Bullet type and weight vs Barrel twist rate

[1075 tech]

In my limited experience, I found that my 1/7 16" 5.56 won't shoot 55 gr with any kind of accuracy. With 62 gr, I can damn near shoot golf balls at 100 yds. The 55 gr were 223 Rem rather than 5.56 Nato. I haven't had the opportunity to try 55 gr 5.56 Nato to see if it makes a difference.

Anyway, question is, does the twist rate in the Grendel make that much difference with different bullet weights? My 16" AA has 1/7.5.

[LRRPF52]

No matter what bullet I have shot in my 16" AA barrel with 1/7.5 twist, I see the same basic groups of anywhere from .75 MOA to 1.2 MOA.

It has been that way with:

90gr TNT factory load
100gr Nosler Ballistic Tip hand loads
120gr Sierra Match King hand loads
123gr SST factory
123gr AMAX factory and hand loads
123gr Lapua Scenar
129gr SST hand loads

I haven't even bedded that barrel into that upper either, or done any of the accuracy enhancing techniques on it other than free-float.

Also, in my 5.56 and 223 Wylde AR15's with 1/7 or 1/8 twist, 55gr M193 shoots very well. Usually within 2", and often 1.5" or less, but I normally shoot steel or CQM tables with it, so more standing shooting for close quarters.

[fivewhy]

In my limited experience, I found that my 1/7 16" 5.56 won't shoot 55 gr with any kind of accuracy. With 62 gr, I can damn near shoot golf balls at 100 yds. The 55 gr were 223 Rem rather than 5.56 Nato. I haven't had the opportunity to try 55 gr 5.56 Nato to see if it makes a difference.

Anyway, question is, does the twist rate in the Grendel make that much difference with different bullet weights? My 16" AA has 1/7.5.

This is my understanding of twist and its effects on accuracy; those more-knowledgeable than I can let me know if I’m talking above my paygrade.

First, you want enough spin to stabilize your bullet when taking into account a) your bullets weight and length, b) your muzzle velocity, and c) your intended distance to target. Obviously you cannot change your rifles twist (i.e., you cannot change the rpm spin rate of your bullets), so you generally have to build a rifle with an intended class of bullets, velocities, and distances already in mind.

Second, there is a concept known as a bullet “going to sleep” as it progresses downrange. I will explain my understanding of this concept below, but first check out these two links.

Link 1 – If you look at the last post here, a guy says: “With light bullets it would shoot 1.5" at 100 and about 1.5" at 200. With heavier bullets it did the standard 1" at 100, 2" at 200, etc, etc. Has to do with over-spinning the bullet, and how long it takes for the bullet to stabilize once it leaves the bore.” ..............Your story with the 556NATO is the same; you’re not alone in the world....in other words, that guy was overspinning his lightweight bullets when shooting at short 100y distances. As his light bullets went to sleep or calmed down, his group size shrank in relation to MOA. The heavy bullets kept the same 1 MOA size because they were never overspun, but the lightweight bullets dropped their MOA group size over a given distance, from 1.5 MOA at 100y to 0.75 MOA at 200y.

Link 2 – Here is a Litz video diagramming a bullet-nose’s pitching and yawing at it travels downrange, not the whole bullet, just it's nose…notice how the pitch/yaw calm down over time as the bullet’s spin decreases over time.

Here is the concept that I think your question asks: For a given bullet weight/length and velocity, the bullet will have a certain Miller Stability Factor (which is call “SG”) at the muzzle. As the bullet’s rate of spin decreases over time, the SG decreases over time as it travels downrange. The ideal SG is just enough, but not too much spin stability…under spinning or over spinning are bad….but what matters most is the spin or SG at your target distance (or in the vicinity of your target distance).

So let’s assume that you intend to shoot 123gr AMAXs or other bullets in the 120gr range, out of your Grendel…and let’s start with a 1:8 twist. At 2600fps, the 123AMAX has an SG of 1.967 at standard air temp/pressure (as air temp increases, SG increases; buuut as air pressure increases, SG drops).

