Twist Rate for Short Barrels

[BoomDeBoom]

I'm thinking of adding a center fire barrel to my Thompson Center Contender. .17,.20, and .22 calibers are all being considered. In these smaller calibers will bullets be stabilized in short barrels (10" to 15")? Are cartridge efficiencies significantly reduced? What might be the difference in accuracy over longer barrels (20" to 24")?

 

[B23]

Unless you are shooting iron sights, accuracy is a moot point, and even if you are shooting iron sights, length only makes it easier for you to shoot accurately, nothing really about the inherent accuracy of the gun itself.

As for the other, just make sure you order a barrel with enough twist to stabilize the bullets you want to shoot at the speeds you'll be shooting them. Also, since you'll have a fairly short-ish barrel, IMO, it wouldn't make a whole lot of sense to select something that is largely overbore.

 

[BoomDeBoom]

Unless you are shooting iron sights, accuracy is a moot point, and even if you are shooting iron sights, length only makes it easier for you to shoot accurately, nothing really about the inherent accuracy of the gun itself.

As for the other, just make sure you order a barrel with enough twist to stabilize the bullets you want to shoot at the speeds you'll be shooting them. Also, since you'll have a fairly short-ish barrel, IMO, it wouldn't make a whole lot of sense to select something that is largely overbore.

B23 In your opinion to avoid overbore would the 221 Fireball or it's 17 and 20 derivatives work better then 222-223 derivatives in a 15" barrel? I am looking for excellent 100-200 yard accuracy for shooting paper. The Hornet derivatives seem to be too difficult to load for a novice reloader and may not have the inherent accuracy or I would consider those.

 

[biggun2869]

My 2 favorites are .223 and 7mm TCU They are both std TC bbls 14 inch and don't know what the twist is, maybe someone will chime in. (KEN LEWIS????) Luckil I have a 10 inch K hornet and I like it but wouldn't recommend for best accuracy. I use 50grn BTs in the 223 and 120grn Nosler solid base in the 7mm ( they haven't made these for a long time) Lucky for me I have almost 500 of them and no more than I shoot it, will easily last me.
You'll be amazed at how well they will shoot!

 

[Evan]

B23 In your opinion to avoid overbore would the 221 Fireball or it's 17 and 20 derivatives work better then 222-223 derivatives in a 15" barrel? I am looking for excellent 100-200 yard accuracy for shooting paper. The Hornet derivatives seem to be too difficult to load for a novice reloader and may not have the inherent accuracy or I would consider those.

I can't speak to anything but the 20 Vartarg, which I've got in a full bench rifle, but enough people have had exceptional accuracy with the 221 and the 20Vartarg that I think it's safe to say that it is an inherently accurate design. That said the 222 and many of its variants were the winning guns for quite a while in benchrest.

Your concern over a 10-15" barrel with this class of cartridges isn't necessary. Both 222 and 221 will burn completely, give good velocity, and have great accuracy potential with good components. They have a long history of doing so in everything from bolt action pistols up to big bench rifles.

 

[Oso]

I'm thinking of adding a center fire barrel to my Thompson Center Contender. .17,.20, and .22 calibers are all being considered. In these smaller calibers will bullets be stabilized in short barrels (10" to 15")? Are cartridge efficiencies significantly reduced? What might be the difference in accuracy over longer barrels (20" to 24")?

You need a certain length of barrel to achieve a desired velocity, but you can play with a shortened barrel by using a powders with a faster burn rate. The XP100 (bolt action pistol) was designed for the 221 Fireball and its barrels were relatively short. Accuracy was good.

The barrel length, twist rate and bullet ogee is more the balance. To offset the shortened barrel length a faster twist rate is needed to stabilize the bullet. Also, a bullet profile that provide more baring surface (flat based) improves the stability in a short barrel. 300Blackout is good example. Shooting a 200gr .308 match bullet (long tapered profile with minimal baring surface). In a .308 with 20"-30" barrel the twist rate is 1:10 and the powder is slower burning, such as varget. Same bullet used in 300BO with a 8" short barreled would be a 1:8 twist and use pistol powder to ensure full burn and near max velocity by time it reached the muzzle. Once you determine how much powder you can burn within that barrel length you dial down you reload charge so that you aren't throwing a bunch of unburned powder out the muzzle to then be ignited in a Fireball.

