Will 62gr work fine in a 1:9?

Yes.

Many say that twist rate is best compromise for both 55 and 62 grain rounds.

Yes, have done it frequently. Have also shot 55gr M193 in 1:7 without issues.

62gr in a 1:12 or 1:14 though...never tried that. Probably won't.

62 grain bullet fired from a semi-automatic AR-15 with a 1:9" twist barrel.



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No problem up to 75gr

Only if its jacketed lead core bullets.  Long for weight monolithic bullets with no lead, often solid copper are long for weight.  A 70 grain Barnes TSX is longer than a 77 grain SMK.  That 70 grain Barnes will absolutely keyhole badly in my 1:9 barrel.  

But even the long-ish 62 grain Barnes shoots fine in a 1:9.  So any 62 should be fine.  So, if you must shoot lead free ammo, or are shooting monolithic hunting/defensive bullets anything much beyond 62 might not fly straight in a 1:9.

Makes sense, I’ve never tried monolithic ammo

It seems like many companies are moving to 1:7 these days and that most debates about twist rate are not really valid because we don't run 20" barrels very much. For very specific uses you may want to look more into twist rate, but I would say 1:7, 8 or 9 should work.

Why would a 20" barrel matter?

That makes nice holes  just the shape of the bullet sideways. Keyholing. I know from experience, tried one of combos you listed a long time ago.

75 grain Hornady A-MAX fired from a 1:9" twist AR-15 at 25 yards . . .




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Yes..so will the 68-69 grain bullets.

It should work fine however every barrel is different. The best way is to shoot it and see how it groups.

Someone asked about the 20" barrel.  Is that better for heavier bullets, will that work better ??  Answer - yes it works better.  The bullet has more barrel length in which to spin up.  More spin, more stability.
So that's probably an over simplification, but you get the picture.  I also should have said longer bullet.  Not necessarily heavier bullet.  I wondered why my new RR upper in their .223 cal had a 1/9 twist.  I thought "what's the point in that".  So I called RR up and asked.  The helpful tech explained it is because my barrel is 20" in length.  Made sense to me....

16-20" barrel makes about 0.02 SG difference. If that's your margin, you're at the whim of the weather anyway.

People talk about bullet weight because it's convenient, but the twist / spin rate requirement is not a function of the weight of the bullet, but rather the length, weight distribution and bullet shape. "62 gr" trips the alarm bells because 62gr M855 is not a traditional lead core bullet.

A 62gr lead core flat base is very different in stability from a 62gr M855, partial steel core boattail.

This is why the copper monolithic bullets are difficult to judge, for a given weight they're much longer than a traditional lead core jacketed bullet.

Weather conditions also change stability, colder air is denser and increases the need for stability... so a combination that is marginal in hot weather may fall apart in cold temperatures. Then again if you're not dealing with any cold environments you might not worry about having extra stability to handle a weather extreme.

A longer barrel increases velocity which increases spin rate, but also increases aerodynamic forces on the bullet so the net increase in stability is pretty small.

The 1:9 barrel is a relic of institutional inertia from the 1980s and was intended for M855 and the derivatives of that bullet type like the 68 & 69 grain OTMs, which at the time were the longest bullets commonly available. When the 75 & 77 grain OTMs came on the market, a few 1:8.5 barrels were made but this should have become the industry standard. A 1:8.5 will shoot any standard ammo that will fit in a magazine in reasonable weather conditions. But AFAIK no one makes that twist anymore. It was rapidly replaced by the 1:8 in competition use with the advent of the very long 80 grain Sierra OTM, used single-loaded since it's longer than magazine length and needs the faster 1:8 twist.


If you mean M855, then 1:9 is the ideal twist rate according to the traditional rules of thumb.

If you have a 1:9, the 75 and 77gr OTM type bullets will likely be marginal at best, and probably best avoided. The Berger 73gr OTM is supposedly ok in a 1:9, at least in reasonable temperatures. 69 & 68gr OTMs are good.

