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9/22/2017 12:11:25 AM
Posted: 9/15/2005 5:55:44 PM EDT
[Last Edit: 9/15/2005 6:08:17 PM EDT by C5TT]
How much loss in accuracy do you get at 100 yards when going from 62gr. SS109 to 55gr M193 when using an M4 1:7 twist barrel?
Link Posted: 9/15/2005 7:40:46 PM EDT
[Last Edit: 9/15/2005 7:45:02 PM EDT by JasonC]
Don't hold me to it, but I have heard about 1/2 inch. I would like to know the real answer myself. I usually only shoot 62gr. but when I do shoot 55gr. its mostly just plinking and nothing serious..
Link Posted: 9/15/2005 7:48:01 PM EDT
A Colt HBAR with a 1 in 7 twist from 1989 shoots both 62 and 55 gr bullets very well. It will group 1.5 to 2" all day long with a 3X9 leupold. It does shoot 75 gr Hornadys and AA2520 under an inch though.
Link Posted: 9/15/2005 9:03:37 PM EDT

Originally Posted By CaptSchofield:
A Colt HBAR with a 1 in 7 twist from 1989 shoots both 62 and 55 gr bullets very well. It will group 1.5 to 2" all day long with a 3X9 leupold. It does shoot 75 gr Hornadys and AA2520 under an inch though.



I think what he is asking is what the difference in drop is between a 62gr and 55gr at 100 yds?
Link Posted: 9/15/2005 10:22:35 PM EDT

Originally Posted By JasonC:

Originally Posted By CaptSchofield:
A Colt HBAR with a 1 in 7 twist from 1989 shoots both 62 and 55 gr bullets very well. It will group 1.5 to 2" all day long with a 3X9 leupold. It does shoot 75 gr Hornadys and AA2520 under an inch though.



I think what he is asking is what the difference in drop is between a 62gr and 55gr at 100 yds?



Group sizes.
Thanks for the replies. Keep em coming.
Link Posted: 9/16/2005 1:06:03 PM EDT
just depends on the barrel. 1/7 twists are designed to shoot the heavier bullets and might tend to over stabilize lighter weight bullets. I have a 1/7 that shoots the 55 grains as well as the 75gr a-max's that I use. At 100 yds I do not see to much of a difference. I don't see much of a difference until out about 300 or more yards. Just my two cents. RG
Link Posted: 9/16/2005 1:27:13 PM EDT
The 1 in 7 should shoot 55 grainers with no problem. The point of impact will be different though (in regards to elevation).
The 62 grain bullet will stabilize in a 1 in 9, but the military (Colt) went with the 1-7 for an additional reason.
The weight of a projectile, if increased, requires additional "spin" to stabilize it in flight. ALSO, additional LENGTH requires the same.
Pull a new tracer round, and see just how long it is. The 1-7 will stabilize the new tracer.
At first we thought that 1-7 barrels would get roasted super quick with 62gr Ball, but held their own just fine.
It is true that 55 grainers seem to like 1-12 and 1-10 alot, but I would not sweat it.
Link Posted: 9/16/2005 1:37:39 PM EDT
Link Posted: 9/16/2005 6:46:01 PM EDT
Link Posted: 9/17/2005 3:49:23 AM EDT
[Last Edit: 9/17/2005 3:50:34 AM EDT by Yojimbo]
I noticed no accuracy difference between my chrome lined 1/9 and 1/7 barrels when shooting lighter bullets.

I've also shot a lot of 45gr Winchester Valuepak ammo through my 1/7 and it shoots them great. With proper ammo selection a 1/7 barrel will shoot 40gr-77gr+.

I heard the same BS about accuracy loss too but ain't it funny how it usually comes from someone who doesn't even own a 1/7 barrel.
Link Posted: 9/17/2005 8:55:14 AM EDT

Originally Posted By ErzulisBoat:
The weight of a projectile, if increased, requires additional "spin" to stabilize it in flight. ALSO, additional LENGTH requires the same.



