Is there a significant amount of effectiveness in the .223 when you jump up from a 20" AR from a 16" AR carbine to make it really all that much worth the while? Or am I just better off using something in a larger caliber ?

Considering a 20" AR build to get a little more effectiveness at range vs just using my 16" for "one gun to rule them all" if you will?

Looking at adding a telescopic optic also to make it easier to see at distance too, rather than just a red dot; and still running a collapsible stock on the 20" too keep things a handy as possible if needed.


Figured I'd try and keep as much compatible between the two guns as far as mags, caliber and such vs using two different calibers and or platforms.

Thanks!

20" rifle, with rifle buffer/spring/RE  shoots faster and smoother than a 16" midlength.

The Magpul rifle stock is sweet.

Ultimate ballistic performance and /accuracy?  Yeah, the 20" has the edge.  But yes, as said above, 20", rifle-length gas system, rifle buffer and spring, make for a sweeter shooter.

I have two 20" ARs, primarily used as my "KD" rifles, and I've used them as far as 600 yards with good results.  One has a 4 x 32 ACOG, the other has a 2.5-10 FFP variable scope.

A 16" is still my "go-to"/tactical rifle for HD, shorter distances, but I can and have made good hits with it out to 400 yards.

So, to use the car analogy, the 16" is my "daily driver" and the 20" are my weekend track cars.

FPNI

The 20” with rifle gas system is the smoothest-shooting AR15 available. No more hearing the “sproing” in your ear from the buffer spring. No recoil whatsoever and not such a loud “crack,” more like a “pow.”  I have shot some carbines that were downright unpleasant, with more recoil and noise than my M14.

Edit: As large as I am the 16” and a collapsible stock just seems like a toy

100+ fps velocity gain, softer recoil, heavier and longer.....


Hunting?   20" pencil or fluted barrel....carry it all day...


Home defense....16" or pistol  is way to go. Even a 16"  is hard to maneuver in a vehicle or tight places . Add 4" of barrel  and it's a pita.

Timely, I'm just today thinking about a compromise for a distance shooter by going 18" .223 Wylde.
I don't have any AR's setup for distance and I'm thinking the 18" should be enough in Wylde.

From my extensive testing, 20" barrels are roughly 200 FPS faster than 16" barrels with lighter bullets.  So say you have plain jane 55 grain 223 ammo, the difference in firing it in a 20" from a 16" would be like firing 5.56 from the 16" vs. firing .223.  So you're in effect turning 223 ammo into 5.56 (but with the extra length and weight of the longer barrel).  

With custom self defense ammo (fusion, etc), the terminal performance difference between the two is negligible.  With FMJ, the difference is significant.  20" are also softer recoiling and easier on parts long term if that is a concern, but we're talking over thousands of rounds.  20" barrels will have much less noise, blast and flash, especially at night, due to a more complete burning of the powder.  

If you want a rifle that will be paired with pet self defense ammo load, 16" is more than fine. If you want a rifle that will perform well with almost any load you could scrounge up ala post SHTF or extreme ammo shortage, you may want the extra velocity and long term reliability of a 20".

I personally like 18" barrels because I think they split the difference well between the 16" and the 20" barrels.  Get close to the handling of the 16", but with the ballistics of a 20".

I have my trusty HBAR but I'm grabbing the .300Winmag if I need to reach out a little more. I like overkill.

Longer iron sight radius, more velocity, less perceived recoil. 20" is the way to go.
If you want compact, you can get a bullpup.

There is an article on this site about bullet fragmentation with m193 ball ammo, which is pretty much the most common used AR ammo. It found that a 20” barrel gets reliable bullet fragmentation to 200yds, and that a 16” barrel will get fragmentation out to 150yds. So 6” reduction in package cost you only 50yds in fragmentation. The shocker was going down to 14.5 from 16, only 1.5 less cost you another 50yds down to 100yds. So 16” is a good compromise for terminal performance.

I read a lot about how soft rifle gas uppers shoot, but didn’t really notice it. I’m guessing that’s only true when you also use the rifle stock/buffer in the lower which I did not do.

My first configuration was as you suggest: the collapsible stock carbine lower with the rifle upper, and that didn’t feel like much softer of a shooter than a mid length 16” upper on a carbine lower.

Plus it was super front heavy with a light mounted, not fun to do any standing, target transition type shooting with.

