incorrigible
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Is anyone aware what twist rate would be required to stabilize a 7.62x39 round in 7"?
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What twist rate is required to stabilize a 7.62x39 round in a 7" barrel
- Thread starter incorrigible
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Weight of bullet is an important factor. Since it's 7.62x39 in a 7 inch barrel I'm assuming you're planning on shooting surplus, so accuracy isn't going to be a major factor regardless... and even if they're handloads, the cardtridge itself isn't noted for accuracy, deserved or undeserved, outside 150-200 yards.
Since twist rate is dependent on a number of factors you'd need to supply more info to get an accurate answer. Even with a short barrel there'll still be factors in getting it to stabilise. If you can dig up an old sks barrel that might be a good starting point. Should be dirt cheap and pre-chambered. Can't remember the exact legalities of cutting it down though, I think it'd be a no-go for under 18 inches off the tip top of my head.
Since twist rate is dependent on a number of factors you'd need to supply more info to get an accurate answer. Even with a short barrel there'll still be factors in getting it to stabilise. If you can dig up an old sks barrel that might be a good starting point. Should be dirt cheap and pre-chambered. Can't remember the exact legalities of cutting it down though, I think it'd be a no-go for under 18 inches off the tip top of my head.
incorrigible
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Thanks for the update a-munch. Rounds - not cartridges - are popular nomenclature for firing one 'down the pipe' (ie a barrel to dumb it down for you). If we travel further down the rabbit hole allow me to connect the dots... 7.62mm is the bullet (happy now?) diameter, length is variable however surplus is a wise and helpful assumption and velocity is dictated by the burn rate and barrel length which is... wait for it... 7"
Save me lord from the wholier than thou. Its painful sifting through the slackjawed in life in order to deal with the many reasonable people out there.
I agree, my understanding is that you're correct with the barrel cutting... at least to the best of my information to date, so it would have to be of new manufacture. I know that the AK74SU which is 5.45x39 requires a 1 in 6 rate of twist to stabilize or you start looking at keyholing or fragmentation.
Surplus "CARTRIDGES"
are a good assumption as there's not going to be much in the way of accuracy or range with a 7" barrel anyway. I'm just not aware of what rate would be required for a 7.62 round to stabilize over the same barrel length. I would think you'd be able to get away with a bit less due to the increased mass and certrifugal forces excerted on the bullet
The rate of twist necessary to stabilize a bullet is pretty much the same regardless of the barrel length.Unless you have a gaintwist barrel the bullets spin will be the same after one inch of travel or 18 inches.As a wild assed guess I'm sure a 1-10" or 1-12" twist would be more than adequate to stabilize a 123gr bullet.
incorrigible
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That's interesting. I'd have thought that with the reduced barrel length/burn time/bullet velocity it would require an increase rate of twist to compensate for the slower round?
I think you will be OK with whatever the standard twist is assuming its a cut down barrel or blank that this firearm is manufactured with. There is a formula to determine the correct twist rate for a given BC at a given velocity. But please don't ask me to do math and I won't give you any bad answers.
I think that if the shorty's muzzle velocity is within 500fps of the 14.5" barreled gun's, I wouldnt bother worrying about it any further. But suppose you did, here's what I would do.
Chrony the 7" barrel with your favourite bullets and compare the average m. velocity with that from a control barrel. Also compare accuracy. If there is a difference, you need to check with another 7" and 14.5" barrel to eliminate variables in barrel construction and firearm assembly.
If for whatever reason you found that accuracy wasnt good enough for you and you suspected it was the barrel's length giving you grief, you could always try with some lighter bullets or a warmer load. (if you are a reloader) If the faster bullets are more accurate and chrony in at the 14.5"ers M. velocity, you have probably found your culprit.
So lets say the accuracy is comparable when you are shooting off a rest.
You may find that accuracy will increase or decrease with a velocity change. (An increase would be pretty good news huh?)
