Building a precision AR-15: Questions about barrel, bolt, twist, etc...

[HKGuns]

Sierra's doppler radar would disagree with you

Say what?

[opsoff1]

In general you're right, of course, opsoff1, but there are bullet options above 69 grains that "appear" able to be stabilized in a 1/9 twist barrel.

On a cursory look only, it appears the Miller stability breaks at around less than 1" in length projectile for stability in the 1/9 twist barrel and there are valid options above 69 grains one could use with that twist. Of course, not all are going to work and they are on the fringe of being acceptable, but they do fall within the acceptable range.

For example, a .990" 77gr bullet shows a Miller stability of 1.44 which is deemed stable, although admittedly on the edge.

Am I missing something? I've not shot anything above 69 grains yet as my distance shooting options are limited.

Edit: To be clear, I'm not just talking about "fictional" bullets. The Sierra 77 gr Matchkings are .994 and appear to be judged stable if you believe the Miller formula. I suppose experience might be different from theory. Not trying to argue with you, hoping you have some experience that says different.

Sure do - and there's a joke about engineers and therories that comes to mind, but I can't remember the whole thing. LOL, no arguing at all! Healthy exchange of ideas!! Anyway - I've shot bullets from 50gr FBHP all the way up to 90gr VLD experimentals in twists from 1/14 all the way to 1/6.5. Cut rifled, pull button, push button, broach cut, hammer forged, 4 groove, 3 groove, 6 groove and 8 groove bbls.
In my experience - as far as a 1/9 - I have never, ever got anything above a 69 to shoot in that twist - ever. Especially a 75 (HPBT or AMAX) or 77's. An 8 twist is HIGHLY marginal with 80's - it is right on the hairy edge. I've seen a lot of theoretical formulas - but as they say - proof is on the target.
I can give you a lot more on this later - but given any situation - I'd always opt for a tighter twist.

[SWThomas]

Just ordered my barrel. I went with opsoff1's recommendation and got a CLE Douglas. Should have it in a couple weeks!

[MistWolf]

Say what?

Sierra uses a dopler radar to measure the ballistic coeffecient of bullets in flight. They found that the more stable a bullet, the better it's BC. In testing the 69 gr Matchking HPBT, they found the bullet had the best BC with a 1:7 twist. Slower twists resulted in a lower BC due to precession. Precession is basically nose wobble. Sierra determined that the slower the twist, the less stable the bullet, the greater the precession, resulting in greater drag.

Real life shooting shows that shooting longer 22 caliber bullets in a 1:9 twist doesn't work well if the shooter is chasing after accuracy, regardless of Miller Stability

[HD1911]

Sierra uses a dopler radar to measure the ballistic coeffecient of bullets in flight. They found that the more stable a bullet, the better it's BC. In testing the 69 gr Matchking HPBT, they found the bullet had the best BC with a 1:7 twist. Slower twists resulted in a lower BC due to precession. Precession is basically nose wobble. Sierra determined that the slower the twist, the less stable the bullet, the greater the precession, resulting in greater drag.

Real life shooting shows that shooting longer 22 caliber bullets in a 1:9 twist doesn't work well if the shooter is chasing after accuracy, regardless of Miller Stability

Sound info. Agreed, bullets that are longer than normal (think VLD style), generally do better with a Faster Twist.

[opsoff1]

Sierra uses a dopler radar to measure the ballistic coeffecient of bullets in flight. They found that the more stable a bullet, the better it's BC. In testing the 69 gr Matchking HPBT, they found the bullet had the best BC with a 1:7 twist. Slower twists resulted in a lower BC due to precession. Precession is basically nose wobble. Sierra determined that the slower the twist, the less stable the bullet, the greater the precession, resulting in greater drag.

Real life shooting shows that shooting longer 22 caliber bullets in a 1:9 twist doesn't work well if the shooter is chasing after accuracy, regardless of Miller Stability

AS I wrote last night, I'd add more today and MistWolf provided a great segue to this addendum;

HKGuns provided some interesting info derived from the Miller Stability Calculations - but there is always a story behind the story
.
On the surface - the handy dandy calculator is easy and provides a ESWAG (Educated Scientific Wild Ass Guess) at the stability of a projectile in a specific twist.
This gives a lot of us a warm and fuzzy feeling that engineers / ballisticians get when something works as hoped. In the grander scheme of things however, we as trigger pullers and reloaders are really relagated to the bottom of the food chain in firearms/ammo/bullet design/testing, basically "knuckle draggers" (and I'd gladly stand in that celebrated line.) The point is, that warm feeling we get after plugging in all the numbers and see that our Earschplittinloudenboomer 5 twist barrel will (should) stabilize our 507 gr FBHP bullets may actually be poop - when we get to the range and can't hit the barn from inside it - the "plan" turned out to be a guess in a dress.
In order to really get warm and cozy with these theories, there are several data points and "ya, well, but" issues that need to be considered.

1. The calculations and formulas are based on a "proposed" semi-imperical formula - semi imperical being based on observation & experiment. He based much of the calculations on correlating experimental Army projectile data. There is also a hefty amount of mathmatical theory here as well.

2. This proposed formula is based on a set of standard conditions - some of which are exceedingly important and WILL effect the outcome: A velocity of 2800fps AND standardized atmospheric conditions, namely 59deg F, 78% Humidity, 29.53 inche of Mercury and at sea level. All your inputs correct off of these numbers.

