The 1:8 barrel twist

[Slater]

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?

[markm]

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 is really what almost everyone should be running. 1/7 is military spec to stabilize the long ass M856 tracer, but it's overkill for almost every mag length round the average guy would ever use.

[1168]

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.

[markm]

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.

[498cm3]

I don't really know, but I did notice most of the match barrels at Fulton are 1 in 8".

[markm]

I don't really know, but I did notice most of the match barrels at Fulton are 1 in 8".

I think my White Oak might be 1/8 too, but I can't remember off the top of my head.

[danieljmaunder]

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.

[Johnny Rico]

Sionics uses 1:8.

[gaijin]

^^ Yes. I think Criterion does too.

[titsonritz]

^^ Yes. I think Criterion does too.

They do.

[MSW]

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?

[Wake27]

BCMs KD4 barrels are 1/7.7 and optimized for MK262 as well.


Sent from my iPhone using Tapatalk Pro

[498cm3]

^^ Yes. I think Criterion does too.

Come to think of it, I believe Fulton gets most of their barrels from them.

[SteveL]

SOLGW match barrels are also 1:8.


BCMs KD4 barrels are 1/7.7 and optimized for MK262 as well.


Sent from my iPhone using Tapatalk Pro

When I took a scoped rifle class with him a few years back these uppers had just hit the market. He talked about them in the class and about how 1:7.7 performed better with MK262 than 1:7 or 1:8.

[Molon]

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

...

[Molon]

SOLGW match barrels are also 1:8. When I took a scoped rifle class with him a few years back these uppers had just hit the market. He talked about them in the class and about how 1:7.7 performed better with MK262 than 1:7 or 1:8.

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/mediaFiles/28568/black_hills_mk262_mod_1_best_group_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/mediaFiles/28568/77_smk_10_shot_group_lothar_walther_barr-2750461.jpg




...

[vicious_cb]

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?

Zip, zero, nada, none over a 1/7. Barrel quality matters far more so just pick the best one you can afford. In fact I prefer the faster twist rate when you see how long the top of line performing ammo like 77gr TMK, Brown Tip ect are.

[georgeib]

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.

[markm]

Sionics uses 1:8.

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.


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.


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.

[Molon]

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..


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/mediaFiles/28568/barnes_70_tsx_vs_sierra_69_matchking_003-2751730.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/mediaFiles/28568/70_grain_TSX_handload_group_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/mediaFiles/28568/70_grain_TSX_at_50_yards-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/mediaFiles/28568/70_grain_TSX_keyhole_at_100_yards_01_res-2751741.jpg


….

[constructor]

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.

[Molon]

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.




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/mediaFiles/28568/hornady_556_70_gmx_TAP_01_resized-2632992.jpg


.....

[markm]

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.

We run a 30 cal cut rifle barrel made a thowy or so under-bored. It's chambered in 300WM and definitely bucks a bit harder.

[Slater]

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?

[constructor]

We run a 30 cal cut rifle barrel made a thowy or so under-bored. It's chambered in 300WM and definitely bucks a bit harder.
I have had a small bore Krieger 300RSAUM Palma contour barrel since 2008, one of the most accurate barrels I have ever owned. It's a perfect match for Lapua bullets.

[georgeib]

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/mediaFiles/28568/hornady_556_70_gmx_TAP_01_resized-2632992.jpg


.....

Thank you for that correction!

[DG23]

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?

Going to cost me a pile of money and time to wear out one if I have all three to shoot from... :)

1/7, 1/8, 1/9

[markm]

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?

I remember that notion when ARs started to evolve from the old A1 1/12 to 1/9 and finally to 1/7. The reality is that throat erosion is the barrel killer, and twist rate has no impact on that wear.

[vicious_cb]

I remember that notion when ARs started to evolve from the old A1 1/12 to 1/9 and finally to 1/7. The reality is that throat erosion is the barrel killer, and twist rate has no impact on that wear.

Yep, and that pretty much has to do with the material property of the steel, how hot you get your barrel and the flame temp of the powder you're burning of which there is very little data on.

[lysander]

Yep, and that pretty much has to do with the material property of the steel, how hot you get your barrel and the flame temp of the powder you're burning of which there is very little data on.
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.

