HMR, legal barrel length

rrpc

Vegeta wrote:

I don't mean to come across arguementative but this is just plain incorrect. Yes target shooters use sub sonic ammo but the faster a bullet travels from A to B the less time wind can effect it and therefore cannot throw it off course as much.

Not being argumentative either , but although it seems counter-intuitive the opposite is true. Wind doesn't actually 'blow' the round. What actually happens is that air pressure increases on the side of the round the wind is blowing from causing the bullet to veer towards the area of lower pressure on the opposite side. With a higher speed round, the veering effect is more pronounced due to lower pressure on the leeward side and the bullet is deflected more. This is more pronounced at the muzzle (where a small deviation has far greater effect on the point of impact than would be found were the deviation to occur closer to the target). The movement isn't left/right either, as the spin of the bullet changes the area of high pressure so that a wind from the right will cause the bullet to veer left and up and from the left, right and down.

The only thing I can think of is that by chopping off a few inches from the barrel is that the expanding gas will be in greater volume at the muzzle increasing yaw angle and taking longer to damp out. Other than that none of the above explain it.

That's what I said at the start. The longer barrel will help the bullet 'settle down' in flight quicker.

Vegeta

rrpc wrote:

Not being argumentative either , but although it seems counter-intuitive the opposite is true. Wind doesn't actually 'blow' the round. What actually happens is that air pressure increases on the side of the round the wind is blowing from causing the bullet to veer towards the area of lower pressure on the opposite side. With a higher speed round, the veering effect is more pronounced due to lower pressure on the leeward side and the bullet is deflected more. This is more pronounced at the muzzle (where a small deviation has far greater effect on the point of impact than would be found were the deviation to occur closer to the target). The movement isn't left/right either, as the spin of the bullet changes the area of high pressure so that a wind from the right will cause the bullet to veer left and up and from the left, right and down.

That's what I said at the start. The longer barrel will help the bullet 'settle down' in flight quicker.

The magnus force in't it

but in general for ammo above the speed of sound (the .17 HMR for example) what I have said is correct, the faster and heavier the bullet the better when it comes to wind.

but the .22lr sub sonic ammo is a special case because air turbulance and pressure changes occur above the speed of sound. 22lr match ammo is specifically designed not to go into the upper transition range of the speed of sound as drag increases disproportionately with velocity in this velocity range

So if I was John Galway I would have no problem cutting the barrel of the .17HMR when I had researched how efficiently standard ammo would burn powder in the new length and that the re-crown is a top notch job so it doesn't add to the initial yaw angle.

rrpc

Vegeta wrote:

The magnus force in't it

but in general for ammo above the speed of sound (the .17 HMR for example) what I have said is correct, the faster and heavier the bullet the better when it comes to wind.

There's one thing that I didn't make clear in my previous post. When wind exerts force on a bullet, it doesn't shift the bullet sideways but changes its direction. This is why the wind deviation at the muzzle is much more detrimental to the point of impact than a wind deviation closer to the target.

It's a bit like the change you make to your windage adjustment screw on your sights. If one click is 1/6 moa that's an angular change from muzzle to target hence the difference at the target will be much greater the farther the target is from the muzzle. One click on my sights is .5mm at 25 yards, 1mm at 50 metres and 2mm at 100 yards. The wind will have the same effect. If you are shooting at 100 yards and a 1/6 MOA gust of wind pushes the bullet at the muzzle the round will strike the target approx 6mm off. If it hits the bullet 25 yards from the target the impact will only be perhaps 1.5mm off.

And you're right about the weight of the bullet, A heavier bullet will defeat the wind far better than a faster bullet. That's a bit simplistic obviously as there's a weight/velocity balance. The shape of the bullet also makes a difference with the FMJ type being better in the wind than a soft point.

With .22lr, a 40gr target round with a muzzle velocity of 325m/s has 30% less wind drift than a 40gr 390m/s round at 100m. The only difference is the speed of the round.

It's harder to compare centrefire rounds because of the many different calibres and weights, but taking two 55gr rounds; a 222 Rem (880m/s) and a 6.5x55 (780m/s) the slower round shows 26% less wind drift at 300m than the faster one.

Vegeta

rrpc wrote:

With .22lr, a 40gr target round with a muzzle velocity of 325m/s has 30% less wind drift than a 40gr 390m/s round at 100m. The only difference is the speed of the round.

but under normal atmospheric conditions the speed of sound is 344m/s so that comparison is not valid

It's harder to compare centrefire rounds because of the many different calibres and weights, but taking two 55gr rounds; a 222 Rem (880m/s) and a 6.5x55 (780m/s) the slower round shows 26% less wind drift at 300m than the faster one.

Ok well there'd be the problem of the BC's not being equal. Do both rifles have the same twist rate? Wouldn't a 6.5x55 rifle usually have a slower twist rate than a .222

Its not that I don't believe you rrpc (and i am enjoying the education I am getting) but having re-read the chapter on wind deflection in my ballistics book it definitely says that everything else equal (a statement not to be taken lightly) that for ammo in the super sonic range a faster bullet is effected by wind less. It does mention the .22lr sub sonic ammo as a special case which I have tried to explain above.

bullets

Vegeta wrote:

Its not that I don't believe you rrpc (and i am enjoying the education I am getting) but having re-read the chapter on wind deflection in my ballistics book it definitely says that everything else equal (a statement not to be taken lightly)

Damn interesting thread. I really got to start reading that ballistics book
myself I only got to the first chapter in it and never got the time to
continue reading it.

My HMR has a 20.9 inch barrel. I have been using 17grains all along
but yesterday switched to 20grain after re-zeroing the difference was
noticeable. All day long was firing 5 Round groups that were about
10-13mm at 50m. Which was better than I was shooting before.
(If I could get that at 100m consistently I would be content)

CZ 452 American BBl has 16" barrel and the scout version is a
little over 16". Only appear to chamber it is .22LR though
have not seen a rimmy for a .17hmr with a very short barrel


~B

rrpc

Vegeta wrote:

but under normal atmospheric conditions the speed of sound is 344m/s so that comparison is not valid

Why is it not valid? The supersonic round is more susceptible to wind drift than the subsonic one, both the same weight. I'd accept there being a difference across the sound barrier, but 30% is a compelling figure.

there'd be the problem of the BC's not being equal. Do both rifles have the same twist rate? Wouldn't a 6.5x55 rifle usually have a slower twist rate than a .222

OK I've found some figures I had for the .308 Rem. Again it's very difficult because of different bullet weights and formats, but this is the closest I could find. 150gr, 150m/s Vs 165Gr, 165m/s The difference between the two is 15% less wind drift for the slower round, which can't be accounted for totally by weight as the weight difference is only 9%.

Its not that I don't believe you rrpc (and i am enjoying the education I am getting) but having re-read the chapter on wind deflection in my ballistics book it definitely says that everything else equal (a statement not to be taken lightly) that for ammo in the super sonic range a faster bullet is effected by wind less. It does mention the .22lr sub sonic ammo as a special case which I have tried to explain above.

And what i'm trying to say is it's not as simple as that. In fact, I can't find any ballistics figure that show a faster bulllet being less susceptible to wind than a slower one. I can find the opposite, but in most cases the rounds are different weights. I've even found that faster lighter rounds show more wind dirft than slower heavier ones, especially at longer distances.

Incidntally, the figures I gave in my previous post were from a 600mm barrel (.222) against a 740mm barrel (6.5x55). So the longer barrel with the heavier/slower round gave the best wind drift figures.