The spin on .223 twist rates: 1:7? 1:9? One-in-whatever? What's the real dope on the subject of "stabilizing influences?".
Whether we're talking .223 barrels with a 1:7 or 1:9 twist, both are a far cry from the MI6's original 1:14 or its replacement, the 1:12. With the popularity of both the AR-15 and the .223 cartridge, there is a wide variation of available bullet weights in both loaded ammunition and components. The question I had was this: How much difference does twist rate actually make? This would require a test of not just different barrel twists, but bullet weights.
A brief primer on .223 barrel twist rates: Faster twists are generally required to stabilize longer, heavier bullets. The original M16 1:14 twist was designed to barely stabilize a 55-grain bullet. It worked fine in certain environments but not in others, which was why it was replaced with a 1:12 in the M16A1. When the military switched from a 55-grain bullet to a noticeably longer 62-grain projectile (the M855), they increased the twist rate to 1:7. The faster the twist rate of a barrel, the more resistance the bullet encounters while traveling down it. This can result in higher pressures and/or higher velocities, depending on the gun and barrel length. It will also speed up the wear on your barrel, but most people won't shoot enough to notice. Instead of accuracy starting to degrade after 15,000 rounds, groups from a 1:7 barrel might start expanding after only 10,000 rounds.
The lion's share of .223 barrels available today--whether we're talking bolt-action rifles or semiautos--have either 1:7 or 1:9 twists. To do some testing, I acquired two rifles from Savage, both Model 12 Varmints with laminated stocks and 26-inch fluted stainless barrels. They are identical except for twist--one has a 1:7 barrel, the other has a 1:9. As a huge number of .223s in this country are fired out of ARs, I also acquired a rifle for testing from Alexander Arms. The first has a 20-inch fluted stainless barrel with a 1:9 twist. The company also sent me a second top end with a 16-inch fluted stainless barrel with a 1:7 twist.
Upon first opening the box of the first Savage, I realized I've been spending too much time with black rifles. The Savage was simply beautiful, a piece of functional art. The grain of the stock was gorgeous, and the stock on the second rifle was just as pretty. Bolt action was smooth with very little play. Trigger pulls on both bolt guns were excellent, due to the Savage Accu-Trigger system. Crisp and just a hair over two pounds, they allowed me to shoot up to the rifles' potential--well, minus the ultrasonic skeletal vibration I've been nurturing with my Diet Coke LV. drip.
Alexander Arms just recently started making AR-15s in conventional calibers, having made their bones with oddball ones like the .50 Beowulf and 6.5 Grendel, as well as their newest venture, ARs in .17 HMR. The rifle and spare top end I acquired for testing are actually chambered in 5.56 NATO, which works fine for .223. What's the difference? Dimensionally, none, but the 5.56 is loaded to slightly higher pressures. 'The rifle was equipped with AA's standard "tactical trigger," a single-stage design that provides a crisp, 4 1/2-pound pull.
Following the weird nonlogic of the firearms world, the bullets used in the .223 Remington/5.56 NATO are actually .224 in diameter. The range of bullet weights available in that caliber is vast, from tiny, 35-grain, thin-jacketed varmint zappers to 90-grain match BTHPs. Factory ammunition is loaded to the SAAMI-spec overall length of 2.26 or shorter, and the AR-15 magazine is designed around cartridges that length. The heaviest bullets you'll find in loaded ammunition, which will fit inside an AR magazine (just barely), are 77-grainers. If you try to stuff a heavier bullet deep enough into a case to make that 2.26 length, all sorts of things start going wrong. Depending on the length of ogive, some 75- and 77-grain bullets are too long and won't allow you to sneak inside that AR magazine. Loading cases with anything heavier than 77-grain bullets will guarantee their overall length is too long to work with even in most bolt-action rifle magazines, and they'll have to be single-loaded into the chamber.
Why use heavy-for-caliber bullets? They do much better bucking the wind at distance. Black Hills helped develop the Mk 262 round for the U.S. military, which features a 77-grain OTM bullet loaded to AR mag length. That was quite an accomplishment, and the story behind that cartridge is interesting enough for a separate article. This heavy bullet has been found to not just be more accurate at distance in Afghanistan, but more effective on enemy combatants. Camp Perry and target shooters have found 80-and 90-grain bullets much preferred on the 600-yard line.
