Advantages of a 1/8 5R barrel?

[9x19]

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.

 

[Art Eatman]

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.

 

[Brian Pfleuger]

The only advantage of a faster twist is to stabilize heavier bullets. Otherwise, it's a disadvantage.

 

[jimbob86]

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?

 

[Brian Pfleuger]

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.

 

[zukiphile]

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.

 

[Theohazard]

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.

 

[2damnold4this]

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.

 

[Theohazard]

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.

 

[Brian Pfleuger]

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.

 

[zukiphile]

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.

 

[9x19]

Thanks for the info and links folks...

 

[Justice06RR]

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.

 

[Theohazard]

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.

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).

 

[chris in va]

I'll also point out the 5r Sport rifling is 'gain twist'.

 

[cryogenic419]

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.

 

[Bart B.]

I cannot believe any rifle maker would put a gain twist barrel in one. Maybe they assemble them with gain twist screws, too.

 

[454 Super Redhawk]

I bought a M&P sport a while back it has the 1/8 twist and it is super acurcate

 

[chris in va]

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.

 

[Bart B.]

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?