decide to do so. My AR has produced good accuracy out of a 1:7 barrel with most of the 40 and 55 grain bullets I have tried. If there is any loss in accuracy from the faster than optimal twist, it would only be noticeable from the bench. If you just want to shoot gnat-sized groups from the bench, a bolt action is the better choice.
 
FYI a lot of the 1:7 over stabilizes the 55gr has been debunked -- my old boss before he retired from KAC, Lt Col Dave Lutz USMC (ret.) was the M16A2 Program Officer -- and as a gun guy he was intimately involved in the program. The story that came out on the M16A2 not doing well with the M193 55gr was a result of guns that where so fouled that there where blockages at the gas port, at the same time they also found that many of the older M16A1 'bent barrels' (that lead to the heavier up front M16A2 barrel) where actually just terribly fouled.

Many entities are going for even faster than 1:7 twists - because based on testing the faster twist caused more terminal effects when the bullet hits tissue -- think of the rotational forces that are increased in a 1:5 or 1:4 twist compared to 1:7 or 1:8 - when the bullet hits tissue at longer ranges when the forward velocity has significantly decayed - the rotational velocity is what helps the bullet come apart.
 
KevinB said:
FYI a lot of the 1:7 over stabilizes the 55gr has been debunked -- my old boss before he retired from KAC, Lt Col Dave Lutz USMC (ret.) was the M16A2 Program Officer -- and as a gun guy he was intimately involved in the program. The story that came out on the M16A2 not doing well with the M193 55gr was a result of guns that where so fouled that there where blockages at the gas port, at the same time they also found that many of the older M16A1 'bent barrels' (that lead to the heavier up front M16A2 barrel) where actually just terribly fouled.

Many entities are going for even faster than 1:7 twists - because based on testing the faster twist caused more terminal effects when the bullet hits tissue -- think of the rotational forces that are increased in a 1:5 or 1:4 twist compared to 1:7 or 1:8 - when the bullet hits tissue at longer ranges when the forward velocity has significantly decayed - the rotational velocity is what helps the bullet come apart.
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So a 55gr in a 1:7 12.5" barrel is sufficient to acheive respectable grouping? Im not talking about 100+ yards here im talking within 75-100yds is the max distance ill be stretching my short ar
 
SksA1 said:
So a 55gr in a 1:7 12.5" barrel is sufficient to acheive respectable grouping? Im not talking about 100+ yards here im talking within 75-100yds is the max distance ill be stretching my short ar
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Yes

I've grouped 11.5" guns to 300m (using a magnified optic - I can't shoot for groups at 300m with my Aimpoints - I just cannot make out the tgt/dot correlation well enough)
 
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