Twist Rate for Short Barrels

[mikecr]

For those thinking they can use high MVs, or higher RPMs to bail themselves out of gyroscopic stability issues, let me show how this fails a test (and is therefore not a truth):

For this I load an unmodified classic Berger 140VLD, in software more advanced than rules of thumb. I'm using ICAO std. atmos, which is sea level, 59deg, 0%RH.
The software calls for 8:1 twist for full stability, giving me Sg=1.56 at 2500fps.
This happens to be 225 KRPMs.

Now, for testing, I swap to a 10:1 barrel, and I trim meplats to make them all the same. This, opening the meplats from 0.062" to 0.075". Same atmos, as I do actually live on the beach.
Here, my resultant Sg at 2500fps is 0.93 [UNSTABLE]. This is now 180 KRPMs.
Can I overcome this issue with more velocity, or RPMs?

Well lets see:
2500fps, Sg0.93, 180 KRPMs *unstable
3000fps, Sg0.96, 216 KRPMs
4000fps, Sg0.99, 288 KRPMs
5000fps, Sg1.02, 360 KRPMs *marginal stability, bullet failures at 4800fps.
6000fps, Sg1.04, 432 KRPMs
From this I see that velocities and RPMs never overcome the barrel twist disparity from 8:1 to 10:1.

What if I hadn't raised drag(lowered BC) by trimming meplats, but left them at 0.062"?
6000fps, Sg1.11.
And if I point meplats to 0.020"?
6000fps, Sg1.39
If I swap barrels again to 9:1tw with meplats left alone at 0.062:
6000fps, Sg1.38
At 9:1 with meplats at 0.020":
2500fps, Sg1.52 [back to fully stable][but only with meplats pointed].

By messing with meplats, changing drag, I'm affecting relative displacement per turn.
The complete turn, needed to overcome overturning forces from that displacement.

 

[BoomDeBoom]

For those thinking they can use high MVs, or higher RPMs to bail themselves out of gyroscopic stability issues, let me show how this fails a test (and is therefore not a truth):

For this I load an unmodified classic Berger 140VLD, in software more advanced than rules of thumb. I'm using ICAO std. atmos, which is sea level, 59deg, 0%RH.
The software calls for 8:1 twist for full stability, giving me Sg=1.56 at 2500fps.
This happens to be 225 KRPMs.

Now, for testing, I swap to a 10:1 barrel, and I trim meplats to make them all the same. This, opening the meplats from 0.062" to 0.075". Same atmos, as I do actually live on the beach.
Here, my resultant Sg at 2500fps is 0.93 [UNSTABLE]. This is now 180 KRPMs.
Can I overcome this issue with more velocity, or RPMs?

Well lets see:
2500fps, Sg0.93, 180 KRPMs *unstable
3000fps, Sg0.96, 216 KRPMs
4000fps, Sg0.99, 288 KRPMs
5000fps, Sg1.02, 360 KRPMs *marginal stability, bullet failures at 4800fps.
6000fps, Sg1.04, 432 KRPMs
From this I see that velocities and RPMs never overcome the barrel twist disparity from 8:1 to 10:1.

What if I hadn't raised drag(lowered BC) by trimming meplats, but left them at 0.062"?
6000fps, Sg1.11.
And if I point meplats to 0.020"?
6000fps, Sg1.39
If I swap barrels again to 9:1tw with meplats left alone at 0.062:
6000fps, Sg1.38
At 9:1 with meplats at 0.020":
2500fps, Sg1.52 [back to fully stable][but only with meplats pointed].

By messing with meplats, changing drag, I'm affecting relative displacement per turn.
The complete turn, needed to overcome overturning forces from that displacement.

For me as a beginner with loading shorter barreled guns, all of the responses have not been totally understood, but have helped me gain what I think might be a good starting point. Frist, I need to determine the caliber I would use for the application. Second, choose a bullet that fits the needs of that application. Third, choose a twist rate that works with a rifle for bullet weight and application. Fourth, get some power that works well with data supplied by bullet manufactures and other experts. Fifth, start building a load that provides best results using best reloading practices. Does that sound like the path to go, or are there more considerations I have missed?

Thanks for all the input and help. This is a great and helpful forum!

 

[fyrewall]

For those thinking they can use high MVs, or higher RPMs to bail themselves out of gyroscopic stability issues, let me show how this fails a test (and is therefore not a truth):

For this I load an unmodified classic Berger 140VLD, in software more advanced than rules of thumb. I'm using ICAO std. atmos, which is sea level, 59deg, 0%RH.
The software calls for 8:1 twist for full stability, giving me Sg=1.56 at 2500fps.
This happens to be 225 KRPMs.

Now, for testing, I swap to a 10:1 barrel, and I trim meplats to make them all the same. This, opening the meplats from 0.062" to 0.075". Same atmos, as I do actually live on the beach.
Here, my resultant Sg at 2500fps is 0.93 [UNSTABLE]. This is now 180 KRPMs.
Can I overcome this issue with more velocity, or RPMs?