There is a trade-off with barrel length. Even with faster powders, it is difficult to replicate the same velocity that a traditional length barrel provides...but you can get close. Generally, longer barrels are more accurate in that the bullet has more time to stabilize and also sometimes a slower powder is what the gun prefers. Slow powder and extreme ogee bullets aren't good combo for short barrels. However, you can achieve acceptable accuracy coupled with the portability, which may be more important than winning a benchrest competition.

 

[JSH]

Pistols are not for everyone. Takes a bit more to become proficient with one.

My 15” 308 gives up very little to a sporting rifle. Same with my 223.

There are those here that are shooting handguns way farther than I tend to anymore. I concentrate more on 300 and under.

 

[B23]

B23 In your opinion to avoid overbore would the 221 Fireball or it's 17 and 20 derivatives work better then 222-223 derivatives in a 15" barrel? I am looking for excellent 100-200 yard accuracy for shooting paper. The Hornet derivatives seem to be too difficult to load for a novice reloader and may not have the inherent accuracy or I would consider those.

I'm partial to the triple deuce so, for me, I'd go 222 and take advantage of the MUCH bigger bullet selection of the 22 caliber versus the much smaller bullet selection of the .17 and .20 cals.

 

[mikecr]

Generally, longer barrels are more accurate in that the bullet has more time to stabilize

This is not true. Stability is not tied to time.
Twist requirements are set at relative displacement per complete turn.

 

[Oso]

This not true. Stability is not tied to time.
Twist requirements are set at relative displacement per complete turn.

Twist is tied to distance...how far bullet travels in barrel while making 1 rotation. There is a "tendency" for higher BC bullets to perform better in longer barrels because additional barrel length provide additional space for rotations inside barrel, which generally translates to greater stability for high BC bullets. Assuming same twist rate, a 16" barrel will do better stabilizing a high BC bullet then will an 8" barrel. Lower BC bullets (squatty, flat bottom, large baring surface) are more forgiving and stabilize easier at slower twist rates.

Like the Pirates' Code...this is a guideline, not a rule... No crazy math just common sense.

 

[dmoran]

You need a certain length of barrel to achieve a desired velocity, but you can play with a shortened barrel by using a powders with a faster burn rate. The XP100 (bolt action pistol) was designed for the 221 Fireball and its barrels were relatively short. Accuracy was good.

The barrel length, twist rate and bullet ogee is more the balance. To offset the shortened barrel length a faster twist rate is needed to stabilize the bullet. Also, a bullet profile that provide more baring surface (flat based) improves the stability in a short barrel. 300Blackout is good example. Shooting a 200gr .308 match bullet (long tapered profile with minimal baring surface). In a .308 with 20"-30" barrel the twist rate is 1:10 and the powder is slower burning, such as varget. Same bullet used in 300BO with a 8" short barreled would be a 1:8 twist and use pistol powder to ensure full burn and near max velocity by time it reached the muzzle. Once you determine how much powder you can burn within that barrel length you dial down you reload charge so that you aren't throwing a bunch of unburned powder out the muzzle to then be ignited in a Fireball.

There is a trade-off with barrel length. Even with faster powders, it is difficult to replicate the same velocity that a traditional length barrel provides...but you can get close. Generally, longer barrels are more accurate in that the bullet has more time to stabilize and also sometimes a slower powder is what the gun prefers. Slow powder and extreme ogee bullets aren't good combo for short barrels. However, you can achieve acceptable accuracy coupled with the portability, which may be more important than winning a benchrest competition.

Don't want to get into a pissing match, but just have to say I strongly disagree with much of what your saying, or believe to be factually true.

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[Oso]

Twist is tied to distance...how far bullet travels in barrel while making 1 rotation. There is a "tendency" for higher BC bullets to perform better in longer barrel provides additional rotation for bullet while inside barrel, which generally translates to greater stability for high BC bullets. Assuming same twist rate, a 16" barrel will do better stabilizing a high BC bullet then will an 8" barrel. Lower BC bullets (squatty, flat bottom, large baring surface) are more forgiving and stabilize easier at slower twist rates.