According to Hatcher (Hatcher's Notebook, 3rd edition, page 556 - really!), it's the length of the projectile that affects the twist rate. Usually, though, that is directly tied to weight, but weight *isn't* in the calculation for twist rate. Other factors are density of the medium the projectile is fired in (more dense medium requires higher twist rates -- water requires 30x higher twist rates than air) as well as the specific gravity of the material that the projectile is made of.

This is more info than is really required, but I've been reading Hatcher's Notebook and find some of it fascinating. Off-topic trivia: According to Hatcher, the first person to fire a Browning Automatic Rifle in combat in France in 1918 was Val Browning, John Browning's son.
Link Posted: 9/17/2005 9:33:29 AM EDT
I would think should gain a little accuracy going from 62 to 55 grain with a 1/7 barrel. 62 grain ammo is inheratntly less accurate due to the penetrator tip being slightly off compared to an all lead/copper bullet.
Link Posted: 9/17/2005 9:36:07 AM EDT
Link Posted: 9/17/2005 12:27:41 PM EDT
[Last Edit: 9/17/2005 12:28:42 PM EDT by Yojimbo]

Originally Posted By MP5_guy:

Originally Posted By ErzulisBoat:
The weight of a projectile, if increased, requires additional "spin" to stabilize it in flight. ALSO, additional LENGTH requires the same.



According to Hatcher (Hatcher's Notebook, 3rd edition, page 556 - really!), it's the length of the projectile that affects the twist rate. Usually, though, that is directly tied to weight, but weight *isn't* in the calculation for twist rate. Other factors are density of the medium the projectile is fired in (more dense medium requires higher twist rates -- water requires 30x higher twist rates than air) as well as the specific gravity of the material that the projectile is made of.





Yes, it's really the length that's the issue but we can't add extra mass and increase the bullet's width without changing the caliber so naturally a heavier bullet is longer...
Link Posted: 9/17/2005 3:20:47 PM EDT
You can make the one or tail more or less tapered to change length/weight. You can add an air pocket, steel penetrator tip, aluminum core, polymer tip etc. Al tis changes length or weight without changing caliber.
Link Posted: 9/17/2005 3:24:43 PM EDT

Originally Posted By Yojimbo:
Yes, it's really the length that's the issue but we can't add extra mass and increase the bullet's width without changing the caliber so naturally a heavier bullet is longer...



True, but....

Hypothetically, you could use aluminum for a core and still have a copper jacket. A 55 grain bullet with an aluminum core would be much longer (therefore needing a faster twist rate per Hatcher) than a conventional lead-core bullet. That's probably a better example of how the calculation would be used.

Yes, all things being equal (i.e the materials used for the bullet), a heavier bullet in the same caliber will inherently require a faster twist rate because it is longer.
Link Posted: 9/17/2005 5:44:34 PM EDT
Thanks for the info!
Link Posted: 9/17/2005 5:53:23 PM EDT
I believe the actual name of the formula to calculate minimal twist for a projectile is the "Greenhill Formula."
Link Posted: 9/18/2005 9:15:17 AM EDT
You guys are exactly right, in regards to conical projectiles, a weight increase typically will elongate the profile, thus a "faster" twist rate is required to induce stabilization.
On an interesting but relevant side note- a spherical projectile (muzzle loader round ball) additional mass (weight) requires LESS revolutions in the bore to stabilize it!
Example- .32cal = 1-40 / .50cal = 1-77 / .75cal = 1-100 (basic figures)
Since the portion in contact is basically a "ring" the more massive projectile will stabilize with less imparted spin.
So the formulas for conical projectiles MUST take into account length.
Thanks for calling me out on that one!
On a side note- shooting a flintlock on the same day you shoot your tricked out AR is an education in itself.
Link Posted: 9/18/2005 10:48:24 AM EDT
My 14.5" 1/7 shoots a tighter group with XM193 than it will with XM855. Both, with the POA/POI set for XM193 will group minute of man on a ISPC target out to 300 meters. The equipment is up to the task, often times it is the shooter who lets the equipment down. Get some of each, go to the range, and see what the practical accuracy difference is for you. That way if the SHTF in your area you will know where the different ammuntion impacts, using your battle sight zero.
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