When you look at a reloading manual, say, with a 9mm Luger, its not too hard to go from 1200 fps to 900 fps using the same bullets. Accuracy is usually acceptable across the range of velocities, but best around the middle of the range. So I wouldnt find this finding too hard to believe.
I think that if the shorty's muzzle velocity is within 500fps of the 14.5" barreled gun's, I wouldnt bother worrying about it any further. But suppose you did, here's what I would do.
Chrony the 7" barrel with your favourite bullets and compare the average m. velocity with that from a control barrel. Also compare accuracy. If there is a difference, you need to check with another 7" and 14.5" barrel to eliminate variables in barrel construction and firearm assembly.
If for whatever reason you found that accuracy wasnt good enough for you and you suspected it was the barrel's length giving you grief, you could always try with some lighter bullets or a warmer load. (if you are a reloader) If the faster bullets are more accurate and chrony in at the 14.5"ers M. velocity, you have probably found your culprit.
So lets say the accuracy is comparable when you are shooting off a rest.
You may find that accuracy will increase or decrease with a velocity change. (An increase would be pretty good news huh?)
When you look at a reloading manual, say, with a 9mm Luger, its not too hard to go from 1200 fps to 900 fps using the same bullets. Accuracy is usually acceptable across the range of velocities, but best around the middle of the range. So I wouldnt find this finding too hard to believe.
lejarretnoir
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For example, 1-16" twist is normal for a .22 rimfire whether it's a 24" or 6" barrel.
CZ uses a 1-9" in their carbine 7.62x39. Ruger and most others use a 1-10". I have a custom Rem 700 7.62x39 with a 1-12" barrel. With Czech ammo I know that 1-14" would be fine in a 20" barrel. With a 7" barrel, play it safe and go with at least 9" or 10" twist.
A real fire breather you'll have.
CZ uses a 1-9" in their carbine 7.62x39. Ruger and most others use a 1-10". I have a custom Rem 700 7.62x39 with a 1-12" barrel. With Czech ammo I know that 1-14" would be fine in a 20" barrel. With a 7" barrel, play it safe and go with at least 9" or 10" twist.
A real fire breather you'll have.
Well that is technically correct IF flight stability was marginal with the longer barrel. However you will find that most typical twist rates have a fair "safety margin" in them and to accomodate heavier bullets. So in practical terms the standard twist rate should be fine, especially with light 123gr bullets.
Here's what you need:
Bullet Length in inches / Twist
0.704 to 0.739 / 1 in 20
0.740 to 0.779 / 1 in 19
0.780 to 0.823 / 1 in 18
0.824 to 0.873 / 1 in 17
0.874 to 0.930 / 1 in 16
0.931 to 0.994 / 1 in 15
0.995 to 1.067 / 1 in 14
1.068 to 1.153 / 1 in 13
1.154 to 1.253 / 1 in 12
1.254 to 1.372 / 1 in 11
1.373 to 1.517 / 1 in 10
1.518 to 1.695 / 1 in 9
1.696 to 1.922 / 1 in 8
1.923 to 2.217 / 1 in 7
2.218 to 2.620 / 1 in 6
Bullet weight is irrelevant, it's the bullet length that matters. Even though the two are related, a 123 gr spire point boat tail would be quite a bit longer than a 150 gr flat point. Barrel length is irrelevant as is muzzle velocity, although high MV's (above 3000 fps) can compensate a bit for too slow a twist.
So what it comes down to is measure the length of the bullet you intend to use and go by the table.
