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Can the stock ACR barrel actually be a 1:8 and not a 1:9?
7/10/2011 10:52:38 AM
I recently read an interesting thread where the OP concluded that the stock ACR barrel is actually a 1:8 and not a 1:9 despite the barrel stamping of 1:9 and BM stating it is a 1:9.
Not saying the OP's findings are wrong but I was hoping someone here may have the tools and know how to follow up this claim and give their findings as well.
Not saying the OP's findings are wrong but I was hoping someone here may have the tools and know how to follow up this claim and give their findings as well.
7/10/2011 11:07:21 AM
IF someone showed me proof their barrel was 1/8 I would believe them.
7/10/2011 11:30:23 AM
Originally Posted By rotorblade:
IF someone showed me proof their barrel was 1/8 I would believe them.
IF someone showed me proof their barrel was 1/8 I would believe them.
I would prefer to see various people, using proven tools / techniques for testing a barrels twist rate come to the same conclusion that the stock ACR barrel is truly a 1:8 and not a 1:9 before concluding that BM is wrong based on one persons findings through using a technique I don't know to be reliable or unreliable.
No offense to the OP of the thread I am referencing, he did a great write up. I just would like to see other people test it as we have many knowledgeable and skilled people on this site.
I admit I find it hard to believe that BM is stamping 1:8 barrels as 1:9's and marketing a weapon as a 1:9 when it is truly a 1:8.
Fair?
7/10/2011 1:30:01 PM
That could explain people's success using heavier bullet weights in their ACRs.
7/10/2011 2:40:06 PM
Originally Posted By WarthogA5:
That could explain people's success using heavier bullet weights in their ACRs.
That could explain people's success using heavier bullet weights in their ACRs.
I am one of those people. I have successfully shot 75g and 77g ammo out to 300 yds with my ACR.
That's why this theory intrigues me.
7/10/2011 3:03:31 PM
They'd have to be almost certifiable to NOT be hyping the fact that they have a 1:8 barrel, if they actually did.
7/10/2011 6:04:50 PM
Originally Posted By Gamma762:
They'd have to be almost certifiable to NOT be hyping the fact that they have a 1:8 barrel, if they actually did.
They'd have to be almost certifiable to NOT be hyping the fact that they have a 1:8 barrel, if they actually did.
+87
7/10/2011 6:21:35 PM
When Spike's Tactical did a limited run of fluted ACR barrels, they were 1/8.
I suppose an accuracy/ballistics comparison between one of those and a stock ACR barrel is in order, no?
I suppose an accuracy/ballistics comparison between one of those and a stock ACR barrel is in order, no?
7/10/2011 6:58:44 PM
There has GOT to be some way to simply measure this and post results...I just do not know it...lol
7/10/2011 9:40:58 PM
Easy to do if you have a tipton or dewey cleaning rod or one of the others that has the rod handle on ball bearings.
Insert rod with a fat (tight) patch into the barrel.
Pull it out a little, maybe 3 or 4"... then mark a spot on the rod thats flush with the end of the flash hider with some white-out.
Continue slowly pulling the rod out until the spot has made one, complete, 360 deg. revolution.
Then measure from the mark to the end of the flash hider... it should be damn close to 9" if the barrel is marked correctly.
You can do this to any barrel to determine/confirm the twist.
-ZA
Insert rod with a fat (tight) patch into the barrel.
Pull it out a little, maybe 3 or 4"... then mark a spot on the rod thats flush with the end of the flash hider with some white-out.
Continue slowly pulling the rod out until the spot has made one, complete, 360 deg. revolution.
Then measure from the mark to the end of the flash hider... it should be damn close to 9" if the barrel is marked correctly.
You can do this to any barrel to determine/confirm the twist.