If you’re dead set on a 20”, instead of government profile, try a .625 gas block pencil profile barrel, a fluted barrel or maybe a hybrid like Ballistic Advantage’s Hanson or Faxon’s Gunner if those brands do their hybrids in 20”.  

Any small amount of extra weight saved out front will go a long way.

All depends on purpose of said item.

Keep in mind the original plans for the AR-15 called for a 18.5" barrel and was made longer so accommodate the use of a bayonet.
This barrel length was used again for the later designs, the AR18 and Mini 14. (Can't recall if it were Eugene or Jim) but if you do any reading on the development of the 5.56mm
it was thought that 18.5" was the optimal for the parameters of the cartridge and anticipated engagement ranges. The AR15 was meant to be a 300 meter gun, lightweight with
lightweight ammo and magazines, capable of multiple hits on full auto, with lessons learned from Korea.

Fast forward 60 years, and just as the others have said, the 20", especially set up as a M16A1 style with round handguards is probably the best all around for average height, A2 stock if you are over 6ft
If you want shorter, the midlength gas system is superior to carbine in e 16"

Regardless marvel you have a choice and have fun!

Depends on the ammo and what you are using it for.

What are you shooting at?
How far are you realistically shooting?

20" will shoot softer....but is of course 4 inches longer.
I have a 20".  I don't shoot it.  But I keep it around because 20" can penetrate armor (up close) shorter barrels can't.

The established velocity threshold for the reliable significant fragmentation of the bullet used in M193, as determined by actual experts in the field of terminal ballistics using genuine M193 ammunition, is 2700 FPS.  Using the established G1 ballistic coefficient of .243 for the M193 bullet and average muzzle velocities of 3270 FPS and 3125 FPS for 20” and 16” Colt barrels respectively, a round of M193 fired from a 20” barrel will fall below the 2700 FPS threshold at 144 yards at standard atmospheric conditions.  A round of M193 fired from a 16” barrel will fall below the 2700 FPS velocity threshold at 109 yards at standard atmospheric conditions.











Even if the M193 bullet impacts at a velocity above the 2700 FPS velocity threshold, there is only a 15% chance that it will have significantly fragmented before reaching a penetration depth of 7”.  The anteroposterior diameter of the upper thorax of the average adult American male is 9.5”.













....

It’s an honor to be corrected by you! A bit disheartening to know that common 5.56 ball ammo is out of fragmentation range between 109-144yds for the most common barrel lengths though.

But that’s why I’ve got some Black Hills 77gr OTM for when I need to do something other than ring steel or punch paper. Hard to believe what I paid for that a year ago is what M193 costs now.

EDIT: “5.56 ammo” in first sentence changed to “common 5.56 ball ammo” around the same time as RJeff21’s response which makes the response seem out of context.

I typed out a really long response but deleted it because you really need to ask yourself what distances you’re trying to shoot to and what it is you’re specifically wanting to do with the rifle, both how you want to use it (from a bench/static position, or more mobile/carried around) and what you’re wanting to shoot it at (paper/steel, hunting/defensive use, etc).  

Answers to those questions will better help us answer what may be the best option for your situation, because the vague terms in your OP (like “at range”) can mean different things to different people.

Just a correction, but “5.56” is not out of the fragmentation threshold past those distances, M193 is, so we’re only talking about one specific cartridge.

Another thing to note, the 77gr SMK (the projectile in your BH loads, unless you have the newer stuff loaded with TMKs) has a relatively high “fragmentation threshold” as well, although it is lower than M193 (remember, it’s starting out slower than M193 as well).  Realistically, you’re not increasing your range of “reliable fragmentation” all that much (maybe 200-225ish yards), so just know it isn’t this magical easy button that will fragment out to the effective range of the gun.

You can choose to go with a load utilizing a hunting/defensive oriented projectile that is actually designed for expansion/fragmentation, which will usually increase the range where the projectile will reliably perform, but there may be some tradeoffs there as well depending on what you choose.

JMHO, but while “reliable fragmentation” in certain loads is certainly nice, I wouldn’t consider those loads/the gun ineffective past those distances.

Certain lighter projectiles traveling at high velocities out of longer barrels can penetrate some/certain types of armor up close, not all.  

Seeing as how you’ll have no clue what type of armor someone may be wearing (or if they’re actually within the exact ranges where velocity remains high enough for reliable penetration), you’ll likely be better served by training yourself in failure drills if your shots aren’t having the desired effect.

