.223 twist - just how much does it affect velocity

[C h r i s]

It's widely known that a faster twist can slightly slow down a bullet, compared to the same round of ammunition in an otherwise identical rifle with slower twist (so same barrel length, bore size, material). I'd always assumed this drop to be very small and read somewhere that for a 30cal its a loss of 1.25 fps per inch-unit of barrel twist (Quoting Bryan Litz).

So after picking up a new Tikka T3x .223 with an 8" twist 22.4" barrel I did a series of chrono tests, as I have some data from when I had the same rifle in a 12" twist.

The results were surprising, and so I'm thinking that there are other significant factors at play here, I was expecting a 5 ft/sec muzzle velocity reduction.

Average MV from 3 shots of each ammunition type as follows, these are all factory rounds, same boxes of ammunition for both rifles, same chrono (magneto V3), both rifles had 10 rounds from new (plus proof) and cleaned before the tests were conducted.

22.4", 12" twist, Tikka T3x, stainless, at 11 Deg C

Hornady 40gn vmax 'varmint express' : 3760 ft/sec, SD 36.2
Hornady 53gn vmax 'superformance': 3336 ft/sec, SD 13.6
GGG 69gn HPBT 2911 ft/sec, SD 11.2 (unstable)

22.4", 8" twist, Tikka T3x, stainless, at 15 Deg C

Hornady 40gn vmax 'varmint express' : 3628 ft/sec, SD 14.5
Hornady 53gn vmax 'superformance': 3373 ft/sec, SD 11.0
GGG 69gn HPBT 2905 ft/sec, SD 9.1

Notice how the 40gn bullets are travelling approx 130ft/sec faster from the slower twist rifle. Where as the 53gn and 69gn are not a million miles apart between rifles.

Obviously not a 'scientific' test, but interesting none the less. As the chrono is strapped to the end of rifle it cant be the air density difference due to temperature variation, but I suppose some of this could be down to temperature of the ammunition.

 

[User00040]

Not a sufficient test as you have failed to account for:

1. nominal bore diameter and subsequent constriction/expansion along it's length

2. barrel wear affecting the above

Most reports are of Tikka barrels being 'slow' relative to other makes, could be down to a minute amount of variation between barrel internal dimensions.

Also, barrels tend to speed up with use.

 

[C h r i s]

Agreed, although both were brand new so no wear. However I do take your point that one barrel can be "faster" than another, just surprised by how much!

 

[User00040]

Agreed, although both were brand new so no wear. However I do take your point that one barrel can be "faster" than another, just surprised by how much!

I think Bryan Litz determined that a faster twist had very little effect on muzzle velocity.

Chamber variance is probably the culprit here.

 

[Miki]

I imagine you have bought a 1:8 as you want to fire heavy or long (monolithic) bullets ?
@caberslash beat me to it .... there is little difference in muzzel velocity relative to twist rate, several other factors bear down on the bullet to a greater extent.

 

[C h r i s]

I imagine you have bought a 1:8 as you want to fire heavy or long (monolithic) bullets ?
@caberslash beat me to it .... there is little difference in muzzel velocity relative to twist rate, several other factors bear down on the bullet to a greater extent.

Both actually, it shot the 40gn incredibly well, so can do vermin as well as longer distance paper target

 

[Bowland blades]

more to do with bore finish internally , twist won't be the issue imo

 

[C h r i s]

Strange that it was only the 40gn's that exhibited this difference.

I'll put some more rounds down it and test again, out of pure curiosity as it shoots very well.

 

[ChesterP]

New Tikka barrels are notorious for speeding up after about the first 200 shots through the barrel. A degree of work hardening of the steel has been previously cited as one possible cause meaning pressure rise for similar charge weights due to less barrel expansion (but whether this is true I don't know) so I would not be too conclusive on MV tests until you have 200 rounds through that barrel. Be careful with the loads as they can go from looking ok on a fresh barrel to overpressure on one with several hundred down the tube. All my tikkas have been like this. The MVs on my 6.5 jumped almost 50fps from out of the box to 200 rounds fired.

 

[ChesterP]

Strange that it was only the 40gn's that exhibited this difference.

I'll put some more rounds down it and test again, out of pure curiosity as it shoots very well.

It's a Tikka!