-ZA
7/10/2011 10:17:30 PM
This is actually fairly common and totally explainable. Most 1/9 AR barrels, of which the ACR uses, can actually have a twist rate which is slightly faster (1/8.2 - 1/8.7). This explains why some 1/9 stamped AR barrels can successfully handle heavier (75/77 gr) projectiles. Since the ACR is using these same barrels the fact that some of them can handle heavier bullets is not a surprise.
7/11/2011 2:13:12 PM
So, since the ACR has a 16" barrel, it if is a 1:8 wouldn't inserting something marked on a rod that will spin produce 2 visible rotations from barrel entrance to barrel exit?
I was hoping for a more scientific method, someone has to have tested a barrels twist rate before using something other than a patch and rod.
I was hoping for a more scientific method, someone has to have tested a barrels twist rate before using something other than a patch and rod.
7/11/2011 9:44:18 PM
Originally Posted By DeltaBravo555:
So, since the ACR has a 16" barrel, it if is a 1:8 wouldn't inserting something marked on a rod that will spin produce 2 visible rotations from barrel entrance to barrel exit?
I was hoping for a more scientific method, someone has to have tested a barrels twist rate before using something other than a patch and rod.
So, since the ACR has a 16" barrel, it if is a 1:8 wouldn't inserting something marked on a rod that will spin produce 2 visible rotations from barrel entrance to barrel exit?
I was hoping for a more scientific method, someone has to have tested a barrels twist rate before using something other than a patch and rod.
That method works... why don't you try it out and see. The patches lock in the lands and grooves and rotate without slipping if the patch is nice
and tight. Why make the experiment harder than it has to be?
-ZA
7/12/2011 9:05:25 AM
Originally Posted By ZA206:
That method works... why don't you try it out and see. The patches lock in the lands and grooves and rotate without slipping if the patch is nice
and tight. Why make the experiment harder than it has to be?
-ZA
Originally Posted By DeltaBravo555:
So, since the ACR has a 16" barrel, it if is a 1:8 wouldn't inserting something marked on a rod that will spin produce 2 visible rotations from barrel entrance to barrel exit?
I was hoping for a more scientific method, someone has to have tested a barrels twist rate before using something other than a patch and rod.
So, since the ACR has a 16" barrel, it if is a 1:8 wouldn't inserting something marked on a rod that will spin produce 2 visible rotations from barrel entrance to barrel exit?
I was hoping for a more scientific method, someone has to have tested a barrels twist rate before using something other than a patch and rod.
That method works... why don't you try it out and see. The patches lock in the lands and grooves and rotate without slipping if the patch is nice
and tight. Why make the experiment harder than it has to be?
-ZA
I see your point but the way I see it, some random guy shouldn't be able to sit in his living room with a rod and patch and be able to prove that BM not only stamped their barrels wrong but also marketed their weapon wrong.
Being that I posted this in the BM subforum, I was hoping that someone from BM would post up about this or maybe test a couple of their barrels.
With that said, if that method works, I will try it and post my findings.
7/12/2011 10:19:28 AM
Just remember the ACR barrl is 16.5" not 16".
7/12/2011 11:45:24 AM
Originally Posted By GreenM4Guy:
Just remember the ACR barrl is 16.5" not 16".
Just remember the ACR barrl is 16.5" not 16".
Thanks, good catch. I saw a spec sheet that listed it as an even 16" and never double checked it.
I am surprised no one else here is interested enough to check theirs and no comment from BM either.
7/12/2011 11:50:12 AM
Originally Posted By DeltaBravo555:
Thanks, good catch. I saw a spec sheet that listed it as an even 16" and never double checked it.
I am surprised no one else here is interested enough to check theirs and no comment from BM either.
Originally Posted By GreenM4Guy:
Just remember the ACR barrl is 16.5" not 16".
Just remember the ACR barrl is 16.5" not 16".
Thanks, good catch. I saw a spec sheet that listed it as an even 16" and never double checked it.
I am surprised no one else here is interested enough to check theirs and no comment from BM either.