Training yourself to shoot around armor when/if needed is going to be a lot more foolproof than hoping a certain piece of equipment will perform marginally better in an extremely narrow set of circumstances that you have no control over.

As stated somewhat differently earlier, let the mission dictate the equipment.

If "effectiveness" is measured in terminal ballistics terms, then distance matters as a part of the 16 vs 20 debate, and bullet construction certainly does.  And, a battlefield/combat mission is much different than home defense or typical law enforcement duty scenarios.

For me, my HD AR does not need a 20" barrel for maneuverability or distance, which is not ever likely to see beyond 200 yards, even on a 20 acre farm.

That pushes me in the direction of the 16" choice as between 16 and 20 in the OP question.  It also pushes me away from fragmentation ammo, given a choice.   There are much better choices today.  

I lean toward relatively light 55-62-64 grain bonded core jacketed softpoints like Speer Gold Dot, or Fusion, or Trophy Bonded Bear Claw, and monolithic solid copper hollowpoint Barnes TSX or TTSX even as light as 50 grains, or 55 and 62 grain loadings.  These loadings continue to provide reliable, consistent expansion from a 16" barrel well beyond the distances I will likely experience with a 5.56 .223 AR.  I don't see a 20" offering material improvement in that use.  I have heavier stuff (larger caliber) readily available, if needed.

For sight radius, the stereotypical 20" has a rifle length sight radius...I get that with my dissipators. I run them with midlength gas and they're quite pleasant to shoot and I get great accuracy with the irons. Hard to beat an A2 upper 16" mid length gas dissipator for reliability, durability, accuracy and handling.

I like 18", rifle length gas with rifle extension/buffer/spring too. That's a very soft shooting, not quite as unwieldy as the 20" and still has a very long sight radius and has higher muzzle velocity.

Interesting that you said that.  I own a copy of "The Black Rifle" book and I don't recall them saying that the AR was designed as a 18.5" rifle.  Even the original proto rifles had 20" barrels.  I DO notice how almost all of the non AR 5.56 15 designs since have had 18.5" barrels.  

The funny thing is, that through my own personal testing over the years, I have discovered the same thing.  18.5" is the ideal barrel length for an AR15, you gain little to no velocity from 18.5" to 20", it shoots even softer than 20", is just as reliable, lighter, handier, and you can STILL mount a bayonet (the attachment ring just goes over the end of the flash hider instead of the bottom, but it is still just as secure).

It's too bad that no AR companies make 18.5" barrels, I had to cut down the 2 that I have from 20" barrels.  18" barrels are almost as good, however, you have to enlarge the gas port slightly for optimum reliability, which makes it a bit harsher shooting than the 18.5" which doesn't require gas port enlargement.  Also, if you want to mount a bayonet, you need to use an extended length flash hider and / or a spacer.  If you care about mounting a bayonet, you're better off having that extra half inch length be barrel vs. muzzle device.  The half inch gives you a slight velocity boost (~25 fps avg) and the extra powder burn from the longer barrel gives you the same advantage of a better flash hider anyway.

@Blain I'm trying to remember where I read it, my instinct was from the book "World's Assault Rifles and Carbines" by Musgrave and Nelson, but also an article in SAR (Small Arms Review) seems to be the most likely suspect. I don't recall the issue, but my guess is early 2000's and it was an interview with Jim Sulivan. I think also the Stoner 63 series was around a 18.5" pew tube as well :)

With all due respect to Feckler's (sp?) data, that is only one study.  There have been other studies and testing that show further fragmentation range and lower velocity threshhold than the 2,700 fps.

Also, some M193s (like South African) have higher BCs than others, and will retain that velocity threshold out to longer distances.

Also, for the above reference picture that shows M193's average fragmentation TC range beginning at 4", that is well within the vitals of most human beings.  Basically, that pictures shows a cavity starting around 4" and extending to around 11" for 70% of the shots, so I don't see what the issues is with that.

Also, it's not just about max range but greater terminal effect at contact distance.  At a given self defense distance, a target shot with a 20" barrel will experience greater (more explosive) fragmentation than if that same target was shot with a comparable load through a 16" barrel.  As the study and corresponding fragmentation graph below indicates, more complete fragmentation at given velocities and thus greater terminal effect.  As a few youtube video comparisons demonstrate when shooting anything from ballistics gel to water melons at the same distance, the 20" rifle produces much more "dramatic" results, which is ideally what you want.    