3. APROXIMATE corrections are provided for velocity and temperature.

4. Specific bullet types - boattails, hollow points and bronze bullets are lighter in relation to length ~ they are less stable and need tighter twists.

5. The calculations utilize a constant to determine "density" in relation to weight. However, this number is specific ONLY to bullets at 2800fps @ 59deg F., 78% Humidity, 29.53 inches of Mercury and at sea level.

6. Assumptions are universal that a stated twist on a barrel is exactly that - not so. It is a well known fact that button barrels (and other mfring methods as well) can slip and provide slower than advertised twists. None of that is addressed in the calculations.

7. The is also an inherent presumption that we have gyroscopically stable bullets to begin with - meaning there are no manufacturing anomalies - jacket thickness is exactly the same 360 deg, there is no core shift, meplats are identical etc - we all know this is pie in the sky stuff. Just doesn't happen - it the reason we knuckle dragger accuracy nuts sort our bullets... <guilty> None of that is addressed in the calculations.

When calculations are run for low temps - the safety margin for stability jumps up to 2.0 as minimum due to cold air's density.

Those of you who's eyes have not glazed over should sit up and pay attention to this part. The plug & play calculation is based on a VERY rudimentary correction of muzzle velocity based on a constant of 2800fps. Your velocity input "mildly" corrects the stability value.

So - the question then becomes - What is a safe twist for bullets fired in non standard conditions? Like Quantico in August ~ 95deg & 95% humidity.
If one were to actually run the calculations or just plug in your known data - you'd get a number that may be something like 1.46 and he color codes the number - I believe red is unstable, yellow is marginal and green is stable. In theory and supported by semi-impirical data (derived from the Army Research Lab) stability factors above 1.0 are deemed acceptable - Miller has pushed the threshold up a bit to incorporate a safety margin. His acceptable range is 1.3 to 2.0 The Army however considers 1.5 to 2.0 as acceptable. This extra margin is factored in to account for gyroscopic instability (say that 10x fast) - basically lower quality bullets AKA GI Ball. Add to this the issues from #7 above as well as #6 and we can fast forward to the range as we stand in front of our 600 yd target and look at perfectly shaped holes made by bullets going sideways - it's really kinda cool - as long as they are on paper (preferably yours)
So - in the end, it is important to remember that Miller formula is a proposed rule based on Standard Conditions - it does not take into account projectile anomalies, actual barrel twist deviations and wildly different atmospheric conditions. And again - proof is on the target.
I've never been able to get a 75 or a 77gr .224" bullet to stabilize in any 9 twist bbl - ever.

Hope this helps.

ETA - I ran the Miller calculations on a 90gr JLK VLD in .223 (using my known load data) - plugged in conditions that would aproximate a day on the range in the summer and it showed me that it would stabilize in a 1/7.5 twist bbl. Which is not even close. A true cut rifled 1/7 ....maybe... but a 1/6.5 definitely, which is what I use (3 groove) Its an MOA gun with irons at 1K.

[opsoff1]

Sound info. Agreed, bullets that are longer than normal (think VLD style), generally do better with a Faster Twist.

Bear in mind that "over spinning" a bullet generally has no ill effects. The rare occasions when over spinning rears it's ugly head is using light bullets/thin jackets in really fast twist bbls and at hyper velocities - I have shot some 40 gr'ers out of a 8.5 twist 28" bbl (22-250) and they literally came apart - poof!! little grey puffs of smoke a 10-20 yds from the muzzle...gone, nothing on target.

Also consider the implications of velocity and twist.
We have all seen a spinning top on a table top - the faster you spin it the more stable it becomes - same same (to a degree) with bullets. BUT! Introduce any anomaly (put a piece of tape on one side and see what happens) and it won't spin right or for long. Same with a bullet that has jacket / core issues.
A bullet in flight looses velocity far quicker that it's spinning slows down. Because of this a bullet actually becomes more stable the farther it goes, further underscoring the use of appropriate twist rates.
Ballistic coefficient is really a comparrison of a bullet to a "theoretical perfect bullet" i.e. one that carries a BC of 1.0. Higher BC bullets don't loose velocity as quickly as a low BC bullet. As MistWolf added, wobble - aka precession hurts stability / BC and obviously accuracy. Another anomaly occurs as well, known as nutation - this is when the bullet rotates (spins) on an axis that is not centered in the core. Both of these issues CAN occur when bullets have insipient flaws AND are over spun - one huge reason why hard core Bench Rest shooters generally opt for the slowest twist possible that will stabilize their bullets - they do not want to introduce any other issue that will open a group.

[guitarist1993]

Just ordered my barrel. I went with opsoff1's recommendation and got a CLE Douglas. Should have it in a couple weeks!

Excellent choice. I've been incredibly impressed with the accuracy I'm getting from my CLE recon. What length did you decide on?

[TomD]

I'd just get one of WOA's varmint uppers and go shoot!

[HKGuns]

Sierra uses a dopler radar to measure the ballistic coeffecient of bullets in flight. They found that the more stable a bullet, the better it's BC. In testing the 69 gr Matchking HPBT, they found the bullet had the best BC with a 1:7 twist. Slower twists resulted in a lower BC due to precession. Precession is basically nose wobble. Sierra determined that the slower the twist, the less stable the bullet, the greater the precession, resulting in greater drag.

Real life shooting shows that shooting longer 22 caliber bullets in a 1:9 twist doesn't work well if the shooter is chasing after accuracy, regardless of Miller Stability

Gotcha. Thanks for the clarification.

ETA: Opsoff, again amazing information! Thanks.