The Firing Line Forums

Go Back   The Firing Line Forums > The Hide > The Art of the Rifle: Semi-automatics

Reply
 
Thread Tools
Old June 4, 2014, 06:04 AM   #1
9x19
Senior Member
 
Join Date: October 15, 1998
Location: Sherman, TX USA
Posts: 2,607
Advantages of a 1/8 5R barrel?

I've been thinking of taking advantage of S&W's $100 rebate on their M&P 15s.

Their mid-length MOE version can be found for $875 before the rebate, and it has the 1/8 5R barrel. Is better accuracy with the heavy bullets its only real advantage over the standard 1/9 barrel?

Thanks in advance for any info.
__________________
Make mine lean, mean, and 9x19!
9x19 is offline  
Old June 4, 2014, 09:04 AM   #2
Art Eatman
Staff Lead
 
Join Date: November 13, 1998
Location: Terlingua, TX, USA
Posts: 22,332
SFAIK, accuracy with heavy bullets is pretty much the only reason to have a twist faster than 1:10. So, my usual question has to do with your intended use.

My use is just for varmints and plinking, so target accuracy with heavy bullets is not a factor for my decisions.
__________________
You're from BATFE? Come right in! I use all your fine products!
Art Eatman is offline  
Old June 4, 2014, 09:19 AM   #3
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,343
The only advantage of a faster twist is to stabilize heavier bullets. Otherwise, it's a disadvantage.
__________________
Still happily answering to the call-sign Peetza.
---
The problem, as you so eloquently put it, is choice.
-The Architect
-----
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.
Brian Pfleuger is offline  
Old June 4, 2014, 09:23 AM   #4
jimbob86
Senior Member
 
Join Date: October 4, 2007
Location: All the way to NEBRASKA
Posts: 6,702
Quote:
The only advantage of a faster twist is to stabilize heavier bullets. Otherwise, it's a disadvantage.
The disadvantage being what?

Increased pressures/lower velocity limit?

Overstabilizing?

If you push light, fragile bullets too fast in a tight twist barrel, they'll fly apart from excessive rotational forces ..... but with a carbine length AR, how likely is that?
__________________
TheGolden Rule of Tool Use: "If you don't know what you are doing, DON'T."

http://nefirearm.com/
jimbob86 is offline  
Old June 4, 2014, 10:15 AM   #5
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,343
Not likely, and those disadvantages are small.

BUT the *only* advantage to faster twist is stabilizing heavy bullets. If you don't need that advantage, why accept ANY disadvantage no matter how small?

If you want that gun and that's the twist they've got, no big deal. It's not a deal breaker. If you have choices, get the slowest twist that will stabilize the bullet weights you plan to shoot. No reason not to.
Brian Pfleuger is offline  
Old June 4, 2014, 10:28 AM   #6
zukiphile
Senior Member
 
Join Date: December 13, 2005
Posts: 1,608
Quote:
BUT the *only* advantage to faster twist is stabilizing heavy bullets. If you don't need that advantage, why accept ANY disadvantage no matter how small?
Money. I have some 1/7 barrels because that's what a vendor had at a price I could stomach.

Like 9x19, I see claims about 5R and polygonal rifling advantages and I am intrigued. I see accuracy claims about the 1/8 5R, but don't know enough to be able to rule out that these are just higher quality barrels generally.
zukiphile is offline  
Old June 4, 2014, 10:29 AM   #7
Theohazard
Senior Member
 
Join Date: April 19, 2012
Location: Western WA
Posts: 2,096
Quote:
Originally Posted by jimbob86
The disadvantage being what?
Over-stabilization. Sure, an over-stabilized bullet is much better than an under-stabilized bullet, but it still tends to be less accurate than a properly-stabilized one.

Those S&W Melanited 1/8 5R barrels are accurate. I have an acquaintance who bought a S&W M&P 15 Sport as a cheap truck gun (this was back when they came with Melanited 1/8 5R barrels). He was surprised with its accuracy, so he decided to see what it was fully capable of. He free-floated it and threw on a Nightforce scope he had, and he said it was just as accurate as his Larues.
__________________
0331: "Accuracy by volume."
Theohazard is offline  
Old June 4, 2014, 10:30 AM   #8
2damnold4this
Senior Member
 
Join Date: August 12, 2009
Location: Athens, Georgia
Posts: 1,274
Brian, that's interesting. It seems like I read in the latest issue of American Rifleman, in the article on the new m855a1 ammunition, that a 1/8 or 1/9 twist was more accurate than 1/7 for both m855 and the longer m855a1.


edit to add: The article stated that the only reason 1/7 was selected was to stabilize tracers.
2damnold4this is offline  
Old June 4, 2014, 10:37 AM   #9
Theohazard
Senior Member
 