[markm]

However, in simplistic terms, ball propellants are the mildest for throat erosion, then single based stick, and last double based stick.

Wait what? Ball is easiest on throat erosion??? I've read that the ball powders are "barrel killers". But in that context is was the harder bore fouling.. not so much throat erosion.

[lysander]

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".

[markm]

I've surely read that ball powder is more robust than extruded... which makes sense for military ammo. Interesting info though.

[1168]

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".

Why the difference between single-base and double-base extruded? Given the burn characteristics you’re describing with ball powder, I’d expect double-base to be closer to ball?

[vicious_cb]

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.

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.

[markm]

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 they are using.

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!

[vicious_cb]

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/image/v2/BDCK_PRD/on/demandware.static/-/Sites-ctd-master-catalog/default/dwfe81ce2b/large/2-h80274.jpg?sw=800&sh=800

[lysander]

I've surely read that ball powder is more robust than extruded... which makes sense for military ammo. Interesting info though.
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

[georgeib]

Another great post. Thank you, Lysander.

[markm]

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.

[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.
Basically, yes.

[vicious_cb]

Well it looks like at least 1 military is going be using double based powder, barrel life be damned. Wouldnt be surprised if M855A1 is atleast a single based since they were chasing velocity since its inception probably some kind of special St. Marks blend.

https://www.thefirearmblog.com/blog/wp-content/uploads/2023/03/20230316_112635-1024x768.jpg

[Molon]

Well it looks like at least 1 military is going be using double based powder, barrel life be damned. Wouldnt be surprised if M855A1 is atleast a single based since they were chasing velocity since its inception probably some kind of special St. Marks blend.



The powder used in M855A1 is a double base propellent.


https://www.ar15.com/media/mediaFiles/28568/m855_powder_022-2423083.jpg


....

[lysander]

Well it looks like at least 1 military is going be using double based powder, barrel life be damned. Wouldnt be surprised if M855A1 is atleast a single based since they were chasing velocity since its inception probably some kind of special St. Marks blend.

As stated above, even though ball propellants are double based, they have some of the lowest flame temperatures and are the easiest om throats.

The propellant used in the M855A1 is SMP-842, a variation of CFE223.

[constructor]

1/8 is really what almost everyone should be running. 1/7 is military spec to stabilize the long ass M856 tracer, but it's overkill for almost every mag length round the average guy would ever use.
Agree and the 7 twist buttons are just plain old mil spec Enfield rifling with a 50:50 L-G ratio. You know what they say about opinions, I would much rather use a 8 twist 5R with a 30-70 L-G ratio that puts out 60-90 more FPS.

[lowprone]

Great thread !

[vicious_cb]

Agree and the 7 twist buttons are just plain old mil spec Enfield rifling with a 50:50 L-G ratio. You know what they say about opinions, I would much rather use a 8 twist 5R with a 30-70 L-G ratio that puts out 60-90 more FPS.

Im not following here, are you saying 1/8 gets more FPS because of less rotations or is it some kind of special rifling combined with a slower twist that does it. Is this like a Triarc track barrel that has some kind of special rifling that boosts FPS or something? I believe the track series barrels are also 1/7 so Im guessing the mfg'er doesnt 1/8 would benefit the barrel.

[vicious_cb]

Agree and the 7 twist buttons are just plain old mil spec Enfield rifling with a 50:50 L-G ratio. You know what they say about opinions, I would much rather use a 8 twist 5R with a 30-70 L-G ratio that puts out 60-90 more FPS.

Im not following here, are you saying 1/8 gets more FPS because of less rotations or is it some kind of special rifling combined with a slower twist that does it. Is this like a Triarc track barrel that has some kind of special rifling that boosts FPS or something? I believe the track series barrels are also 1/7 so Im guessing the mfg'er doesnt 1/8 would benefit the barrel.

[jwfuhrman]

Really wish they’d sell them separately, the KD4 barrels that is

[lysander]

Im not following here, are you saying 1/8 gets more FPS because of less rotations or is it some kind of special rifling combined with a slower twist that does it. Is this like a Triarc track barrel that has some kind of special rifling that boosts FPS or something? I believe the track series barrels are also 1/7 so Im guessing the mfg'er doesnt 1/8 would benefit the barrel.
Slower twist theoretically would give higher velocities for the same pressure profile as less energy is used to rotate the projectile. It was easily measured in old tank guns.