I really don't care. Mine shoots 75's & 77's just fine (handloads suppressed & unsuppressed), so that's really
all that matters to me.
-ZA
7/12/2011 7:36:42 PM
From http://www.futurefirepower.com/myths-about-the-nato-556-cartridge :
And from http://anarchangel.blogspot.com/2007/01/stabilization-mythology.html :
And from http://ammo.ar15.com/ammo/project/perf_whattwist.html :
For those of you that don’t want to read that much, let me break it down. Bullet weight is not directly what requires twist rate. Bullet length is. Given, the only way you’re going to add weight to a 5.56 bullet is to change the shape or the length. Anyway, the 1:9 is pretty much your “do-anything” twist rate. The military uses 1:7 because of their use of tracers (and steel core penetrators, as steel is lighter than lead, you have to have a longer bullet). Furthermore, they use 1:7 for stabilizing these rounds at distances OVER 500 YARDS.
Now why do most people say they want to use a nice, heavy 70+ grain round? Self-defense right? The difference in twist rate won’t be evident until 500 yards. That’s over a ¼ mile. I see no potential scenario for a legitimate defensive shoot at ¼ of a mile. Police snipers won’t operate at those distances. And if you’re going to say for SHTF where you’re not worried about legally justifying a shoot, I’d recommend a better plan than announcing your position while trying for ¼ mile shots (or you're just beggnig for a reason to shoot someone and you need to see a professional).
So, unless you’re doing long range matches (and the ACR isn’t a match grade rifle), 1:9 will do everything you need it to. If you think 1:7 is the end-all-be-all solution to the universe, you need to put the laptop away and get outside and shoot. Just being “mil-spec” doesn’t make something magical.
I’m sorry for being logical…feel free to commence flaming.
But when tested in frigid Alaska, accuracy was decreased because of the increased friction from the denser, colder air. Therefore, the barrel twist was eventually increased from 1:12 to 1:9 and eventually to the 1:7 you see it today. Although some bull-barreled AR-15’s and Stoner Sniper Rifles can be found in a 1:9, most issued M-16’s and M-4;s are primarily a 1:7 twist.
This change increased the accuracy of the 5.56 round out past 500 meters…
This change increased the accuracy of the 5.56 round out past 500 meters…
And from http://anarchangel.blogspot.com/2007/01/stabilization-mythology.html :
The first common myth about stabilization, is that the heavier a bullet is, the faster it must spin to be stabilized. In fact this isn't really true, heavier objects gyroscopically stablize at lower rotational velocities than lighter objects (the flywheel effect); and though the aerodynamic stabilization componenst required for heavier objects are greater in magnitude than for lighter objects, the differences in weight and relative difference between force components, between different examples of the same diameter bullet loaded for the same cartridge, are generally small enough that the aerodynamic component of the stabilizing forces required do not change significantly.
The issue with bullet stabilization is actually length not weight; but because the diameter of the bullet is fixed (we are after all talking about different bullets in the same caliber), there are really only three things which generally change the weight:
1. Profile: if the bullet is less tapered, then it will be heavier for a given length, but generally less ballistically efficient (though not always).
2. Construction: If the bullet is solid copper it will weigh less for a given length than a jacketed lead (the 37gr copper solid varmint bullets for example, are the same length as 45gr jacketed lead bullets). If the bullet is a tracer, or steel penetrator type armor piercing it will also weigh less than a solid lead jacketed bullet for a given length. Also if a bullet is hollow (or partially hollow such as some MilSurp .303 or 7.62r loads), it will obviously weigh less for a given length.
3. Length: The longer the bullet, presuming construction and profile remain the same, the heavier it will be
Generally, this means that a change in length is the same as a change in weight; and since bullet length isn't commonly discussed or published; and because weight is a more important component in interior ballistics, we mostly refer to different bullets and loadings by their weight.