This is a great real world test that was done around 15 years ago, where gel blocks were actually shot out to 200 yards.  It shows a different story in regards to M193 fragmentation threshold.

http://www.ar15.com/ammo/project/Ballistic_Gel_Experiments/BTAmmoLabsTest1/Test1.html



"We now believe a more reasonable figure for a "reliable" fragmentation velocity threshold is 2600 fps.


Acknowledgements:

Though B & T Ammo Labs does not accept payment for services or endorsements for products we receive such consistently outstanding moral and reader support that, though it seems repetitive, it bears repeating that:

This experiment was made possible by the outstanding support of the AR15.com community, Eric the Ammoman, and viewers like you.

If we forgot to mention you specifically, please let us know.

Specifics and Conditions:

Ammunition:

Federal 5.56, 55 grain Copper Alloy Jacketed rounds with cannelure. (XM193 Lot 030).
and
Winchester 5.56, 55 grain Copper Alloy Jacketed rounds with cannelure. (Q3131a Lot SC12).

Rifle: Sig Sauer 551 with optional 1:7" twist, 6 groove, 16" overall length barrel.

Temperature: 30 C (87 F)
Humidity: 74%
Pressure: 1013 hPa (29.85" Hg)
Altitude: 24 meters MSL (79 feet)

The targets were FBI spec blocks (20x20x50cm) of 10% ballistic gel (9 parts water, 1 part gelatin by weight). Vyse ballistic gel was prepared according to FBI ballistic gel testing protocols. Additionally, gel temperature was not permitted to exceed 37 degrees C at any time during preparation.

Following preparation, gel was stored at ~4 degrees C and permitted to cure for 72 hours before testing. Gel was tested within 15-20 minutes after removal of blocks from refrigeration. Gel blocks were calibrated with a .177 caliber BB immediately before testing. (Optimum = 8.5cm @ 590 fps). Our results were within accepted calibration standards for ballistic testing without penetration measurement adjustments.

Procedure:

10 velocity tests on each round type were conducted using The Entirely Ubercool Oehler Model 43 Personal Ballistic Laboratory (hereinafter "The Entirely Ubercool Oehler Model 43 Personal Ballistic Laboratory") set 15 feet from the muzzle. Distance zeroing of the rifle was also performed during this stage.

The Entirely Ubercool Oehler Model 43 Personal Ballistic Laboratory was then set up at varying distances to produce measurements of different impact velocities with gel blocks. Gel blocks were protected from stray shots via a cut out steel plate permitting only direct, near center gel hits to be recorded.

35 shots to bare gel at varying distances were performed with Federal XM193.
35 shots to bare gel at varying distances were performed with Winchester Q3131a. (Shot 36 which was to begin testing of 69 grain SMK destroyed all three muzzle skyscreens and resulted in the data collection stage of the experiment being terminated in favor of the frozen adult beverage drinking stage, which always precedes the "data analysis and quantitative modeling" stage.

Several years of experimentation by B & T Ammo Labs has shown that the high levels of cognitive task importance during analysis phases make them ideally suited for the slot just after excessive adult beverage consumption).

Immediately after the screen incident, sudden rain showers caused severe damage to The Entirely Ubercool Oehler Model 43 Personal Ballistic Laboratory. Several years of experimentation by B & T Ammo Labs has shown that crying over broken lab equipment only results in the replacement of that equipment if Dr. Tatjana's (Hee!) brother is present to witness the crying. As he was not, data collection was closed for the remainder of the experiment.

Fragments were extracted from blocks and their penetration depth measured and recorded. Recovered fragments were then weighed together to arrive at both recovered mass and largest single fragment mass. Penetration depth and wound cavity volume are beyond the scope of this work but will be reviewed at a later date.

Results, Notes and Observations:

Federal XM193 Calibration Velocities:

3110
3102
3092
3113
3117
3119
3085
3115
3099
3091

Average: 3104.30 fps
SD: 12.18

Winchester Q3131a Calibration Velocities:

3095
3089
3082
3079
3098
3096
3075
3090
3070
3102

Average: 3087.6 fps
SD: 10.66

Fragmentation results:

(Ordered by range)
Velocity Fragmentation
Averages

SO = "Split only." The round only split in half and did not substantially fragment.