For this project I ordered a number of different bullet weights from both Sierra and Hornady. The heavier projectiles came with twist rates recommended right on the boxes-1:7 to 1:10 for the 65-grain spitzers from Sierra, 1:8 for the Hornady 75-grain BT, 1:7 to 1:8 for the Sierra 77-grain HPBT, and 1:6.5 for the Sierra 90-grain HPBT. Following the twist-rate recommendations on the Sierra bullet boxes was the word "only" in red. But part of what I wanted to discover was what would happen if I didn't follow those recommendations.
FACTORY AMMUNTION ALEXANDER ARMS (20-IN.BBL., 1:9 TWIST) LOAD BULLET VELOCITY STD. AVERAGE WEIGHT(GR.) (FPS) DEV. GROUP (IN.) HORNADYNTX 35 3,813 33 1.33 BLACK HILLS FMJ 55 3,112 22 1.40 FEDERAL BTHP 69 2,788 31 1.44 HORNADYBTHP 75 2,755 34 2.31 BLACK HILLS OTM 77 2,734 21 1.31 ALEXANDER ARMS (16-IN. BBL, 1:7 TWIST) HORNADYNTX 35 3,622 31 1.41 BLACK HILLS FMJ 55 2,987 23 1.79 FEDERAL BTHP 69 2,681 34 1.64 HORNADYBTHP 75 2,615 37 1.73 BLACK HILLS OTM 77 2,614 19 1.68 FACTORY AMMUNITION ALEXANDER ARMS (20-IN. BBL, 1:9 TWIST) BULLET/WEIGHT (GR.) CHARGE COL VELOCITY STD. AVERAGE WEIGHT (IN.) (FPS) DEV. GROUP (GR.) (IN.) SIERRA BLITZKfNG/40 25 2.20 3,674 23 1.38 SIERRA 5PITZER/55 25 2.25 3,108 21 1.45 SIERRA 5PfTZER/65 23 2.25 2,899 27 1.35 HORN. A-MAX/75 22.5 2.39 2,805 24 1.82 5IERRA BTHP/90 21.6 2.55 2,522 18 6.28 ALEXANDER ARMS (16-IN, BBL, 1:7 TWIST) SIERRA BUTZKING/40 26 2.20 3,567 27 1.43 SIERRA SPITZER/55 25 2.25 3,022 23 1.57 SIERRA SPITZER/65 23 2.25 2,743 33 1.34 HORN. A-MAX/75 22.5 2.39 2,683 14 1.52 SIERRA BTHP/90 21.6 2.55 2,581 18 2.65
It was interesting to me that while the 35-grain Hornady NTX and 40-grain Sierra BlitzKing are both light for caliber, there was no caution on the boxes about what twist rates they should be fired through. Why? Because spinning a bullet faster than it requires to stabilize doesn't harm anything.
I talked with a lot of people smarter than me, including Bill Alexander of Alexander Arms. While I didn't have time to actually try it myself, Bill told me that I would see a distinct difference in both accuracy and velocity between button-rifled stainless steel AR barrels and standard chrome ones, all other things being equal. In fact, there are so many things that affect accuracy and velocity (especially in semiautos) that I tried to eliminate as many variables as possible.
When handloading, I used only new Homady brass, Winchester Small Rifle primers and Alliant Powder's relatively new AR-Comp powder. From the name you can probably guess it was designed specifically for loading .223/5.56 for use in AR-15s. The Alliant reloading guide had "recipes" for bullet weights from 52- to 90-grain bullets, and I adjusted accordingly when loading for the lighter 40-grain bullets I had on hand (don't try that at home). I was not trying to develop max-pressure loads, just consistent ones to judge accuracy and velocity and so loaded everything a little light.
Savage model 12 Varmint (26-in, bbl, 1:9 twist) Load Bullet Velocity Std. Average Weight (pps) Dev. Group (gr.) (in.) hornaovntx 35 32 0.98 black hills fm 55 3,159 22 0.84 federalbthp 69 2.874 29 1.02 hornadybthp 75 2,813 30 1.11 black hills otm 77 2,774 19 1.22 savage model 12 VARMINT (26-IN, BBL., 1:7 TWIST) hornady ntx 35 3,972 29 0.93 black hills fmj 55 3,162 23 0.92 federalbthp 69 2,864 25 0.98 hornadybthp 75 2,831 38 1,08 black hills otm 77 2,788 19 0.89 SAVAGE MODEL 12 VARMINT (26-IN, BBL., 1:9 TWIST) BULLET/WEIGHT (GR.) POWDER COL (IN.) VELOCITY STD. AVERAGE CHARGE (FPS) DEV. GROUP (CR.) (IN.) SIERRA BLITZKING/40 26 2.2D 3.859 29 1.08 SIERRA 5PITZER/S5 25 2.25 3,224 23 0.95 SIERRA SPITZER/65 23 2.25 2,951 37 0.89 HORNADY AMAX/75 22.5 2.33 2,889 22 0.99 SIERRA BTHP/90 21.G 2.55 2,601 18 6.45 SAVAGE MODEL 12 VARMINT (26-IN. BBL., 1:7 WIST) JLIRRA BLITZ K1N 26 2.20 3,866 41 1.21 G/40 SIERRA SPITZER/55 25 2.25 3,187 28 1.05 SIERRA SPITZER/65 23 2.25 2,972 29 0.35 HORN ADY A-MAX/75 22.5 2.39 2,933 24 0.97 5IERRA BTHP/90 21.6 2.55 2.613 23 1.45 All ammunition used was .223 Remington. All handloads loaded on new Hornady brass with Winchester Small-- Rifle Primers using Alliant AR-Comp powder. Accuracy results are the averages of four five-shot groups at 100 yards from a sandbag rest. Velocities are averages of 10 shots measured with an Oehler Model 35P 12 feet from the muzzle.