Assuming a MV of 2600fps, your velocity drops to 1400fps at 800y. At 800y, your SG is 1.600 at standard pressure. This still puts you above the necessary SG of 1.5 to be considered stable. Moreover, your SG at 800y is not overly high such that it should negatively affect your group size at 800y. Your SG, with the 123AMAX and a 1:8 twist, does not drop below 1.5 until you hit 1100fps, which is well past 1000y if your initial MV is 2600fps.

Therefore, I conclude that a 1:8 twist is pretty close to ideal for a 123gr AMAX (and similarly sized .264 bullets) for shooting at 800 yards or so if you can get your muzzle velocity up to 2600fps.

Now, taking your 16” bbl at 1:7.5 twist, you could push a 123 AMAX to around 2375fps or 2400fps without great difficulty or parts-wear. Let’s take the worse of the two…with a MV of 2375fps and a 1:7.5 twist, you get an SG of 2.172. At 800y, your velocity would be around 1250fps and give you an SG of 1.753….which is all pretty good.

Therefore, I conclude your slightly faster twist of 1:7.5 is probably a little fast for a Grendel twist rate, but this faster twist helps to account for the decreased velocity that you would get from a 16” bbl in comparison to a 24” bbl. Which indicates your bbl is pretty close to ideal given its length….but this conclusion ASSUMES that you use heavy 123gr pills.

In contrast, if you push a 100gr AMAX at 2700fps MV (which you could do in a 16” bbl), you would have an SG of 4.944 at the muzzle. This is, imo, too much twist for the shorter range that the 100gr pill would be used for. In other words, in your Grendel, you would have the exact same situation you had with the 55gr pills in your 556NATO rifle…lightweight bullets aren’t great at close range but heavier bullets would work well in the fast twist of your Grendel rifle. However, there is always a fix...if you slow down the velocity of your lighter bullets and thereby reduce the SG at short ranges, then you could obviate this problem. Or just ignore it completely, and shoot at targets where the SG has already calmed down enough, given whatever is your bullet and muzzle velocity.

…….or what does the community think? I’m trying to learn too, here.

[customcutter]

Fivewhy welcome aboard! All I can say is if it's above your paygrade it's definitely above mine. I was going to call BS after looking at the first link. I mean the guy admitted he was shooting from two different benches at two different ranges. Then I clicked on the second link, and it seems there might be something too it. Not saying I'm ready to drink the whole glass of Kool-Aid, but I've never had anything happen like it, in my limited shooting experience. Still can't wrap my two brain cells around the idea of a gun shooting better groups at 200yds than 100yds....

Edit: By better groups I mean smaller MOA

[fivewhy]

As a side note, based on what I've read from Litz (who is a great teacher, btw), any SG / stability factor above 2.0 is probably too high. Basically, if you start at the muzzle with a 1.5, it will gradually decrease to 1.0 as the spin decreases...then once you drop below 1.0 you start having problems with actual flight-path stability and you could have tumbling in the worst case scenarios...but this typically occurs right as you enter into the transonic velocities ranges. Litz loves talking about transonic velocities...and I frankly will never have to worry about that.

[fivewhy]

Thanks Cutter, I appreciate the nice thoughts. I'm learning and growing in this sport, so I'm trying to figure it out as a I go. We will see how it goes. Thanks again for the welcome.

[LRRPF52]

AA determined that a 1/7.5" twist is optimal for 18" and shorter barrels for this very reason, while using a looser twist for many of the 20 and 24" barrels.

Velocity does affect spin stability. The other problem is that probably 95-98% of shooters are only going to look at accuracy where it barely counts, at 100yds, while those who shoot long range prefer to see what's going on with groups at 300yds.

The tighter twists can produce spin stability that hasn't yet gone to sleep at close range, but shoot great at distance.

When I was challenged to shoot my 16" Grendel at 1200yds the first time, it really surprised me how predictable the impacts were with factory 123gr AMAX. I have a 1/7.5" twist on that barrel. Todd Hodnett also has seen predictable results with .308 and .338 LM at Extreme Long Range by using really tight twists for those guns, but stiffer, shorter barrels.

