AR-15 ammo selection
I will be building an AR-15 in 5.56mm over the next couple of months. I am inclined to go with a 20" upper for maximum power. If I want to shoot the hotest / heaviest rounds available for the gun accurately, what twist rate should I go with?
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If Heavier/longer bullets only is what you intend to use, the fastest barrel twist is required. If there is any possibility that you will be using light or intermediate weight bullets, go for a 1 in 9 twist as a compromise. 0 -
In actual use, longer/heavier bullets are typically advantageous in circumstances where a longer barrel is suitable. If your proposed use of the rifle is mainly for specific uses where longer/heavier bullets are needed, a 20" barrel with 1/7 twist would be suggested.
The 223 is actually very limited in power and using a short barrel simply reduces that limited potential even more.0 -
7.62,
That's a no brainer, of course the 1 in 7 is the way to go. You can use up to and including the 90 grain VLD's in the 1 in 7. You don't loos as much velocity with a 4 inch decrease in barrel length, as one would think.
Mobuck,
quote:In actual use, longer/heavier bullets are typically advantageous in circumstances where a longer barrel is suitable.
I am not sure where you are getting this from, but typically it is only the cartridges that are extremely overbore capacity, that benifit from a longer barrel. As stated above, the 223/5.56 does not drop as much velocity going from 24" to 20", or 20 to 16". Furthermore, using a heavy bullet in the short barrel, enhances the meaningness of the rifle/load combination. I have a 16" varmint with a 1 in 8 twist, that loves the Barnes 62, and 70 grain TSX bullets. It likes the Hornady 68 grain Match as well, and the 77SMK's. When you have the accuracy to precisely place your shot, power becomes a fad. But these still have lots more than the SS109.
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I have a 1/8 twist that loves 69+77gr. It puts them into one hole at 100yds all day long [:)] 0 -
quote:Originally posted by 7.62x39Lover
I will be building an AR-15 in 5.56mm over the next couple of months. I am inclined to go with a 20" upper for maximum power. If I want to shoot the hotest / heaviest rounds available for the gun accurately, what twist rate should I go with?
The key thing in your question is, "AR-15". The longest/heaviest bullets you can run through a mag in this rifle are stabilized by a 1-8" twist. There are heavier bullets out there, but you cannot load them to AR-15 Mag length. If you wanted to single load them you might want to opt for the 1-7" twist. Then again, why not just go with a bolt gun? The real reason, the military went with the 1-7" twist was to stabilize the tracer round that matches the ballistics of the M855 (62 gr. FMJ)
One thing that always kills the AR-15 as far as lengthening the barrel is the gas port. There is very little continuous push after the bullet passes the gas port. In the different length gas systems, velocity increases dramatically as they get longer. After the gas port has been passed by, by the bullet, adding barrel length doesn't do a whole lot for velocity. Most all of the maximum velocity can be reached with a 20" barrel in an AR-15. In a bolt gun you can increase the velocity significantly over the AR-15 by adding barrel length. In my 26" Savage, I was able to obtain velocities of 3150fps with 77 gr. SMK's/Nosler CC's. In my 20" AR-15 I could obtain velocities of 2940fps. In my 16" I can only obtain velocities of 2690fps.0 -
Thanks for all the info guys. Looks like the manufacturers are right on the money producing so many uppers in 1:7, 1:8, and 1:9.
From what you guys are telling me I might be leaning towards 1:7 or 1:8.0 -
quote:Originally posted by 7.62x39Lover
Thanks for all the info guys. Looks like the manufacturers are right on the money producing so many uppers in 1:7, 1:8, and 1:9.
From what you guys are telling me I might be leaning towards 1:7 or 1:8.
Unfortunately, the lions share of bulk/surplus ammo that is reasonably available, is still the 55 grain ammo. While I have heard that you cannot over stabalize a bullet, I know an M4 with a 1:7 twist that shot 40 grain TAP ammo into a 6" group at 100 yards. When the rifle was given 69 grain match ammo, the group was around an inch. The sight was an EOTECH.
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quote:Originally posted by tsr1965
quote:Originally posted by 7.62x39Lover
Thanks for all the info guys. Looks like the manufacturers are right on the money producing so many uppers in 1:7, 1:8, and 1:9.
From what you guys are telling me I might be leaning towards 1:7 or 1:8.
Unfortunately, the lions share of bulk/surplus ammo that is reasonably available, is still the 55 grain ammo. While I have heard that you cannot over stabalize a bullet, I know an M4 with a 1:7 twist that shot 40 grain TAP ammo into a 6" group at 100 yards. When the rifle was given 69 grain match ammo, the group was around an inch. The sight was an EOTECH.
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True story. The 55 gr. that is hitting the market was produced in mass for the military as much as 40 years ago. Back when we didn't have anywhere near the manufacturing tolerances of today. Bullet concentricity (bore center vs. weight center) is the big issue. A bullet that is not made concentric will not fly true. The weight being off center from the center of the bore will go around the bore center and throw the bullet off. This effect is increased as the bullet is rotated faster. That is why older rifles usually only had the twist it required to stabilize the most common bullet. "Over-twisting" isn't near as critical with today's manufactured bullets.
I can't say true story to the 40 gr. vs. 69 gr. bullet, because I wasn't there. But, I believe it.[;)][:D]0 -
Most of mine are 1/9. So far all good with bullets up to 69 gr. One has a 1/7, also accurate with the same range of bullets. Check with the barrel manufacturer you plan on using and see what they offer in your price range. 0
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