The 77-grain Sierra BTHP has been successfully loaded to 2.26-inch AR-mag length (see Black Hills' Mk 262 above), but loading both the 75-grain Hornady A-MAX and the 90-grain Sierra BTHP to fit inside either the AR magazines or the Savage magazine was a no-go. The 75-grain rounds I loaded to a 2.39-inch OAL and the 90-grainers to 2.55. For testing, I had to single-load them into the Savage's chamber.
After spending a lot of time handloading, I was able to head out to the range. For accuracy testing I used a 4.5 X-14 X-42mm Burris MTAC scope, which has the company's G2B Mil-Dot reticle. I like Burris scopes, as they have good glass, hold zero and don't require you to sell your first-born to afford one (not that anyone would buy my kids ...).
It would be lying to say that I didn't have some expectations when I started pulling the trigger, but what I was expecting didn't happen--neither the bolt guns nor the ARs in 1:7 or 1:9 showed any distinct preference for any bullets weighing between 35 and 77 grains--with two exceptions. The 1:9 Alexander Arms rifle didn't like the 75-grain A-MAX, and the 1:9 Savage didn't like the Black Hills 77-grainers, but that was it. There was no predictability to the accuracy results--the two most consistently accurate loads across the board were the 35-grain NTXs from Homady and the 77-grain Black Hills loading. Think about that. That's a 42-grain difference--one bullet more than double the weight of the other--and yet they both performed.
When it came time to load the 90-grain rounds I'd put together ... well, that was interesting.
Both the Savage and Alexander Arms guns with 1:7 twists shot these heavy bullets, but not as accurately as the lighter projectiles. Loading them into the 1:9 guns ... wow. I'd heard of keyholing and seen it happen once or twice because of damaged barrels, but every 90-grain bullet I fired out of the 1:9 barrels started yawing and hit the targets sideways. While I did my accuracy testing at 100 yards, I moved the targets closer just to see and found that the long 90-grain bullets had already started yawing badly at 50 yards. The 1:9 twist rate was just not fast enough to stabilize them. Period. The targets looked like they'd been peppered with shrapnel.
ARs with 1:12 barrels are notorious for sending 62-grain M855 bullets downrange sideways. I've seen some AR-15s with 1:9 barrels that shoot 77-grain bullets great at 600 yards, while others with match barrels will struggle to keep those same bullets into a coffee can at 200 yards.
Conclusions? The vast majority of all .223/5.56 ammunition sold in this country features projectiles at or around 55 grains in weight, so whether you have a 1:7 or 1:9 barrel, the twist shouldn't make a difference in your accuracy. When it comes to those barrels, the minor quirks in the individual barrels seem to affect accuracy more than the twist rate, at least when it comes to most ammo.
However, when you start shooting bullets weighing at or over 65 grains, there's no guarantee what kind of accuracy you're going to get if your barrel doesn't have a twist faster than 1:9. All barrels seem to show a preference for one type of ammo or another, and there doesn't seem to be any way to predict it or explain it. I've seen two bolt-action match rifles from the same custom maker shot side by side--one loved a certain load and would shoot one-hole groups all day long, while the other one didn't like it at all.
We're living in the golden age of firearms and have gotten so spoiled we don't realize it. I wasn't surprised--but rather expected--that the Savage rifles with their excellent triggers would shoot sub-MOA with most ammo. And that neither of the Alexander Arms ARs even came close to jamming--even with the wide variety of bullet weights.
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|Publication:||Guns & Ammo|
|Date:||Feb 21, 2013|
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