Well lets see:
2500fps, Sg0.93, 180 KRPMs *unstable
3000fps, Sg0.96, 216 KRPMs
4000fps, Sg0.99, 288 KRPMs
5000fps, Sg1.02, 360 KRPMs *marginal stability, bullet failures at 4800fps.
6000fps, Sg1.04, 432 KRPMs
From this I see that velocities and RPMs never overcome the barrel twist disparity from 8:1 to 10:1.

What if I hadn't raised drag(lowered BC) by trimming meplats, but left them at 0.062"?
6000fps, Sg1.11.
And if I point meplats to 0.020"?
6000fps, Sg1.39
If I swap barrels again to 9:1tw with meplats left alone at 0.062:
6000fps, Sg1.38
At 9:1 with meplats at 0.020":
2500fps, Sg1.52 [back to fully stable][but only with meplats pointed].

By messing with meplats, changing drag, I'm affecting relative displacement per turn.
The complete turn, needed to overcome overturning forces from that displacement.

Velocity will never provide adequate stability for that long pointy bullet for a 10 twist. I think the twist rate is the factor and rpms would be imparted to the bullet when the bullet was still in the barrel. Messing around with bullet points would affect drag for stable bullets but if the bullet was unstable, like wobbling, drag would be increased even for bullets with needle like points.

 

[mikecr]

BoomDeBoom, it all BEGINS with a bullet. Everything is built around it.

 

[Ballisticboy]

There seem to be a lot of misconceptions on what governs gyroscopic stability. Simply put gyroscopic stability is governed by the ratio of inertial and aerodynamic moments about the centre of gravity of a bullet or any other spin stabilised projectile. Gyroscopic stability does not care about bullet spin rate in terms of RPM or radians per metre, it does not care about any forces, be it drag, lift or anything else, it does not care if the barrel is 1 inch or 1 yard long, it does not care what the bullet weight is, it all comes back to the ratio of the inertial and aerodynamic moments. Note that as it is the moments about the centre of gravity which matter, not the forces, you could have an infinite force acting on a bullet which will have no effect on the stability if it acts through the centre of gravity. This is why drag has little or no effect as its moment arm about the centre of gravity is tiny compared to other lateral forces unless your bullet is at silly angles of yaw in which case you are not going to hit anything anyway.
As most people know, once a spin stabilised projectile has left the barrel, in most cases gyroscopic stability tends to increase as the linear velocity falls at a faster rate than the spin rate. This means that the aerodynamic moments fall faster than the inertial moments giving the increase in the ratio of the inertial and aerodynamic moments and thus in gyroscopic stability. Therefore, for most bullets, the gyroscopic stability immediately after leaving the barrel is the critical minimum value.
For a given bullet, the size of the aerodynamic moments is largely a function of the square of the projectile velocity and the aerodynamic overturning moment coefficient. The inertial moments are largely a function of the square of the spin rate in radians per second. However, at the muzzle of the barrel the spin rate is governed by the forward velocity of the bullet and the barrel twist rate so gyroscopic stability, being a function of the ratio of the two sets of moments, can be shown to be independent of bullet velocity, merely a function of barrel twist rate. The fly in the ointment for this of course is that the aerodynamic overturning moment coefficient tends to vary with velocity as well which is why gyroscopic stability changes somewhat with muzzle velocity, generally being at a minimum around Mach 1 for many bullet designs. Bullet weight does not directly affect gyroscopic stability only indirectly in affecting bullet length and the longitudinal and transverse inertia.
Thus, assuming your barrel has a constant twist rate along its length, for a given velocity, the length of the barrel will no effect on the gyroscopic stability. It may affect the bullet behaviour through changes in the muzzle blast, barrel vibration or bullet/barrel interactions but it will not change the gyroscopic stability. So for a short barrel you should use the same twist rate as for a long barrel as a first guess with the possibility that a small change spin may be of benefit due to the increased muzzle blast etc.

 

[fyrewall]

"simply put...." Great Balls of Fire!

how about a "spin stabilised projectile" with zippo RPM?

 

[Ballisticboy]

"simply put...." Great Balls of Fire!

how about a "spin stabilised projectile" with zippo RPM?

It would either have zero forward speed or have been fired from a smoothbore.

 

[fyrewall]

It would either have zero forward speed or have been fired from a smoothbore.

But then our bullet would have no value as projectile nor be spin stabilized having no rotations per time (RPM) and would quickly turn sideways. Should adequate gyroscopic stability be imparted to the bullet it would continue happily spinning at near the same rate until the time of flight would end. I am confused about "Gyroscopic stability does not care about bullet spin rate in terms of RPM or radians per metre".

 

[Mudygmc]

OP good luck with your choice. This has certainly became a complicated thread. Build what suits you and enjoy it. Shoot the barrel off the thing.

Adam

 

[Ballisticboy]

I am confused about "Gyroscopic stability does not care about bullet spin rate in terms of RPM or radians per metre".

All that is important is the ratio between the two types of moments, inertial and aerodynamic. The actual value of the spin rate does not govern the stability, only if it can provide sufficient inertial moment. Some people believe bullets have to attain a certain speed in order to have enough spin to be stable but this is not true, a bullet can be stable at very low speeds as well as very high speeds.
As for the original question in the OP it is as I said before the barrel length does not change the twist rate needed for stability for a given velocity but may change the bullet yaw due to muzzle blast etc.