The issue with bullet stabilization is actually length not weight; but because the diameter of the bullet is fixed (we are after all talking about different bullets in the same caliber), there are really only three things which generally change the weight:
1. Profile: if the bullet is less tapered, then it will be heavier for a given length, but generally less ballistically efficient (though not always).
2. Construction: If the bullet is solid copper it will weigh less for a given length than a jacketed lead (the 37gr copper solid varmint bullets for example, are the same length as 45gr jacketed lead bullets). If the bullet is a tracer, or steel penetrator type armor piercing it will also weigh less than a solid lead jacketed bullet for a given length. Also if a bullet is hollow (or partially hollow such as some MilSurp .303 or 7.62r loads), it will obviously weigh less for a given length.
3. Length: The longer the bullet, presuming construction and profile remain the same, the heavier it will be
Generally, this means that a change in length is the same as a change in weight; and since bullet length isn't commonly discussed or published; and because weight is a more important component in interior ballistics, we mostly refer to different bullets and loadings by their weight.
And from http://ammo.ar15.com/ammo/project/perf_whattwist.html :
The 1:7 twist is used by the military to stabilize the super-long L-110/M856 tracer bullet out to 800 yards, but unless your plans include shooting a significant amount of M856, the 1:9 twist rate is better suited for general use.
For those of you that don’t want to read that much, let me break it down. Bullet weight is not directly what requires twist rate. Bullet length is. Given, the only way you’re going to add weight to a 5.56 bullet is to change the shape or the length. Anyway, the 1:9 is pretty much your “do-anything” twist rate. The military uses 1:7 because of their use of tracers (and steel core penetrators, as steel is lighter than lead, you have to have a longer bullet). Furthermore, they use 1:7 for stabilizing these rounds at distances OVER 500 YARDS.
Now why do most people say they want to use a nice, heavy 70+ grain round? Self-defense right? The difference in twist rate won’t be evident until 500 yards. That’s over a ¼ mile. I see no potential scenario for a legitimate defensive shoot at ¼ of a mile. Police snipers won’t operate at those distances. And if you’re going to say for SHTF where you’re not worried about legally justifying a shoot, I’d recommend a better plan than announcing your position while trying for ¼ mile shots (or you're just beggnig for a reason to shoot someone and you need to see a professional).
So, unless you’re doing long range matches (and the ACR isn’t a match grade rifle), 1:9 will do everything you need it to. If you think 1:7 is the end-all-be-all solution to the universe, you need to put the laptop away and get outside and shoot. Just being “mil-spec” doesn’t make something magical.
I’m sorry for being logical…feel free to commence flaming.
7/12/2011 7:53:44 PM
Originally Posted By BuddyChryst:
From http://www.futurefirepower.com/myths-about-the-nato-556-cartridge :
And from http://anarchangel.blogspot.com/2007/01/stabilization-mythology.html :
And from http://ammo.ar15.com/ammo/project/perf_whattwist.html :
For those of you that don’t want to read that much, let me break it down. Bullet weight is not directly what requires twist rate. Bullet length is. Given, the only way you’re going to add weight to a 5.56 bullet is to change the shape or the length. Anyway, the 1:9 is pretty much your “do-anything” twist rate. The military uses 1:7 because of their use of tracers (and steel core penetrators, as steel is lighter than lead, you have to have a longer bullet). Furthermore, they use 1:7 for stabilizing these rounds at distances OVER 500 YARDS.
Now why do most people say they want to use a nice, heavy 70+ grain round? Self-defense right? The difference in twist rate won’t be evident until 500 yards. That’s over a ¼ mile. I see no potential scenario for a legitimate defensive shoot at ¼ of a mile. Police snipers won’t operate at those distances. And if you’re going to say for SHTF where you’re not worried about legally justifying a shoot, I’d recommend a better plan than announcing your position while trying for ¼ mile shots (or you're just beggnig for a reason to shoot someone and you need to see a professional).
