ar rifling question

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drslav

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can someone direct me to a thread that talks about barrel rifling in ar rifles/
why is there a choice between 1/9 1/7 etc with there things? whats better and why?
i had a norinco and was told it was 1/12. now i am in a market for 14.5 ar and i have no idea what is going on. i want it to be accurate up to 200 yds...
what is the difference between 14.5 and 16" barrel?
 
The faster the twist the heavier bullet it will stabilize and also the faster the barrel will wear out due to the increased friction and forces generated by the tighter twist. Generally 1:9 twist is good for light bullets going up to around 68 grains and some 75 grain bullets, a 1:7 twist will stabilize heavier bullets better and is generally for longer range shooting of longer heavier bullets with a better BC. Given the choice I would go with a 1:8 twist and if it's not an option on the model you are looking at I would go with 1:9 unless you are planning to use it with a longer barrel for longer range shooting.
I've had rifles with both 1:7 and 1:9 and both will work fine with a 55 grain bullet so it's not a really big deal other than a little faster barrel wear on the tighter twist 1:7 twist.

Good luck.
 
What twist rate you need depends on what you want to use the gun for. If you want to shoot targets up to 200 or 300 meters, you don't need long and heavy bullets so a twist rate of 1 in 9" is nearly ideal because it will handle light bullets and 62 gr with good results, even 69 gr will be fine. I consider the 1 in 9" the best general purpose rate. Anything faster falls into the 600 m or longer target category.
 
drslav said:
why is there a choice between 1/9 1/7 etc with there things? whats better and why?
Click to expand...

When the AR was first produced, it came with a 14" twist, probably because that was common in .22 centrefire calibers in sporting use.

Tests showed the 14" did not produce adequate stability with the 55 FMJ bullet being produced for military use, so it was tightened to 12" after about 30 000 AR-15s were produced.

When NATO adopted the 5.56mm cartridge, heavier bullets were introduced. The 12" twist would not stabilize the long 4g FMJ bullet or the even longer tracer. Colt did tests and found that a 9" twist stabilized the FMJ, but it took a 7" twist to stabilize the tracer. They also found that the 7" twist was prone to excessive wear, so they recommended the US military use a 9" twist for general purpose use. But, the military wanted all rifles to be able to use tracer, so they went with the 7" twist rate. To maximize barrel life, most barrel makers went with the 9" twist for the commercial market.

Now, a number of extra-long, super heavy, high-performance bullets are being made to try to get the most lethality out of the 5.56 cartridge. The 9" twist won't stabilize some of these, so 7" is becoming more and more popular in the commercial market, too. Some match barrel makers are making an 8" twist rate as a compromise between the two.

The wear problems Colt found with the 7" twist don't seem to be a problem in the real world, and the 7" twist will stabilize everything, so it is probably the best choice, unless you are 100% certain you will never shoot anything heavier than 62 grains.
 
BattleRife said:
When the AR was first produced, it came with a 14" twist, probably because that was common in .22 centrefire calibers in sporting use.

Tests showed the 14" did not produce adequate stability with the 55 FMJ bullet being produced for military use, so it was tightened to 12" after about 30 000 AR-15s were produced.

When NATO adopted the 5.56mm cartridge, heavier bullets were introduced. The 12" twist would not stabilize the long 4g FMJ bullet or the even longer tracer. Colt did tests and found that a 9" twist stabilized the FMJ, but it took a 7" twist to stabilize the tracer. They also found that the 7" twist was prone to excessive wear, so they recommended the US military use a 9" twist for general purpose use. But, the military wanted all rifles to be able to use tracer, so they went with the 7" twist rate. To maximize barrel life, most barrel makers went with the 9" twist for the commercial market.

Now, a number of extra-long, super heavy, high-performance bullets are being made to try to get the most lethality out of the 5.56 cartridge. The 9" twist won't stabilize some of these, so 7" is becoming more and more popular in the commercial market, too. Some match barrel makers are making an 8" twist rate as a compromise between the two.

The wear problems Colt found with the 7" twist don't seem to be a problem in the real world, and the 7" twist will stabilize everything, so it is probably the best choice, unless you are 100% certain you will never shoot anything heavier than 62 grains.
Click to expand...

very informative, thanks
 
good joke on 1.5" so let me ask it differently...is there benefit to 16" barrel over 14.5?

other than that thank you for the info. it will help me make better choice... i am leaning towards 1/7. i dont shoot enough to worry about wear...and accuracy is important to me with any bullet.

actually i think i just figured something out... 1in 7 better in 14.5 and 1:8 in 16" 1/9 in anything longer does that make sense? 2 full turns in a barrel length to stabilize the bullet...
 
Twist and velocity are two parts of the equation as they determine bullet RPM which is what causes stability. So a bullet out of a 10" 1/7 barrel can have a slower RPM and be less stable than one out of a 20" 1/9. Another is bullet length and balance as both affect nutation.

It is to bad that bullet makers don't state the optimum RPM for stability. Then again the cost of testing must be huge.
 
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