The 1-in-9 twist goes back much further than the 1980s . . .

In 1952, Winchester was contracted to develop a series of "M1 homologous" bullets in .22, .25, .27 and .30 caliber, all using the FA T1E2 cartridge case (later adopted as the 7.62mm NATO case), necked down to accept the various bullet diameters.  They were also to provide 10,000 rounds of each caliber and M70 rifles chambered appropriately.  In the case of the .22 caliber "M1 Homologous", it was a 66 grain bullet, and screaming along at 3,950 fps out of a 22" barrel, the optimum rifling twist was 1-in-9.5.  If you slow the bullet down to more reasonable speed that you could achieve with a .223 Remington case, 1-in-9 becomes the optimum.

In 1969, the Army was looking at improving the long range performance of the 5.56mm in the Stoner 63 Weapon System, and contracted Cadillac Gage to develop such a cartridge.  Cadillac Gage subsequently sub-contracted Les Industries Valcarter Inc (IVI) of Quebec, Canada to actually do the engineering work.  IVI developed the XM287 Ball and the companion XM288 Tracer.

The XM287 was a 68 grain lead core gilding metal jacketed bullet, approximately 0.93" long with 9 caliber secant ogive with a 9 degree boat tail, this was launched at around 3,000 fps out of a 20" barrel.  The XM288 tracer was similar.

Since, the XM23 and XM207 (the Stoner 63 Rifle and LMG) never when beyond studies, the XM287 cartridge disappeared along with them, taking the 1-in-9 twist with it.

I know the twist wasn't invented in the 80, but it became an industry standard thing for AR15 barrels then. My point was that it should have been replaced by 1:8.5.

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've also heard the myth that bullets had to make a full revolution in the barrel in order to be stable, that more than one revolution made them more stable, etc. Which is nonsense but some apparently believe it.

1-in-8.5 wouldn't stabilize the SS110 tracer . . .

If that myth were true, no 9mm pistol bullet would ever be stable.  The standard 9mm twist is 1-in-10, and few pistols have barrels longer than 10 inches.

1:9 isn't going to be better. And how many people are shooting SS110/M856 tracers, from their consumer AR15s, at actual targets where they need a good group? Nobody.

Same with the number of people shooting their AR15s in -60 degree weather. Maybe 5 people in Alaska.

Feel free to get a faster twist barrel if either applies to you. Or buy some other ammo.

But muh milspec, bro.

75-77 grain bullets, depending on construction and design, still require faster than 1-in-9, and a lot of people shoot them.

So, 1-in-8 or 1-in-7 . . .

Sorry if i use this topic for my own good but: I need to use lead free ammo, do you guys thinks a 1 in 8" 16 inch barrel will shoot the 55gr Barnes TTSX and 35gr Hornady NTX well enough for up to 200 yard hunting or is a different twist better for my application?

The recommended optimal twist rate for the the 55 grain TTSX is in 1:8.6", so you're good for stability.  

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It works just fine

Or 1 in 8.5, which I argued should have replaced 1 in 9 30 years ago, which would have ended the need for 99% of AR15 shooters to worry about twist rate.

1-in-9 is sufficient for 62 -68 grain bullets, and that those were as long civilian bullets went from the 1970s until recently.  And, the military wanted 1-in-7 twist for their barrels due to the long tracer and the match shooters also needed 1-in-7 for the long 80 grain match bullets.  The 77 grain bullets only showing up around 1999-2000.

So, for thirty years, up until around 2000, you had 1-in-12 for 55 grain bullets, 1-in-9 for those that shot 62-68 grain bullets and 1-in-7 for the military and the 80 grain match shooters.  All the tooling required to make1-in-12, 1-in-9 and 1-in-7 existed, what market force was there for barrel companies to invest several thousands of dollars for new and different 1-in-8.5 twist tooling?

It's not like faster twist in shorter bullets is detrimental to accuracy.