So, unless you’re doing long range matches (and the ACR isn’t a match grade rifle), 1:9 will do everything you need it to. If you think 1:7 is the end-all-be-all solution to the universe, you need to put the laptop away and get outside and shoot. Just being “mil-spec” doesn’t make something magical.
I’m sorry for being logical…feel free to commence flaming.
From http://www.futurefirepower.com/myths-about-the-nato-556-cartridge :
But when tested in frigid Alaska, accuracy was decreased because of the increased friction from the denser, colder air. Therefore, the barrel twist was eventually increased from 1:12 to 1:9 and eventually to the 1:7 you see it today. Although some bull-barreled AR-15’s and Stoner Sniper Rifles can be found in a 1:9, most issued M-16’s and M-4;s are primarily a 1:7 twist.
This change increased the accuracy of the 5.56 round out past 500 meters…
This change increased the accuracy of the 5.56 round out past 500 meters…
And from http://anarchangel.blogspot.com/2007/01/stabilization-mythology.html :
The first common myth about stabilization, is that the heavier a bullet is, the faster it must spin to be stabilized. In fact this isn't really true, heavier objects gyroscopically stablize at lower rotational velocities than lighter objects (the flywheel effect); and though the aerodynamic stabilization componenst required for heavier objects are greater in magnitude than for lighter objects, the differences in weight and relative difference between force components, between different examples of the same diameter bullet loaded for the same cartridge, are generally small enough that the aerodynamic component of the stabilizing forces required do not change significantly.
The issue with bullet stabilization is actually length not weight; but because the diameter of the bullet is fixed (we are after all talking about different bullets in the same caliber), there are really only three things which generally change the weight:
1. Profile: if the bullet is less tapered, then it will be heavier for a given length, but generally less ballistically efficient (though not always).
2. Construction: If the bullet is solid copper it will weigh less for a given length than a jacketed lead (the 37gr copper solid varmint bullets for example, are the same length as 45gr jacketed lead bullets). If the bullet is a tracer, or steel penetrator type armor piercing it will also weigh less than a solid lead jacketed bullet for a given length. Also if a bullet is hollow (or partially hollow such as some MilSurp .303 or 7.62r loads), it will obviously weigh less for a given length.
3. Length: The longer the bullet, presuming construction and profile remain the same, the heavier it will be
Generally, this means that a change in length is the same as a change in weight; and since bullet length isn't commonly discussed or published; and because weight is a more important component in interior ballistics, we mostly refer to different bullets and loadings by their weight.
The issue with bullet stabilization is actually length not weight; but because the diameter of the bullet is fixed (we are after all talking about different bullets in the same caliber), there are really only three things which generally change the weight:
1. Profile: if the bullet is less tapered, then it will be heavier for a given length, but generally less ballistically efficient (though not always).
2. Construction: If the bullet is solid copper it will weigh less for a given length than a jacketed lead (the 37gr copper solid varmint bullets for example, are the same length as 45gr jacketed lead bullets). If the bullet is a tracer, or steel penetrator type armor piercing it will also weigh less than a solid lead jacketed bullet for a given length. Also if a bullet is hollow (or partially hollow such as some MilSurp .303 or 7.62r loads), it will obviously weigh less for a given length.
3. Length: The longer the bullet, presuming construction and profile remain the same, the heavier it will be
Generally, this means that a change in length is the same as a change in weight; and since bullet length isn't commonly discussed or published; and because weight is a more important component in interior ballistics, we mostly refer to different bullets and loadings by their weight.
And from http://ammo.ar15.com/ammo/project/perf_whattwist.html :
The 1:7 twist is used by the military to stabilize the super-long L-110/M856 tracer bullet out to 800 yards, but unless your plans include shooting a significant amount of M856, the 1:9 twist rate is better suited for general use.