Federal XM193:
Velocity / Fragmentation

0 meters:
3105 42.0%
3110 44.0%
3093 44.0%
3101 49.0%
3117 51.0%
Average:
3105 46.0%

25 meters:
3003 42.0%
3009 42.0%
3014 46.0%
3007 52.0%
3010 40.0%
-
3008 44.4%

50 meters:
2925 38.0%
2925 44.0%
2931 34.0%
2911 36.0%
2921 42.0%
-
2922 38.8%

75 meters:
2820 35.0%
2823 38.0%
2836 41.0%
2816 38.0%
2822 39.0%
-
2823 38.2%

100 meters:
2731 32.0%
2719 35.0%
2735 41.0%
2730 44.0%
2716 38.0%
-
2726 38.0%

125 meters:
2640 34.0%
2632 34.0%
2627 38.0%
2636 32.0%
2641 34.0%
-
2635 34.4%

150 meters:
2530 SO
2579 SO
2534 0.0%
2541 35.0%
2549 33.0%
-
2546 13.6%

Winchester Q3131a:
Velocity / Fragmentation

0 meters:
3082 46.0
3101 44.0
3083 42.0
3075 46.0
3095 47.0
-
3087 45.0

25 meters:
2991 42.0
2996 41.0
2988 43.0
2995 43.0
2969 37.0
-
2988 41.2

50 meters:
2921 34.0
2914 34.0
2885 32.0
2902 34.0
2891 31.0
-
2902 33.0

75 meters:
2791 35.0
2789 32.0
2807 32.0
2810 31.0
2811 35.0
-
2801 33.0

100 meters:
2729 32.0
2698 31.0
2709 31.0
2687 28.0
2676 31.0
-
2700 30.6

125 meters:
2620 SO
2600 32.0
2602 31.0
2597 29.0
2610 33.0
-
2606 25.0

150 meters:
2524 32.0
2575 26.0
2502 SO
2530 31.0
2505 SO
-
2527 17.8



Conclusions:

Our experiments draw us to three primary conclusions.

First, we suspect that the velocity v. fragmentation relationship is not a linear one. This is particularly apparent around 100 - 125 meters where the drop-off of fragmentation flattens significantly.

Second, we speculate for the first time that cannelure shape and form contributes significantly to fragmentation via separation at the cannelure. However, we observe this tendency ONLY near the extreme low end of fragmentation threshold. In particular, the more jagged and deeper cannelure groove exhibits quicker and more complete separation between the nose and bullet base.

Finally, we conclude that a more accurate reliable fragmentation threshold for XM193 and Q3131a (M193) is 2600 fps.

17 of 23 shots (73.9%) under 2700 fps fragmented significantly enough to show fragmentation related tissue damage.

Of these, 6 of 11 shots (54.5%) under 2600 fps fragmented significantly enough to show fragmentation related tissue damage.

Perhaps most significantly, 11 of 12 shots (91.7%) between 2600 and 2700 fragmented significantly enough to show fragmentation related tissue damage."

I even created a thread to re-highlight this information a few years ago.

Cheesy little test but it illustrates the point (it's the test that was used to convice Lemay too).  There are dozens of such comparison videos out there, testing the two barrel lengths on all types of mediums.  

This is even more crucial to understand during the current ammo panic, because you may only have cheap 223 fmj on hand.

Any FMJ that you shoot will produce dramatically better results with the 20" barrel than a 16" shot at the same distance within fragmentation range.

Its 2021.  Why are we even considering frag ammo when bonded soft points and monolithic hollow points offer much mire reliable, consistent terminal ballistics over a broader range of velocity?  And, because such bullets can be made with greater precision than FMJBT, they are more accurate.

My M193 and M855 stash is relegated to practice, training, and "when I run out of the good stuff" status.  Even my 77OTM is for target/match.

Why are we?  Exactly because it's 2021!  I mean, have you been in a cave the past year?  Don't you know what's going on with the ammo situation?

Far from being able to be choosy about premium self defense ammo, right now, people are lucky to find anything they can get!  If all you can get is fmj, you want to know how it will perform!

I agree, but the discussion thus far focuses on frag ammo to the exclusion of bonded soft point and monolithic hollow point loadings, as if they don't exist.  Many of us do have this type of ammo.

I'm just shocked that it was not even entering into the discussion.  These type bullets will enable a 16" barrel to have superior terminal ballistics to a 20" using frag bullets from close in to distances well beyond effective fragmentation.  For instance Barnes TSX from 50, 55, 62 and 70 grains expansion threshold is only 1900 fps, and they reach fully expanded 2x diameter at 2000-2100 fps.  Bonded softpoints have similar expansion thresholds.  And, these bullets break bones and still retain close to 100% of their weight.