Even compared to longer barrels with more mv, but looser twist, he beats them for hit probability with a 16" gas operated .308 vs. 24" bolt gun .308, using a 1/9 twist on the .308 Win. With the .338 LM, he uses a tight twist with a 20" barrel.

These theories about spin stability have been proven in the real world, by folks who focus on practical shooting and mainly look at what they see happening downrange.

[1075 tech]

Thanks five and LR. I suspected the 1/7 was too fast for the 55 gr. I hadn't thought about it settling out at longer ranges. I'm pretty limited here in opportunities to practice at longer ranges very often. Given the chance, I'll see what she does at 200 and 300 with the 55's before I trade them all for 62's.

[Klem]

Optimum twist rate implies the bullet remaining stable until the target. There is a window between not enough spin and too much. Not enough speaks for itself however a bullet can be too stable which results in one or more of the following;

1. The bullet flies apart (unlikely in the Grendel using bullets from reputable manufacturers).
2. Slight imperfections in the bullet alter the trajectory the faster it spins.
3. The bullet resists pointing to the target after apogee.

There is a stability calculator on the JBM website where you can punch in your bullet details and twist rate. It uses the Miller formula. The Greenhills Formula is the older standard.

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[LRRPF52]

In extreme long range, I've seen bullet impacts that didn't nose over after the apogee, and it was very prominent on the aluminum plate reactive targets.

It would be devastating for terminal ballistics, especially since these were 7mm or .338 LM.

[pdq5oh]

Looking at the Litz video the bullet doesn't appear to begin settling down until 125 yds. It doesn't become truly settled until 195 yds. I've suspected this to be the problem with my rifle not shooting the Berger 130 Hybrids well at 100 yds. I haven't been able to shoot them on paper farther than that. I DID have good luck shooting them at steel at 1000 yds, tho. Better hit rate than SST loads that shoot stupid little groups at 100 & 200. This leads me to believe long, high BC bullets don't shoot as well close in, but will perform better at greater distance than shorter, lower BC bullets. I assume this is why bench rest shooters use short, flat base bullets at short range. Some of my best groups at 100 yds have been with 52-62 gr flat base bullets with my 5.56 rifles.

[1075 tech]

Some of my best groups at 100 yds have been with 52-62 gr flat base bullets with my 5.56 rifles.

I'll have to check those 55's I was using. I'm not sure of the profile. Just some factory Federal bulk ammo. The 62's that shot well were ZQI steel core. In my belief that the 1/7 was too fast for the lighter bullets, I did pick up some 69gr Nosler CC to reload but haven't had the opportunity to get out to test.


Regarding my 16'' 1/7.5 Grendel, it would seem to follow that the heavier 120+ pills would give better accuracy wise at shorter ranges. But LR52 seems have decent results with a wider range, 90-129gr.

Given the testing and results posted with the lighter mono's, it is something I would be interested in. I may have to try an 18" or 20" for those?

[sneaky one]

1075- the lighter monos work best with a faster twist- 1-9 is only marginal.

[Rainman]

With a very limited test of handloads so far at 100 yards in my 1:9" twist 18" BHW barrel, groups nearly doubled with both Hornady 129 SSTs and Berger 130 grain bullets to just under MOA. I can pretty consistently shoot 100 to 123 grain factory and handloads at half MOA for five shots at 100 yards. A younger Navy acquaintance shoots my rifle with the same ammo down to .30 MOA. My next step will be to try the 129/130 loads at 200 yards in the 1:9" upper and then the same loads at 100 and 200 yards in my new AA 16" Lightweight upper with 1:7.5" twist. My anticipation is the 200 yard groups from the 1:9" barrel will show even greater deterioration of accuracy and the 1:7.5" AA upper will stabilize both bullets and deliver much better groups at 100 and 200 yards. I'm limited to 200 yards until late March or April when I will again have an oppotunity to shoot paper to 400 yards and steel to 1,100.

[1075 tech]

1075- the lighter monos work best with a faster twist- 1-9 is only marginal.

So.... May work well in the 1/7.5 16"? That would be awesome.