For those of you that don’t want to read that much, let me break it down. Bullet weight is not directly what requires twist rate. Bullet length is. Given, the only way you’re going to add weight to a 5.56 bullet is to change the shape or the length. Anyway, the 1:9 is pretty much your “do-anything” twist rate. The military uses 1:7 because of their use of tracers (and steel core penetrators, as steel is lighter than lead, you have to have a longer bullet). Furthermore, they use 1:7 for stabilizing these rounds at distances OVER 500 YARDS.
Now why do most people say they want to use a nice, heavy 70+ grain round? Self-defense right? The difference in twist rate won’t be evident until 500 yards. That’s over a ¼ mile. I see no potential scenario for a legitimate defensive shoot at ¼ of a mile. Police snipers won’t operate at those distances. And if you’re going to say for SHTF where you’re not worried about legally justifying a shoot, I’d recommend a better plan than announcing your position while trying for ¼ mile shots (or you're just beggnig for a reason to shoot someone and you need to see a professional).
So, unless you’re doing long range matches (and the ACR isn’t a match grade rifle), 1:9 will do everything you need it to. If you think 1:7 is the end-all-be-all solution to the universe, you need to put the laptop away and get outside and shoot. Just being “mil-spec” doesn’t make something magical.
I’m sorry for being logical…feel free to commence flaming.
I read your entire post but missed the part where you tested your ACR's barrel twist rate.
I shoot mostly 55g 193 so the 1:9 was not an issue for me when buying an ACR even though 1:7 is the current preferred cool kids kit to have in any online discussion.
I have shot issued M16A2's at 500yds, I know what they are capable of and I also know firsthand that whatever barrel I have on this ACR will shoot 55g-77g ammo out to 300yds, haven't gone over that yet.
I simply wanted to know if ACR's have 1:9 barrels as stamped / marketed by BM or if they are 1:8 as speculated.
I didn't know a proven way of checking so I asked here, in the BM section.
No worries, I will keep it to myself.
7/12/2011 7:57:52 PM
Isn't it a known fact that heavier loads are more accurate and stabilized out of a 1/7 barrel than a 1/9 barrel? Thus, if the 1/7 is better at this than 1/9, and 55gr and 62gr bullets come out just fine from a 1/7 as they would a 1/9, given the extra stability/accuracy guaranteed out of a 1/7 twist barrel, wouldn't this make the 1/7 twist the "do-anything" twist rate, since it offers stabilization and accuracy over a wider/broader range of ammo types available?
7/12/2011 9:28:19 PM
Originally Posted By Plissken:
Isn't it a known fact that heavier loads are more accurate and stabilized out of a 1/7 barrel than a 1/9 barrel? Thus, if the 1/7 is better at this than 1/9, and 55gr and 62gr bullets come out just fine from a 1/7 as they would a 1/9, given the extra stability/accuracy guaranteed out of a 1/7 twist barrel, wouldn't this make the 1/7 twist the "do-anything" twist rate, since it offers stabilization and accuracy over a wider/broader range of ammo types available?
Isn't it a known fact that heavier loads are more accurate and stabilized out of a 1/7 barrel than a 1/9 barrel? Thus, if the 1/7 is better at this than 1/9, and 55gr and 62gr bullets come out just fine from a 1/7 as they would a 1/9, given the extra stability/accuracy guaranteed out of a 1/7 twist barrel, wouldn't this make the 1/7 twist the "do-anything" twist rate, since it offers stabilization and accuracy over a wider/broader range of ammo types available?
IIRC, 1:7 is too fast a twist for the 45g and lighter varmit bullets b/c it can spin the jackets off in mid-flight.
So, I'd say that if a 1:9 stabilizes 77's and 75's (the upper end of what most people shoot), then it would be
a better "all around" twist b/c it won't spin the jackets off of the light varmit bullets.
-ZA
7/12/2011 9:43:48 PM
I remember Travis Haley saying he prefers 45gr for CQB to avoid over-penetration. I wonder which twist rate he prefers.
I also wonder where/how 1/8 comes into play.
I also wonder where/how 1/8 comes into play.
