There's not a ton of AR's on the market with this particular twist, but I see a few every now and then. What advantage (if any) is there in the 1:8?
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There's not a ton of AR's on the market with this particular twist, but I see a few every now and then. What advantage (if any) is there in the 1:8?
1/8 offers nothing better than 1/7, though.
1/8 isn’t necessarily bad, but does not need to exist, and does exist in part due to how gun people love spreading ignorance.
It's probably more about the individual barrel and load work up than the nominal twist rate. Perhaps a pool of Camp Perry .223 shooters might have a collective preference on twist.
But based on what I've shot, I generally like a faster twist the shorter the barrel. We run a 1/9 twist 20" gun that will stabilize 80gr Noslers, and even 88 gr ELMs. And those suckers are long. Shorter barrels don't get the velocity and thus RPMs to stabilize longer bullets as well.
I don't really know, but I did notice most of the match barrels at Fulton are 1 in 8".
I've run both 1/8 and 1/7 twist on a variety of guns, I haven't noticed any comparable difference between the two twist rates. As markm stated, military guns were specced at 1/7 for stabilization of tracer rounds. My philosophy is this, if its a quality barrel, if its either 1/7 or 1/8 twist it doesn't really make a difference to me. I can shoot up to 77's out of either. I've had douglas 1/7 and 1/8 mk12 barrels and shot them side by side, with 77 smk's they shot virtually the same.
Sionics uses 1:8.
^^ Yes. I think Criterion does too.
FWIW, my Compass Lake built SAM-R clone is a 20” Krieger 1/7.7 twist, CLE chamber. The build record notes the twist. CLE now use 1/7, but when I spoke with the elder owner around 2016, he said the 1/7.7 gave the best accuracy with the Mk262 ammo. Or, maybe he had a bunch of unused 1/7.7 barrels he needed to sell….
Grain of salt time?
BCMs KD4 barrels are 1/7.7 and optimized for MK262 as well.
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It’s largely Internet Commando myth that bullet stability increases with increasing barrel length. Technically, the gyroscopic stability factor does increase with increased barrel length, however the amount that it increases is miniscule and lost in the noise of other variables.
As an example, the 55 grain bullet in M193 will have a gyroscopic stability factor of approximately 4.25 when fired from a 14.5” Colt M4 barrel. When the same M193 round is fired from the 5.75” longer barrel of the 20” Colt A2 barrel, the gyroscopic stability factor of the 55 grain bullet only increases to approximately 4.27.
While the increased RPM due to the faster velocity will act to increase the bullet stability, the increased velocity also “increases the force applied to the nose of the bullet at the center of pressure and strengthens the overturning aerodynamic torque which actually makes the bullet less stable.”* These two opposing dynamics are the reason for the miniscule increase in gyroscopic stability factor.
* From Applied Ballistcs For Long-Range Shooting by Bryan Litz
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I'm sure he provided you with statistically significant shot-group data for MK262 fired from AR-15s with premium barrels of varying twist rates to support his claim, right? Per Ballistic Performance of Rifle Bullets, the "recommended twist for optimal performance is 1:8.7" or faster" for the 77 grain SMK. When fired from a barrel with a 1:8" twist, the 77 grain SMK has a nominal gyroscopic stability factor of 1.75.
The 10-shot group pictured below was fired from an AR-15 with a 1:8" twist Lothar Walther barrel at 100 yards using Black Hills MK262. The group has an extreme spread of 0.96".
https://www.ar15.com/media/mediaFile...02-1342440.jpg
A 10-shot group of hand-loads topped with the 77 grain SMK fired from the same 1:8" twist AR-15 had an extreme spread 0.56".
https://www.ar15.com/media/mediaFile...rr-2750461.jpg
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As a data point I'll add my 2 cents. I only have a single 1:8 barrel in 223 Wylde. It came in a 16" PWS rifle that I have. According to Strelok Pro, the Barnes 70gr VOR-TX has a gyroscopic stability factor of only 1.33. The Barnes 70gr. VOR-TX bullets are solid copper and have a nominal length of 1.073", so I'm sure that's a factor. As it so happens that particular load out of my particular rifle groups like shit.
I can't remember the exact numbers, but I want to say it was around 1-1.5" at 25 yards. Badly enough that I wouldn't want to shoot any of it through a suppressor on that particular rifle, and I didn't bother with further distances. The same rifle will shoot 5 rounds of IMI Razorcore into a ragged hole at 25, and around an inch at 100. Heck, it groups m193 better than it does the 70gr Barnes.
ETA: I remembered that this barrel didn't do great with Black Hills 70gr GMX Barrier either. That ammo uses Hornady 70gr GMX all copper bullets with a nominal length of 0.9" and produces a gyroscopic stability factor of 2.19 according to Strelok Pro. Not as bad as the Barnes, as I recall, but not good by any stretch.
This has put me on the hunt to build a more precision orientated upper, but is shying me away from anything slower than 1:7. Which is posing some difficulty as far as available options.
The Sionics on my lap is marked 1/7. Perhaps current production is different. I heard they also no longer offer the FSB. That sucks.
I did an experiment where I had some relatively long (for the twist) bullets. And although barrel length was the same, the increase in velocity (and thus bullet spin) took the bullet from key holing to stability and modest grouping.
I honestly did not expect the additional velocity to do anything for stability, but I was wrong.