Those of us with hunting background have been using both bonded bullets and monolithic bullets for decades in either factory loadings or handloads.  I could have just as easily said "Its 1990."

Yes, I do load FMJ for plinking, and have a stash of M193 and M855, but, for defense I'm going to resort first to Speer Gold Dot, Trophy Bonded Bear Claw or similar bonded soft points or the .223/5.56 version of the Barnes TSX/TTSX I have been loading for several decades now.

You get more of everything with a longer barrel.
Which also includes more bbl deflection, more gas in bcg, and perhaps a tad more cut into the bullet as it rides rifling for 4 more inches.

More FPS at muzzle also equates to more RPM's, and as we know RPM's of varying weights of bullets can be an issue.

I think there's much more to it than just swapping bbl's and calling it good. I guess there's that window of parameters that will allow many changes to be ok, but one modification and you change the look/shape of that window.

More gas in the BCG?  The AR15 was designed as a 20" barrel, it has the optimum amount of gas....

More rpm’s? 1:7 is 1 turn in 7 inches no matter the velocity. 20” shoot softer as less pressure is hitting the gas port. The rest is....

Well, sort of.  RPM is a function of both twist AND velocity.

This is the formula:

MV x (12/twist rate in inches) x 60 = Bullet RPM

http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/

If you increase velocity and maintain the same twist rate, rpm increases.  

Making a barrel 4" longer in this instance (16" versus 20") typically increases velocity 150-200 fps.  The same ammo fired from a 20" barrel will have higher rpm than when fired from a 16" barrel of identical 1:7 twist.

The higher muzzle velocity will produce higher rpm.  While in an abstract sense it is true that twist rate is still the same, velocity has increased.  With that increase, rpm goes up.  Fact.  Physics.  Science.

Why does it matter?  It might not from the standpoint of terminal ballistics, as distance to target and bullet construction (frag vs. bonded soft point vs. monolithic expanding hollowpoint), but increased velocity does mean flatter trajectory.  Perhaps more important is the relationship between twist rate and velocity in bullet stabilization in flight.

That is why a 1:9 twist barrel might cause a 77 grain SMK bullet to keyhole badly and have poor accuracy when fired from a 16" barrel, but be super accurate from a 20" barrel due to the higher velocity.  But, increase twist to 1:7 or even 1:8 and the 77 grain bullet is stable in flight and is just as accurate from a 16" as if shot from the 20" barrel. It's also why pistol barrels need even faster twist for heavy bullets because velocity is lower.  It is rpm that stabilizes the bullet, and velocity and twist are the two major factors (temperature and altitude are considerations).

Berger's twist rate calculator helps explain this relationship:

https://bergerbullets.com/twist-rate-calculator/

I still think that's more the function of the twist rate variance of button rifling than velocity. Your example barrel probably rounds up to 1:10. I'd like to see a true 9 start at 24"+ and be cut and recrowned until the 77 SMK shows instability at 100yds. I bet that threshold is 10".

It’s largely Internet 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

I never thought to actually measure the twist rate in that barrel labeled "1:9" twist.  Maybe its slower.  It's an old gov't profile CMMG.  I'll check it with a marked cleaning rod and see.  Good point.  It absolutely sprays 70 grain, Barnes TSX, but they are even longer than 77 SMKs, even though being pushed a bit faster.

What Rifleman_556 has noted, gotta switch gas length when going to the 20" to reap the full benefits.

16 or 20" car, I would just stay w/ the 16".

I can't remember ever seeing a carbine gassed 20".

A 16" barrel, moving a 5.56 77 TMK at ~2700 fps results in a frag range of ~400 yards. A 20" barrel would only bump that that to 500 yards max. How often do you shoot at critters at 400-500 yards?

The Thunder Ranch abomination called the "Marksman."






...

You are correct, sir.  I checked that barrel.  It is just a bit over 1:10!   Closer to a 1/4 turn past 10", more like 1:10.2, even though labeled 1:9!  Lesson learned, measure, don't assume label is correct.  While at it I checked the only other "1:9" I have.  That one was actually tighter, 1:8.5 or maybe closer to 1:8.4.  I'll bet the second one can shoot 77s.  Thanks for the suggestion.  Sorry for the semi thread jack.

That should've been a reject! Where's the Footlong lawyer?

Thanks for measuring. Since my twist thread archived a year or two ago, hopefully this post comes up in search if someone has 1:9 trouble.