[ta0117]

As stated above, even though ball propellants are double based, they have some of the lowest flame temperatures and are the easiest om throats.

The propellant used in the M855A1 is SMP-842, a variation of CFE223.

Is this SMP-842 a low temperature powder? Can’t find data on it.

[lysander]

Is this SMP-842 a low temperature powder? Can’t find data on it.
It is a ball propellant so it should be similar to other ball propellants.

[constructor]

Im not following here, are you saying 1/8 gets more FPS because of less rotations or is it some kind of special rifling combined with a slower twist that does it. Is this like a Triarc track barrel that has some kind of special rifling that boosts FPS or something? I believe the track series barrels are also 1/7 so Im guessing the mfg'er doesnt 1/8 would benefit the barrel.
Rifling design, less engraving force and less resistance. Most 1:7 is plain square enfield rifling with 40:60 or 50:50 l-G ratio. Some of the Triarc barrels were produced by us.
The 7 twist buttons are a military design so most just use them as they are. Military didn't design the 8 twist buttons so whoever uses them must design the button. If they have enough sense to use 8 twist then they usually design an improved rifling. 5R, Shilen's ratchet, like Noveske's poly but it isn't really a poly like HK or Glock it's really like a "R" type, Columbia's poly(female type poly) .

[markm]

Military didn't design the 8 twist buttons so whoever uses them must design the button. If they have enough sense to use 8 twist then they usually design an improved rifling.

Maybe why we've had such glorious luck with and love for the Rem 700 1/8 5R rifle we've shot the piss out of for the last 10 years.

[Disciple]

The 7 twist buttons are a military design so most just use them as they are.

Do you know if Ballistic Advantage Hanson barrels use the military design?

[constructor]

Do you know if Ballistic Advantage Hanson barrels use the military design?

No, Just borescope it to see if it has square lands

[DwayneZ]

Slower twist theoretically would give higher velocities for the same pressure profile as less energy is used to rotate the projectile. It was easily measured in old tank guns.

Indeed true, but by how much? Rainier Ultramatch 1:7.5 20" vs say a Wilson 1:8 20" or even some oddball 1:12 twist barrel. Same round, not sure a chrono is gonna pick up the diff.

I think the best config is achieving just enough rpm's to stabilize bullet, from there you have best balance between rpm's and FPS. In that math you can also look to using minimal twist to achieve needed rpm's to stabilize bullet.

For readers, powders transfer energy into the bullet, two types of kinetic energy, 1) velocity FPS, and 2) rotational or angular velocity (rpm's).
By way of energy conservation, the energy transfer to muzzle = FPS + rpm's.
Less rpm's means a tad more FPS. More rpm's means a tad less FPS.

[Disciple]

No, Just borescope it to see if it has square lands

I don't have one.

[constructor]

Indeed true, but by how much? Rainier Ultramatch 1:7.5 20" vs say a Wilson 1:8 20" or even some oddball 1:12 twist barrel. Same round, not sure a chrono is gonna pick up the diff.



I agree, all factory ammo is lower than max pressure so it's doubtful there will be much noticeable difference.
If you had a 12 twist 5R and worked up to a maximum load in it then stuck the same load in a 1:7 twist with 6 grooves and a 40-60 L-G ratio you would probably see a difference. 4-5000psi isn't going to blow the barrel but it may flatten or pierce the primer. Anyone looking for all out performance should look for a barrel with better specs, if taking carbine courses with factory ammo it's not going to matter.

ETA- One benefit - If you are using a SBR and you lose terminal performance due to loss of velocity, a better spec barrel may help gain some velocity and terminal performance. When we were doing all the testing in 2008 we found the 5R Melonite treated barrels were getting 60-90fps more out of the same length(16") barrel, our 12.5" barrels were as fast as some 16" barrels.

[constructor]

Maybe why we've had such glorious luck with and love for the Rem 700 1/8 5R rifle we've shot the piss out of for the last 10 years.

Boots knew what he was doing, funny thing is his design was based on a Russian design.