The 70 grain TSX is a looong bullet. Depending upon variables, the performance of this bullet can sometimes be rather iffy from a 1:8” twist barrel.
https://www.ar15.com/media/mediaFile...03-2751730.jpg
When properly stabilized, the 70 grain TSX is capable of decent accuracy/precision. The 10-shot group pictured below was fired from an AR-15 with a 1:7.7” twist barrel at a distance of 100 yards. The group has an extreme spread of 1.12”
https://www.ar15.com/media/mediaFile...01-2751735.jpg
A bullet that is not properly stabilized can still show decent accuracy at shorter distances. The 10-shot group of the 70 grain TSX pictured below was fired from a Colt 6721 at a distance of 50 yards. The 6721 barrel has a 1:9” twist and it produced a tidy little group at 50 yards with the 70 grain TSX.
https://www.ar15.com/media/mediaFile...ds-2751739.jpg
However, when the 70 grain TSX is fired from the same barrel at a distance of 100 yards, accuracy is degraded and multiple shots show key-holing.
https://www.ar15.com/media/mediaFile...es-2751741.jpg
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Seems like higher quality barrels use 1:7.7 or 1:8. When we were testing barrels in 2007 most of the 1:7 twist 6 groove barrels did not meet the minimum bore area spec. that causes higher pressures and more engraving force.
The Hornady 70 grain GMX has a nominal length of 1.045”. Per Ballistic Performance of Rifle Bullets, the 70 grain GMX has a nominal gyroscopic stability factor of 1.37 when fired from a barrel with a 1:8” twist. The factory loaded 5.56mm 70 grain GMX shot well from my 1:8” twist Lothar Walther barreled AR-15.
https://www.ar15.com/media/mediaFile...ed-2632992.jpg
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I occasionally read opinions that, all else being equal, a 1/9 barrel will have a longer service life than a 1/7 barrel. Any validity to that?
Actually, there is an abundance of data on the subject, it is just that little of it is published in layman's terms, or easily accessed.
However, in simplistic terms, ball propellants are the mildest for throat erosion, then single based stick, and last double based stick.
There is a reason 90% of all military small arms (30 mm and smaller) ammunition is loaded with ball propellant, it is the easiest on barrels. There is a ton of test data showing barrels that use ball propellant exclusively out last other propellant types.
There are two reasons for this, 1) ball propellant is heavily deterred to achieve progressive burn characteristics, so it's flame temperature is a good bit lower than other propellants, and 2) the calcium carbonate left over from processing is very good at reducing throat erosion, but it does make the propellant "sooty".
I've surely read that ball powder is more robust than extruded... which makes sense for military ammo. Interesting info though.
Simplistic is good enough for me, Im not going to start taking apart my factory ammo to see whats inside other than the occasional bullet set back or bent necks but when it comes to double based stuff like vihtavuori I doubt thats going be used in high enough volume for it to matter. I was more concerned about whats in the cheap stuff like Wolf that gets shot in high volume since I have no clue what the characteristics are of the powder.
That Russian stuff is actually high quality ammo. Wolf is surely (but I could be wrong) extruded. I pulled some Herters steel cased apart once and subbed each component one at a time to see what made that Russian stuff tick.
Everything about it was good quality except the shitty bullets. I directly subbed 69gr match bullets over the existing 62 gr herter's charge, case, and primer, and that ammo shot well!
I don't doubt it, also Im pretty sure thats all this stuff is: Russian case, primer, maybe powder topped with a Hornady bullet thats stupidly accurate for the price.
https://www.cheaperthandirt.com/dw/i...?sw=800&sh=800
Small arms propellant needs to have progressive burn characteristics. As the bullet moves down the barrel, the volume increases, in order to keep the pressure increasing as this happens, you need to create gas faster than the volume increases, or at least as fast as the volume increases. There are three ways to do this, 1) by increasing the amount of propellant burning (increase the surface area on fire), 2) have the area that is on fire, burn faster, or 3) a combination of (1) and (2). Number (1) can be done by the geometry of the propellant grain. Number (2) can be done by slowing down the initial speed the material burns, so that the later rate is relatively faster.
To achieve (1):
Look at a cylinder, as the surface burns away, the radius of the grain decreases, which means the surface area decreases, if you put a single hole in the center of the cylinder (single perforation grain), as the grain burns, the outside area decreases, but the inside hole gets bigger so the total area burning remains mostly constant. If you put several holes in the grain (multi-perforation grain) each of the holes will grow as the outside shrinks, and total area increased.
To achieve (2):
If you coat the surface of the grain with a "flame retardant", or "deterrent", the flame will burn slower until it burns through the deterred layer.
For (3):
By using both grain geometry and a deterrent you can get single-perf grains to burn progressively, even though the area is remaining constant.
If you look and a sphere, the area decreases rapidly as the diameter decreases during burning, so the surface needs to be heavily deterred in order to get progressive burning. And, as a result of this thick coat of "flame retardant" to reduce the burning rate, you get a reduction in the heat liberated in relation to time, so the flame temperature is lower, even though ball propellants are all double based.
Also, spheres pack better with less dead air space between the grains so you can physically get more flammable material in the same volume. This makes ball propellants much more "energy dense", so you can get higher velocities from the the same volume, assuming everything else is equal.
Single base propellants are 99% nitrocellulose, double base propellants have about 10% nitroglycerine, giving them a higher energy content, but at the cost of a higher flame temperature.
For example the deterred flame temperatures for selected propellants:
WC 846 - 2244° K
IMR 4227 - 2550° K
IMR 4350 - 2622° K
IMR 4895 - 2287° K
IMR 8138M - 2406° K
EXP 1591A - 2912° K
All of the IMRs are single based, and the EXP is a double based extruded propellant.
Old Army films are fun and informative:
https://www.youtube.com/watch?v=CgXz-njLLV4
Another great post. Thank you, Lysander.
Interesting... and possibly the explanation of how a mil pull down 5.56 has SO MUCH powder inside. If I'm tracking the powder has a certain retardant to allow for more propellant without an early pressure spike that could burst the case or worse.