AR-15 Barrel Question

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CGS1

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I just purchased a new Bushmaster Target model Ar-15, it has the heavy barrel 20" on it(Bigger under the handguard). Cal: 5.56 twist 1-9.
Is the heavier barrel more accurate than the Govt. Profile barrel.
This is the important part,(Dose the weight of the heavier barrel justify the difference in accuracy compared to the Govt. Profile barrel?) I have a new Bushmaster 20" Govt. Profile barrel in my safe that i could switch out.
This will be used for Coyote hunting. I use the Hornady V-Max 55gr. Factory ammunition.
All imput will be appreciated.
 
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rc508pir

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Assuming both have the same accuracy when cold, a thicker bull barrel will stay accurate longer than the gov profile because it takes longer to get hot.

A bull barrel will also take longer to cool down though
 

Shadowrider

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Assuming both have the same accuracy when cold, a thicker bull barrel will stay accurate longer than the gov profile because it takes longer to get hot
^^^ This. A good quality pencil barrel can be a 1/2 MOA barrel for a shot or three. They usually shift POI and open up groups when they heat up. That's why they aren't considered "accurate".
 

ConstitutionCowboy

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According to the theory I have read about, the heaver barrels are less affected by harmonics. Make sure there is no play between your upper and lower receivers or at least have it dampened. Make sure your barrel fits snugly in the upper receiver (No up-and-down or side-to-side play) before you torque the barrel retaining nut. If the space between the barrel and upper receiver can be shimmed, shim it; otherwise, use green Loctite. (Don't use red or you'll have to use a lot of heat to remove the barrel, and blue does not have the 'body' you'll need to fill the gap.)

A lot will depend on what chamber your Bushmaster has, and the twist rate. When your cartridge is chambered, the bigger the space between the ogive of the bullet and the beginning of the rifling the more the bullet will be allow to 'wobble' before engaging the rifling which will then induce erratic flight to the bullet. The heavier - and therefore the longer - a bullet is, the faster the twist rate needs to be. A little research on some of the AR websites will point you in the right direction.

Woody
 

swampratt

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This may not be a fair comparison as I feel a larger diameter and longer bearing surface bullet produces more twist and harmonic or whip into a barrel but here it goes.
My savage axis .223 with pencil barrel shoots tighter groups than my .308 savage heavy barrels have ever shot.
I had a 24" heavy and cut it to 20.5 and increased accuracy and then got an 18" heavy in .308 and it shoots very accurate but the axis pencil barrel .223 in 22" out shoots them.
Talking accuracy to 300+ yards.

Now i do like a short barrel and just because it is short and fat does not mean it will be an accurate barrel.
You must just shoot it and see.

When i got some factory .223 ammo I did some testing with it.
I will see if I can find what i wrote about it.
 

swampratt

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dennishoddy

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When your cartridge is chambered, the bigger the space between the ogive of the bullet and the beginning of the rifling the more the bullet will be allow to 'wobble' before engaging the rifling which will then induce erratic flight to the bullet. The heavier - and therefore the longer - a bullet is, the faster the twist rate needs to be. A little research on some of the AR websites will point you in the right direction.

The used of a contricity tool will greatly reduce the "wobble".

220200.jpg


When looking to buy a new rifle or build one of your own, one of things to look at is rifle twist. This is a characteristic of the barrel that will affect how your bullets perform down range and, to some degree, will dictate what rounds should be fired out of that weapon. Rifle twist is expressed as a ratio. An example would be a 1/7 or 1:7 twist. This means that for every seven inches of barrel, there will be one complete twist or rotation of the rifling, which imparts spin onto the bullet. Without getting too in depth about bullet types, this should explain the basics of how rifle twist influences bullet flight and what a buyer should look for to fit their own needs.

The rifling of the barrel causes a bullet to spin. This spin is directly related to how “tight” or “loose” the rifling in the barrel is. Faster spins are good for longer bullets as they need more spin to stabilize their center of gravity. Often times, shooters choose tighter twist rates for heavier bullets. This can make shooters think that it is the weight of the bullet that requires a certain twist rate. This is not (entirely) true. To make a bullet heavier, the bullet is made longer to add material without increasing caliber. It is this length that needs the twist rate.

Rifling-300x300.jpg
Early M16s used a 1:14 twist rate as they fired light and short bullets. This rate was very limited in what it could do; not even the 55 grain rounds used were spinning fast enough. The military then moved up to 1:12 twist that put a better spin on the bullet. As heavier rounds came out, the twist rate became tighter and tighter. The 1:9, the military found, was good still slow enough for the 55 grain rounds, but it was fast enough for 75 grain bullets as well. As barrels became shorter however, it was noted that even tighter twist rates were needed. This was because a bullet needs at least two rotations in the barrel to become stable. A 16 inch barrel with a 1:9 twist cannot deliver this, so the twist rate was tightened to 1:8. As barrels became even shorter and bullets became heavier, twist rates were again increased. During testing however, it was noted that bullets that twisted too fast could tear themselves apart if the copper jacket had any imperfections. For this reason, the 10.5 inch barrels on AR15s still use a 1:7, sacrificing a full two rotations in order to not over spin the round and cause it to break apart.

With that little history lesson, we can start to see what is needed for our rifle barrels. If you plan to use a rifle for varmint hunting loaded with 45 grain rounds, you are going to want a slow twist rate. If the purpose of the rifle is to have heavy hitting rounds at 300 meters and will be loaded with 70 grain rounds, a twist rate of 1:7 will do better. By matching the proper twist rate to the proper rounds, you can ensure that you will fire stable rounds that will give the most accuracy your rifle has to offer.

https://blog.uspatriottactical.com/rifle-twist-101-basic-understanding/

Bullet RPM Formula just in case you were curious.
Here is a simple formula for calculating bullet RPM:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.

http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/

I'd always heard of bullets coming out of the barrel and then a grey puff, and didn't really believe a bullet could come apart, but they really can. Ultra high velocity rounds only.
 
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lasher

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fanatic accuracy folks also want to load to the "proper" OAL and hornady makes tools to measure the jump (leade) length that is best for that barrel, leade being the distance the bullet jumps prior to engaging the rifling. a short jump is good but you do not want the bullet engaging the lands prior to the primer igniting but if you have say more that 1 barrel then for correct possible OAL on several barrels demanding special length for each barrel and creating a nightmare of keeping track of which rifle gets which length. i have over 10 barrels and just use the standard bump back the shoulder around .003, everything runs well and is accurate to clover leaf and bullets in bullet holes. i ony use the medium weight barrels to handle heat, i don't claim they are more accurate than pencil barrels, i do believe the medium barrels remain more accurate over various time/round count than pencil barrels. all the barrels i have are from one builder (not counting pistols or 22 LR), and all the barrels i used are much more accurate than my current level of skill
 
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