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Posted: 1/22/2002 8:41:34 AM EDT
I understand every rifle is different, but can you usually expect better accuracy from a 24" barrel over a 20"?
Link Posted: 1/22/2002 9:02:25 AM EDT
From what I understand. The "24 will give you better MV. However, it is probably not more inherently accurate. That being said, if you are using a longer barrel you might have a longer sight radius that in turn can help your accuracy.
Link Posted: 1/22/2002 9:13:36 AM EDT
Agree w/ arbubba, one thing though, accuracy at long range (+500 yards) will be enhanced with the longer barrel, IMHO (provided both barrels being equal in construction).

Mike
Link Posted: 1/22/2002 9:31:39 AM EDT
A common misconception is that barrel length afects acuracy. Acuracy could be just as good on a 10" barrel as a 20", but what you get is muzzle velocity. A 24" barrel gives you range, but not acuracy.
Link Posted: 1/22/2002 9:52:42 AM EDT

Originally Posted By Armed_Scientist:
A common misconception is that barrel length afects acuracy. Acuracy could be just as good on a 10" barrel as a 20", but what you get is muzzle velocity. A 24" barrel gives you range, but not acuracy.



...or, more accuracy at longer range?

Longer bbl. provides more velocity in this case, which at longer ranges contributes to better accuracy due to the sustained velocity.
The transition from super to sub-sonic velocity, which is what begins the gyroscopic destabilization of the projectile, is delayed with the longer bbl., thereby allowing accuracy at extended distances.

Now, I either just made one helluva good case, or, it was some kinda "baffle 'em with bullshit" story!
Sounded pretty good.
Link Posted: 1/22/2002 10:05:20 AM EDT

Originally Posted By BusMaster007:

Longer bbl. provides more velocity in this case, which at longer ranges contributes to better accuracy due to the sustained velocity.
The transition from super to sub-sonic velocity, which is what begins the gyroscopic destabilization of the projectile, is delayed with the longer bbl., thereby allowing accuracy at extended distances.




Could you elaborate on this please ? It there something about traveling less then the speed of sound that causes bullets to lose accuracy ?
Link Posted: 1/22/2002 10:22:44 AM EDT

Originally Posted By Philadelphia_GunMan:

Originally Posted By BusMaster007:

Longer bbl. provides more velocity in this case, which at longer ranges contributes to better accuracy due to the sustained velocity.
The transition from super to sub-sonic velocity, which is what begins the gyroscopic destabilization of the projectile, is delayed with the longer bbl., thereby allowing accuracy at extended distances.




Could you elaborate on this please ? It there something about traveling less then the speed of sound that causes bullets to lose accuracy ?



No. I'm not programmed to respond in that area.
Truly, I don't know!
It is something I've read about regarding long range shooting.
Particularly re the .308 in the shorter vs. longer bbl.; 600 yds. vs. 1,000 yds.
That's why I said it could be a "baffle 'em"...
I may have been baffled myself!

Perhaps you can elaborate and unbaffle me.
Link Posted: 1/22/2002 10:24:39 AM EDT
Philly-GM,

As the bullet slows from super-sonic speed to sub-sonic a, "gyroscopic destabilization of the projectile" as BM007 states, occurs. Simply put the slowing to sub-sonic disturbs the bullet enough to cause a loss of accuracy.

For example: I shoot 77SMKs at 2850fps, loaded mag length. At 800 and even at 900 (which we don't shoot) this bullet is moving above the speed of sound and it will hold 0.5 MOA accuracy (given good wind conditions and my ability to hold/guage the wind drift). However this bullet goes sub-sonic between 900 and 1000 yards and no matter how well I hold or how light the wind this momentary disruption makes MOA (10" groups at 1000yds) darn near impossible. Granted the guy I shoot with and mentor can "in good air" shoot sub-moa at a 1000 with this load, I have yet to reach that ability. Probably has something to do w/ the fact he's older and been shooting thousands of rounds a week for more than 30 years and is able to Hold and Dope the wind better than I.

There are others here who can provide more technical language for this occurance, but that's my take on your question.