Join Date: April 19, 2012
Location: Western WA
Posts: 2,096
Here's some great info showing how slower-twist barrels are more accurate with most of the .223/5.56 bullets people shoot:

http://thefiringline.com/forums/showthread.php?t=530927

In my opinion, most people have absolutely no use for a 1:7 and would be better off with a 1:9 or 1:8.
__________________
0331: "Accuracy by volume."
Theohazard is offline  
Old June 4, 2014, 10:38 AM   #10
Brian Pfleuger
Staff
 
Join Date: June 25, 2008
Location: Central, Southern NY, USA
Posts: 18,343
Quote:
Brian, that's interesting. It seems like I read in the latest issue of American Rifleman, in the article on the new m855a1 ammunition, that a 1/8 or 1/9 twist was more accurate than 1/7 for both m855 and the longer m855a1.

I'm not surprised by that, in general terms. Once a bullet is properly stabilized, spinning it faster can only be bad. Well, it could be "neutral", as in no definitive, detectable consequence at all, but if it changes anything it's going to be for the worse.

Specifically with that bullet... can't really say. Having a steel tip and bismuth-tin core pretty much means it will defy most stability calculator assumptions.
__________________
Still happily answering to the call-sign Peetza.
---
The problem, as you so eloquently put it, is choice.
-The Architect
-----
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.
Brian Pfleuger is offline  
Old June 4, 2014, 10:39 AM   #11
zukiphile
Senior Member
 
Join Date: December 13, 2005
Posts: 1,608
2damn, that was an interesting article, and gave me some comfort that with the cheap stuff the occasional fliers from my 1/7 aren't always my fault.

That article made me want a 1/12 pencil barrel and some light .223 varmint ammunition.
zukiphile is offline  
Old June 4, 2014, 11:24 PM   #12
9x19
Senior Member
 
Join Date: October 15, 1998
Location: Sherman, TX USA
Posts: 2,607
Thanks for the info and links folks...
__________________
Make mine lean, mean, and 9x19!
9x19 is offline  
Old June 4, 2014, 11:37 PM   #13
Justice06RR
Senior Member
 
Join Date: December 21, 2010
Location: Central FL
Posts: 1,244
Quote:
Brian, that's interesting. It seems like I read in the latest issue of American Rifleman, in the article on the new m855a1 ammunition, that a 1/8 or 1/9 twist was more accurate than 1/7 for both m855 and the longer m855a1.
From what I've researched and read on other forums like M4C, M855 ammo is not that accurate to begin with . Something like 2-4MOA, acceptable for your basic soldier but not for a sniper or target shooter.

1/8 is a great compromise in between. if you don't plan to run bullets heavier than 69, 1/9 is fine. 70gr and over you want the 1/7. 1/8 is inbetween and will do well for most commercial ammo.

I've read only good things about the Sport's 1:8 5R rifling. The melonite coating is a big plus as well compared to just plain phospate finish. I've owned 2 melonite barrels, one on a Sport and another on a custom Midlength build. Both were excellent shooters.
Justice06RR is offline  
Old June 4, 2014, 11:43 PM   #14
Theohazard
Senior Member
 
Join Date: April 19, 2012
Location: Western WA
Posts: 2,096
Quote:
Originally Posted by Justice06RR
1/8 is a great compromise in between. if you don't plan to run bullets heavier than 69, 1/9 is fine. 70gr and over you want the 1/7. 1/8 is inbetween and will do well for most commercial ammo.
According to the tests in the sticky I linked above, most 1:9 barrels will shoot even heavier bullets than 69 gr., and 1:8 is good for the longest bullets that will fit in an AR-15's magazine.

Quote:
Originally Posted by Justice06RR
I've read only good things about the Sport's 1:8 5R rifling.
About a year ago S&W stopped shipping the Sport with a 1:8 5R barrel and started using a normally-rifled 1:9 barrel (still Melanited though). Unless they switched back to the old 1:8 5R barrels in the last few months, it probably still comes with the normal 1:9 (I haven't seen a brand-new Sport in a few months).
__________________
0331: "Accuracy by volume."
Theohazard is offline  
Old June 4, 2014, 11:44 PM   #15
chris in va
Senior Member
 
Join Date: December 26, 2004
Location: Louisville KY
Posts: 12,492
I'll also point out the 5r Sport rifling is 'gain twist'.
chris in va is offline  
Old June 5, 2014, 05:53 AM   #16
cryogenic419
Senior Member
 
Join Date: March 29, 2009
Location: NW Ohio
Posts: 388
For what its worth I've shot 52, 69, and 77 SMK's through my 1/9 M&P15 out to 200 yards accurately. I've shot 77 and 80 gr SMKS out to 600 yards accurately with a 1/8 RRA NM. I've shot 52, 69, 77 gr SMK's through my 1/7 Colt out to 200 and 300 yards accurately.