Bullet Length in inches / Twist
0.704 to 0.739 / 1 in 20
0.740 to 0.779 / 1 in 19
0.780 to 0.823 / 1 in 18
0.824 to 0.873 / 1 in 17
0.874 to 0.930 / 1 in 16
0.931 to 0.994 / 1 in 15
0.995 to 1.067 / 1 in 14
1.068 to 1.153 / 1 in 13
1.154 to 1.253 / 1 in 12
1.254 to 1.372 / 1 in 11
1.373 to 1.517 / 1 in 10
1.518 to 1.695 / 1 in 9
1.696 to 1.922 / 1 in 8
1.923 to 2.217 / 1 in 7
2.218 to 2.620 / 1 in 6
Bullet weight is irrelevant, it's the bullet length that matters. Even though the two are related, a 123 gr spire point boat tail would be quite a bit longer than a 150 gr flat point. Barrel length is irrelevant as is muzzle velocity, although high MV's (above 3000 fps) can compensate a bit for too slow a twist.
So what it comes down to is measure the length of the bullet you intend to use and go by the table.
Andy is correct about bullet length and its importance.
I haven't pulled any 123gr bullets but I just put the caliper on a 180gr matchking a 168gr TSX and a 220gr Hornady steel jacketed solid and the longest of them would stabilize in a 1-11 twist according to his chart.
I wonder if the choice of 1-9.5" rifling in CZ rifles was because of the lenght of some of the different bullets that military uses,like AP and Incendiary.
I haven't pulled any 123gr bullets but I just put the caliper on a 180gr matchking a 168gr TSX and a 220gr Hornady steel jacketed solid and the longest of them would stabilize in a 1-11 twist according to his chart.
I wonder if the choice of 1-9.5" rifling in CZ rifles was because of the lenght of some of the different bullets that military uses,like AP and Incendiary.
incorrigible
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Thank you good sir, I believe this information/table and those from others goes a long way to determining which blank to order.
And please accept my apologies if I was a tad brusk in my response. Sometimes that the trouble in asking a question in areas one isn't as familiar is they don't provide the proper information required to generate a proper response. There are those on these forums who are all to quick to jump in to ridicule others for a lack of knowledge it had taken them half a lifetime to aquire. I'm not one of those and if you're not as well then you certainly warrant an apology from me, and you have it.
It's cool
Weight is a factor that is covered off, since it's related to density, which is included in the calculations for the table - which is for bullets that are constructed primarily of lead.
BC is a measure that includes among many things, a bullet's weight, and is a measure of how well a bullet overcomes air resistance - it's not related to how fast the bullet must spin to remain stabilized, except that a high BC bullet is typically long and needs to be spun faster than a shorter bullet.
BTW - that table has some rounding and overlap (i.e. they don't make 10.68 twist barrels), furthermore it's not hard and fast concerning what will or will not stabilize. My 0.310" 123 gr bullets are about 0.875" long, so a 1:16 twist should stabilize them, but so might a 1:17 or even a 1:18, but I should not expect them to, and neither should I expect a 1:17 (already marginal) that stabilizes a 123 gr bullet at 2000 fps to stabilize it if I shoot it at sub-sonic speeds.
Of course it is important in stabilisation. See CH10 "Applied Ballistics for long range shooting" by Bryan Litz. The formulae that ignore weight make odd assumptions about bullets. I posted a link to an online calculator earlier.
"Don Miller has created and published a simple, and surprisingly accurate formula for twist rate in Precision Shooting.
The equation is:
S = (30*m)/(t^2*d^3*L*(1+L^2))
S is the gyroscopic stability factor. This is the measure of static stability and 1.4 is the minimal (optimal) value.
m is the bullet weight in grains
t is the twist rate in calibers per turn (for example, a 1:13" twist .308 barrel has t = 13/.308 = 42.2 calibers per turn.
d is the bullet diameter
L is the bullet length in calibers.
This equation is very accurate for a wide range of bullets, velocities and twists. There are corrections for non-standard atmosphere and velocity corrections (the equation is built around MV = 2800 fps). The effect of different muzzle velocity is not as important to stability as most people think.
Anyway, that's the equation I use, exclusively, for calculating static stability. Don's really given us a gem.
Take care,
-Bryan"