Mike
Link Posted: 1/22/2002 10:32:18 AM EDT
Glad to see this thread. I have been debating between a Bushmaster 24 inch , 20 inch, or 16 inch V-match. I dont think that the 24 inch is necessarily going to give more accuracy, as you would most likely be using a scope anyway so iron sights wont be a factor. But I can see where it would give you more velocity extending the range a bit. I think what BusMaster said makes sense regarding the bullet stabilization. Question is, how much more velocity will the 24 inch barrel give over a 20 inch.
One thing that I have found out is that the 16 inch V-match has virtually the same accuracy as the 20 inch (per Bushmaster) because the shorter barrel is stiffer and has less flex to it. The longer you go the more chance you have for flex.
Link Posted: 1/22/2002 10:45:19 AM EDT
Read a lot on this super vs subsonic stuff. As for 20 vs 24 I opt for 24 for a varmint rifle, most do I think, 20 for a competition and 16 for a CQB rifle. I'm no self proclaimed expert. My 40X has a 26" tube AR 10-T a 24" Ar15 NM 20" CQB 16". At 100 yds, in good wind, I can't tell the differnce. Probably why what you are going to do with the rifle dictates its features. My understanding of what I have read has made me spend my $$ this way. There is a point, I have read, where barrel length stops making an accuracy, velocity difference. I'm not baffeled, but could be full of s%#t.
Link Posted: 1/22/2002 10:50:23 AM EDT
Hawkeye asks "Question is, how much more velocity will the 24 inch barrel give over a 20 inch?"

50-60 fps. according to Bushmaster chart.

Mike
Link Posted: 1/22/2002 11:00:38 AM EDT
Thanks for the great answers, guys. I may have gotten more info than I can assimilate. Maybe I'm baffled and full of it.
Link Posted: 1/22/2002 11:00:48 AM EDT

Originally Posted By mr_wilson:
Hawkeye asks "Question is, how much more velocity will the 24 inch barrel give over a 20 inch?"

50-60 fps. according to Bushmaster chart.

Mike



If thats the only difference, then I guess a 20 inch will do for me. I dont plan on using it past 300 to 400 meters. I wouldnt ever have the chance or need to utilize the extra range of the 24 inch.
Link Posted: 1/22/2002 11:20:30 AM EDT
Geez, I spent about an hour looking for something to back up my bullsh--, I mean, statements based on what I thought was correct.

What I found was some good information on snipercountry.com.

The link to the article:
www.riflebarrels.com/twistii.htm

...and a quote from it to help get the idea across:



If a bullet is fired from a barrel that has no rifling, the force exerted on the nose of the bullet by the air will cause it to flip over and its trajectory will be wild.

To overcome this tendency, a bullet is spin-stabilized by rotating it about its long axis. The amount of spin required is determined by the specific gravity, shape and construction of the bullet, bullet velocity, and the density of the air it is going to travel through.

It is interesting to note that this force, or overturning moment, acting on the bullet nose is greatest when the bullet first exits the barrel. If a certain bullet-barrel combination will stabilize a bullet at the muzzle, the bullet will remain stable for the rest of its flight. Why is this so? As a bullet flies on toward the target and beyond, it is losing velocity rapidly as any trajectory table will show. For example a 68 grain bullet from a 6PPC leaving the barrel at 3150 fps will be going 2786 fps at 100 yards and 2449 fps at 200 yards (ballistic coefficient of .265 and standard metro conditions). The rotational speed of the bullet or its RPM's decreases at a much slower rate. The overall result is a lessening force acting on the bullet nose, and that force is being overcome by a proportionately greater spin rate compared to forward velocity. The down-range bullet therefore is more stable than it was at the muzzle. The only exception to this occurs when the bullet passes through the speed of sound. At that velocity, about 1180 fps, it may lose stability.


I wouldn't spout off in an attempt to baffle anyone.
If I'm ignorant of facts and discover something to expose that, I'll admit my ignorance.
Better to do that, than remain a fool.

I hope this is good information for your bbl. length choice.
Look into the snipercountry.com forums for great info on bbl. length/rigidity (OK, you perverts, knock it off!); twist rates; muzzle BRAKES; etc.
Good stuff.

My Bushmaster V-MATCH A3 upper is the standard version, 24" bbl., which at present is the alternate upper for the original A2 Dissipator.

I would someday consider an A3 20" pre-ban upper just to have for the collection.
A new post-ban lower is on the list first, to be the other component in the V-MATCH equation.
Two rifles are more fun than one!




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