The difference between the guns really is the RRA is a 20" barrel, the other 2 are 16". I'm confident I could get the Colt out to 5-600 yards I just haven't sat down and actually tried with it yet.

Is a 1/8 5R more accurate than others? Possibly. Accurate at 100? 200? Further than that? Depending on how far you plan on shooting. I think ammo selection and quality is going to play a big role if you are looking for accuracy further out.

If you like the other features of the gun and its a good deal, go for it.
cryogenic419 is offline  
Old June 5, 2014, 06:02 AM   #17
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,395
I cannot believe any rifle maker would put a gain twist barrel in one. Maybe they assemble them with gain twist screws, too.
__________________
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master
Bart B. is online now  
Old June 5, 2014, 08:45 AM   #18
454 Super Redhawk
Member
 
Join Date: September 4, 2008
Location: Greenville, NC
Posts: 65
I bought a M&P sport a while back it has the 1/8 twist and it is super acurcate
454 Super Redhawk is offline  
Old June 6, 2014, 02:09 PM   #19
chris in va
Senior Member
 
Join Date: December 26, 2004
Location: Louisville KY
Posts: 12,492
Quote:
I cannot believe any rifle maker would put a gain twist barrel in one. Maybe they assemble them with gain twist screws, too.
I didn't believe it either until I called S&W CS. They are Thompson/Center barrels.
chris in va is offline  
Old June 6, 2014, 03:43 PM   #20
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,395
Their marketing folks must have been trained by the same school Winchester's ones went to. Winchester says their barrels are made stiffer by fluting them. Does S&W claim gain-twist barrels are more accurate than constant-twist ones?
__________________
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master
Bart B. is online now  
Old June 6, 2014, 03:50 PM   #21
Jim Watson
Senior Member
 
Join Date: October 25, 2001
Location: Alabama
Posts: 10,901
Harry Pope thought so.

Bartlein will let you make up your own mind in modern calibers.

S&W/TC factory gain twist barrels? Where does S&W or TC say so?

Brian, I think they gave up on the bismuth, the current M855A1 bullet is just steel and gilding metal.
Jim Watson is offline  
Old June 6, 2014, 05:02 PM   #22
9x19
Senior Member
 
Join Date: October 15, 1998
Location: Sherman, TX USA
Posts: 2,607
The 1/8 being gain-twist is news to me as well... I'll have to ask to pull the bolt and look next time I'm at the LGS.

At my level of skill, and from an AR, I doubt I'd notice any accuracy improvement from the gain-twist style of rifling.
__________________
Make mine lean, mean, and 9x19!
9x19 is offline  
Old June 6, 2014, 10:31 PM   #23
Jim Watson
Senior Member
 
Join Date: October 25, 2001
Location: Alabama
Posts: 10,901
I don't know if anybody would.
Harry Pope liked it but Schalk, Schoyen, and Petersen didn't, and they were no slouches at making lead bullet rifles.

The company that started out as the modern Gain Twist Rifle Barrel Co. was not even making gain twist barrels when they were bought out.

There is a guy in Canada making nice ones for the old calibers and, as I said, Bartlein will give you any twist any gain you want.
Jim Watson is offline  
Old June 7, 2014, 10:16 AM   #24
Jimro
Senior Member
 
Join Date: October 18, 2006
Posts: 5,441
That article in American Rifleman was by Maj (ret) John Plaster, where he states that the 1/7 twist will give roughly twice the size of groups as a 1/9 twist. But you have to dig for the actual numbers, which are 1.6 moa and 0.9 moa with M855A1. I really didn't care for the article as it was a lot of hype and not a lot of analysis, specifically I can point you to DTIC reports where increased terminal ballistics were considered as part of the Army's "Green Bullet" program as far back as 2005 when a Canadian projectile was considered.

Considering the accuracy standard for M855A1 is the same as M855, which is roughly 4 MOA from an M4 rifle, I'd say that arguing over twist rate is really picking hairs.

When you shoot MM855A1 through a 1/8 twist barrel, as the AMU did, it performs accuracy wise quite similar to match grade ammo, at least in the lots that were tested.

Jimro
__________________
"Gorsh" said Goofy as secondary explosions racked the beaten zone, "Did I do that?"

http://randomthoughtsandguns.blogspot.com/
Jimro is offline  
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 11:26 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
This site and contents, including all posts, Copyright © 1998-2014 S.W.A.T. Magazine
Copyright Complaints: Please direct DMCA Takedown Notices to the registered agent: thefiringline.com
Contact Us
Page generated in 0.12541 seconds with 7 queries