Questionable comment in Outdoor Life article on AR accuracy

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Greybeard
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Questionable comment in Outdoor Life article on AR accuracy

#1

Post by Greybeard » Sat Feb 08, 2020 9:02 pm

Toward the end of this, the author makes a comment that the point of impact on the last shot from an AR mag can be different due to the bolt locking back. I would think that by the time that happens, any change in "harmonics" should not matter as the bullet has probably already exited the barrel. Whatyathink? Truth or trash?

His statement: "Also, be aware that with your last shot from a magazine, when the bolt locks back, the rifle has different harmonics since it’s only experiencing half the recoil cycle, which can alter your point of impact."

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Re: Questionable comment in Outdoor Life article on AR accuracy

#2

Post by treadlightly » Sat Feb 08, 2020 9:18 pm

My guess is the only differences are the drag the bolt experiences against the follower versus the top of the next round to be fed.

As the magazine empties, the moment (torque) on the rifle should change a little.

However, by the time the bolt has moved out of breech, the bullet is out of the barrel.

My guess - emphasis on guess - is there is no point of impact difference based on what's waiting in the magazine.

I would be fascinated to be shown wrong.


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Re: Questionable comment in Outdoor Life article on AR accuracy

#3

Post by strogg » Sat Feb 08, 2020 9:20 pm

I call shenanigans. It is known that the bullet is still accelerating outside the muzzle. With that in mind, the bullet reaches maximum velocity several inches past the muzzle, so any turbulence in the gasses may affect trajectory a bit by fractions of an MOA. But regardless of that, by the time the gasses from the gas tube start pushing the bcg back, the bullet is already several feet in front of the muzzle. The bullet is long gone by the time the bolt starts moving back. By then, the bolt locking back or the lack of drag by the magazine spring are very irrelevant. It might make a difference for those superhuman zen folks who are one with the AR15 and looking for sub-moa groups whilst shooting at Jerry Miculek speeds (for the follow-up shots), but really?

You can see what I mean by this high speed video here. Do you see any "harmonics"? I don't.


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Re: Questionable comment in Outdoor Life article on AR accuracy

#4

Post by LeonCarr » Sat Feb 08, 2020 9:47 pm

Will the last round impact differently? I don't think so.

I think that the first round hand cycled into the chamber will impact differently since it lays in the chamber differently than the rounds cycled into the chamber by the rifle itself.

Just my .02,
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Re: Questionable comment in Outdoor Life article on AR accuracy

#5

Post by Beiruty » Sat Feb 08, 2020 9:54 pm

The bolt would stay closed until the bullet leaves the barrel. All other "explanations" are bogus.
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Re: Questionable comment in Outdoor Life article on AR accuracy

#6

Post by Greybeard » Sat Feb 08, 2020 10:01 pm

Thanks for the prompt replies, guys. This is not the first time I have seriously questioned what one of the Outdoor Life writers had to say.
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Re: Questionable comment in Outdoor Life article on AR accuracy

#7

Post by Syntyr » Sat Feb 08, 2020 10:20 pm

Beiruty wrote:
Sat Feb 08, 2020 9:54 pm
The bolt would stay closed until the bullet leaves the barrel. All other "explanations" are bogus.
:iagree: This!

Think of the position of the gas tube on the barrel. It is usually a couple of inches from the end. The gas from the burning propellant cant enter the gas tube until the bullet passes the gas tube. The gas then has to go all the way back down the barrel to hit the bolt face. By my "guess " the bullet is long gone by the time the bolt starts moving rearward. Cant see how the the bolt locking back would have any impact on point of impact.
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Re: Questionable comment in Outdoor Life article on AR accuracy

#8

Post by The Annoyed Man » Sun Feb 09, 2020 10:43 am

Bolt doesn’t even come out of battery until the bullet has exited the barrel, so yeah...what everyone else has said. In MY experience, it’s always the first or second shot that’s the flyer—particularly from a cold bore.
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Re: Questionable comment in Outdoor Life article on AR accuracy

#9

Post by WildBill » Sun Feb 09, 2020 11:32 am

strogg wrote:
Sat Feb 08, 2020 9:20 pm
It is known that the bullet is still accelerating outside the muzzle. With that in mind, the bullet reaches maximum velocity several inches past the muzzle ...
Since all of the energy been transferred to the projectile I don't understand how a bullet could accelerate after it left the muzzle.
I know Wiki isn't the final authority, but this article agrees with my thinking:
For projectiles in unpowered flight, its velocity is highest at leaving the muzzle and drops off steadily because of air resistance.
https://en.wikipedia.org/wiki/Muzzle_velocity

I have never heard of harmonics applied to the entire gun, but there are a couple good YouTube videos on AR-15 barrel harmonics.
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Re: Questionable comment in Outdoor Life article on AR accuracy

#10

Post by iratollah » Sat Mar 28, 2020 2:58 am

There's a high power competition called "National Trophy Infantry Team Match" (aka Rattle Battle) where competitors shoot 25 rounds in 50 seconds start at 600 yards, then 500, then 300 yards. Competitors shoot service rifle AR15, prone, iron sights, with a sling. They use full magazines. While target is an Army “E” silhouettes, if last round effected accuracy they we can be certain this bunch that lives for accuracy wouldn't load 20 in a magazine.
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