Like the Pirates' Code...this is a guideline, not a rule... No crazy math just common sense.

 

[Oso]

Don't want to get into a pissing match, but just have to say I strongly disagree with much of what your saying, or believe to be factually true.

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No pissing match - please explain. Completely open to listening and learning.

 

[mikecr]

There is a "tendency" for higher BC bullets to perform better in longer barrels because additional barrel length provide additional space for rotations inside barrel, which generally translates to greater stability for high BC bullets. Assuming same twist rate, a 16" barrel will do better stabilizing a high BC bullet then will an 8" barrel.

Twist rate is established with rifling, regardless of barrel length.
Please expand on this.

 

[Ned Ludd]

The effect of barrel length on twist rate for a given length projectile is pretty straightforward. Barrel length strongly affects the velocity that can be achieved at safe operating pressure. Thus, the optimal twist rate for a given barrel length should be based on the velocity you can reasonably expect to achieve with a given bullet. It is the linear velocity of a bullet through a given twist rate that specifies its rotational velocity, not its rate of acceleration. In practical terms, that means for a specified twist rate, it is how fast the bullet is moving through the very last part of a bore just before exiting the muzzle that dictates its rotational velocity as it leaves the barrel. A very good example of this is found with gain twist barrels.

Regardless of actual barrel length, with a decent estimate of expected muzzle velocity, you can determine the barrel twist rate that will fully stabilize a given length bullet under specific atmospheric conditions using a calculator such as Berger's Twist Rate Stability Calculator.

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

 

[Oso]

Twist rate is established with rifling, regardless of barrel length.
Please expand on this.

Not sure why/how my response double posted earlier.

Yes - sorry I oversimplified my earlier response. Rifling determines twist rate. Barrel length divided by Twist Rate equals number of rotations bullet spins inside a barrel. Higher BC bullet's streamlined profile results in a narrow band of contact (baring surface) with barrel's bore. As such, higher BC bullets take a little more work to stabilize... need a little more rotation. This can be achieved either by faster twist rate for a shorter barrel or slower twist rate in longer barrel, with both scenarios achieving the required amount of rotation to stabilize same bullet.

I should clarify that short vs long comparison can be something as simple a 8" versus 16" barrel.

 

[fyrewall]

My Berger manual shows 31 fps gain or loss per inch for a 24 inch barrel for the popular .243 Win. Other rounds are sort of similar. I keep seeing stuff where barrels are chopped of in 1 inch increments and that sort of matches what the Berger manual sez.

RPM = MV * (12/twist rate inches) * 60.
A 24 inch long .243W 10 twist with a 87 @ 3095 the RPM = 3095 * (12/10) * 60 = 222, 840 (really spinning) - normal barrel
A 20 inch long .243W 10 twist with a 87 @ 2971 (3095 - (31*4)), RPM = 2971 * (12/10) * 60 = 213,912 (still really spinning) -stubby barrel.

Bullets accelerate inside the barrel but decelerate outside (MV) but the RPM's essentially remain constant for the short time of flight.

Calculations using the Berger stability estimator calculator match those of the Miller estimator calculator and come up with some stability number -Sg. The calculations are sort of involved but simple math - MV, diameter, length, weight, temperature, pressure ….. The bullet length is the most important. A Sg number is produced with an advisory of a minimum of 1.4 or 1.5 depending on who does the calculator. The Berger estimator makes references as to optimum ballistic capabilities considering stability. Sometime ago I went on-line and retrieved the Miller calculations and plugged them into an Excel spread sheet so I would not need to mess with various sites. Don't include the plastic tip thing in the length. Yes cartridge/bullet efficiencies would be reduced should the bullet not be adequately stabilized (wobble). Use a bullet having a length and the other considerations to enable stabilization.

The real experts on this forum would elaborate on this - like writing books.

 

[JSH]

Using slow pistol type powders, or fast rifle powders in long barreled pistols, has pretty much been disproven.
Short barrels and twist rates have fairer the same.
Muzzle blast, sure your going to get more. Some of my best loads are blasty and have an enormous fire ball, but the proof is on the paper.
There are those around that have found MOST things that work well in rifles, worked as well in their handguns.

Smaller circle, but I was seeing folks shooting further with handguns 20 years ago than rifles. A fair bunch of us got tired of doubting thomas types. I upset my share of rifle shooters with their pet rigs.

I suggest the OP investigate what rifle shooters were using with his caliber of choice. Then dig on some info from pistol shooters. Most rifle cartridges have been chambered at some time or another in a handgun, wildcat or vanilla.

 

[mikecr]

Ok, we have a common perspective error here.
Stability requirements are not addressed with revolutions per time (RPM), but revolutions per displacement (distance). This isn't a play on words. It's what it is, and all else fails tests.

Did you know that gyroscopic stability typically goes up downrange? This, even while velocity has been dropping like a rock? It's not because of turns per time, it's because of turns per displacement.
Forward speed has slowed a lot, while turn speed has barely slowed. If muzzle released turning once per 8", by 600yds the bullet could be turning once per 6" travel. The Sg at that downrange point can be based on an effective twist rate of 6:1.

Also, whether a bullet is barrel released at 500fps or 5,000fps, or 5,000,000fps, it's displacement per turn is the same. If it's an 8:1 twist barrel, the bullet is turning once per 8" -regardless of velocities. You can't do anything about that. It's not slipping in a bore like a clutch surface,, it's not loose in a bore,, but against hard rifling that will not allow bullets to spin at any other rate.

And you can't assume a longer barrel will mean higher velocity, and/or higher velocity means higher stability. You can't assume a smaller bullet, or a lighter bullet, will be faster or more stable either.
The bullet makers have declared their stability requirements regardless of cartridge, barrel length, bullet size, or velocity. They test and report with only one additional factor: an atmospheric standard. This is needed because displacement is of air, at declared density.
8" of air at 5,000ft above sea level is less dense than 8" of air at sea level. The relative displacing of this is different.

On velocity itself, there is drag through a given displacement (like 8" travel). Drag can be a high overturning force. While drag goes up as squared by the velocity, this has to be adjusted lower thanks to our bullets better aerodynamics. It's handled with drag coefficients.
So a faster bullet can be more stable, given lower drag, which can present lower overturning forces. These are 'cans' not 'musts'. It all has to be tested to see final results.
It's probably possible to correlate relative BC:1, in replacing relative 8":1.

 

[Mudygmc]

I dont have near the experience most here have. I say just go by the general rule for rifling and bullets. If you want to run heavies go fast lighter bullets go slow. I have really started to play with bolt pistols alot lately. And in many cases the pet loads for rifles 24+ barrel lengths shoot great in bolt pistol lengths -16. Couple examples. My 26in 22-250 shoots 40gr vmax over 3900 into 3/8 all day. Exact same load in a 14in striker is under 1/2. Easily under 2in at 250. My go to deer rifle is a 300WM. It has a 28in benchmark 1in9.5 barrel. Im running 77.2gr of h1000 and the 212gr eld-x at 3020fps. It is under 1/2 at 250. I built a 300WBY pistol with a 16in 1in10 pacnor on it. Im running 85gr of h1000 behind the same 212gr eld-x at 2730fps. I consistently put 1in and have a few under 3/4 at 250 with the pistol. I have read lots on this type of issue. Havent got a lot of solid info. Some do great other not so much. What i believe to be true is twist rate per bullet weight is more or less a standard. I wouldnt go faster for a pistol than a rifle. On my experiences. Also i think there is an efficiency range per volume of powder. Ive hands down had better luck with Hodgen powders. Belted mags I will take h1000 all day long. Ive tried faster and slower in both my rifle and pistol but best performance is h1000. Creedmoor sized cases I cant beat h4350. Smaller 223 sized h322 and varget are hard to beat.
Maybe for benchrest type accuracy it is different. But thats not me. I shoot steel and fur. Poking holes in paper is fun now and then but not my cup of tea.

Adam