Quantcast

Does a RB travel with constant rotation, or accelerate?

Help Support Muzzle Loading Forum:

Status
Not open for further replies.

Larry (Omaha)

50 Cal.
MLF Supporter
Joined
Aug 27, 2004
Messages
1,131
Reaction score
726
Two other BP shooters I know have different opinions about RB rotation after it leaves the barrel. So, I am going to ask the experts here. A 50 cal, 48" barrel, with 1 revolution in 48" twist is fired using 60 grains of 2F.
When the RB leaves the muzzle it has already rotated 1 complete turn. Here is the question: Does the RB continue rotation of 1 turn every 4', or does it accelerate to a faster rotation?
Flintlocklar 🇺🇲 :dunno:
 

rdlowe

Pilgrim
MLF Supporter
Joined
Dec 11, 2015
Messages
236
Reaction score
185
Location
Dallas TX
Good question, Larry. The bullet won’t spin faster or slower until/ unless an external force acts upon it. This usually happens when it impacts something. If you could fire a round ball in outer space, it would keep spinning and spinning and spinning at the same rate as when it left the muzzle.
 

Carbon 6

Cannon
Joined
Nov 2, 2018
Messages
6,544
Reaction score
3,063
The moment it leaves the barrel it begins to slow down.

The confusion comes from how you measure rotation. Often it is converted to RPMs
For example a round ball exiting the muzzle doing 1500 fps out of a 1/66 twist gun is rotating just over 16000 rpms.
 

tnlonghunter

40 Cal.
Joined
Aug 10, 2005
Messages
540
Reaction score
123
Location
Maryland
I believe it's Newton's 1st law - an object in motion stays in motion until acted upon by an outside and equal (or greater) force. In this case, the force of the expanded gas from your main charge produces the forward motion, and the rifling induces spin. But once the ball leaves the barrel, the explosion of the charge no longer exerts any force. Rather, gravity becomes the more powerful force, along with wind resistance against the surface of the ball. In combination, they will slow the ball's rate of rotation to some degree until it hits the target or the ground.
 

Larry (Omaha)

50 Cal.
MLF Supporter
Joined
Aug 27, 2004
Messages
1,131
Reaction score
726
The moment it leaves the barrel it begins to slow down.

The confusion comes from how you measure rotation. Often it is converted to RPMs
For example a round ball exiting the muzzle doing 1500 fps out of a 1/66 twist gun is rotating just over 16000 rpms.
What is the formula to convert rate of twist to RPMs?
Flintlocklar 🇺🇲
 

Zonie

Moderator
Staff member
Moderator
MLF Supporter
Joined
Oct 4, 2003
Messages
31,510
Reaction score
4,511
Location
Phoenix, AZ
To convert a rate of twist to RPM's you have to know the velocity of the ball and the rate of twist.
Since velocity in firearms is usually measured (in the USA) in feet per second and barrel twist is usually measured in inches we have to convert the twist rate to feet by dividing by 12. For instance a 1:48 twist equals 48 inches per revolution. 48/12 = 4 feet per revolution.

If the velocity of the ball was 1400 feet per second and the rate of twist was 4 feet, dividing 1400 fps by 4 will give the rotation of the ball per second. In this case, 1400/4 = 350 rotations per second.
Because RPM stands for Revolutions Per minute and there are 60 seconds in a minute we need to multiply that 350 RPS times 60 to get, 350 X 60 = 21,000 RPM.

Putting both of these calculations together we end up with (V/T) X 60 = RPM (Where V is the velocity in feet per second and T is the rate of twist in feet).

Let's do it again with a 1600 fps velocity and a rate of twist of 1:60. 60 inches/12 = 5 feet.

(V/T) X 60 = (1600/5) 60 = (320) 60 = 19,200 RPM.
 

tenngun

Cannon
MLF Supporter
Joined
Jan 27, 2008
Messages
13,079
Reaction score
3,512
Location
Republic mo
Throw in how GUI key the ball looses velocity and that would slow the rpm
 

hawkeye2

58 Cal.
Joined
Apr 12, 2004
Messages
3,793
Reaction score
809
Location
Winchester, VA
Throw in how GUI key the ball looses velocity and that would slow the rpm
Sorry but I respectfully disagree. The velocity of the ball will decrease mostly due to air resistance but the rate of rotation will remain constant as there are no forces acting on it to slow it's rotation. That holds true in Physics 101 as we were told to ignore friction in pulleys and things like that. Actually there is air resistance acting on the surface of the ball in opposition to its rotation which will slow it somewhat however if it left the muzzle at 1500 fps and 20,000 rpm it will still be spinning at (almost) 20,000 rpm when its velocity has dropped to 900 fps. The only place forward velocity influences rotational velocity is while the projectile is still in the barrel.
 

tenngun

Cannon
MLF Supporter
Joined
Jan 27, 2008
Messages
13,079
Reaction score
3,512
Location
Republic mo
Um, you think?
If it turns one turn in four feet and it is going 1600 fps it would take .0025 seconds to make one turn, cover that four foot distance. 24000 revolutions in one minute.
But it will slow in a short time to 1200 fps .0033 of a second to cover four feet and make one turn. 18000 revolutions per min.
So I’m thinking even with a ball keeping a spin rate of 1 in 48 the rpm would slow as the time it takes to go four feet increases
Of course a ball will rarely be one second in flight.
 

hawkeye2

58 Cal.
Joined
Apr 12, 2004
Messages
3,793
Reaction score
809
Location
Winchester, VA
The RPM is entirely independent of the forward velocity once the ball leaves the muzzle. The RPM or angular velocity is established by the rate of twist and the velocity of the ball at the time of firing but has nothing to do with the distance it travels any time after leaving the barrel. If we have a barrel with a twist of 1 turn in 48" (constant twist and not gain twist rifling) and the barrel is 48" long It uses that length to reach the maximum velocity and after the ball exits the muzzle it begins to loose velocity and to drop due to air resistance and gravity. Those two forces however do not oppose the rotation of the ball and in turn slow it. The momentum of its spin (see: spin angular momentum) will continue to keep its angular velocity constant and that momentum must be overcome by something to change the rate of spin. If the RPM were to drop the projectile would loose stability. That would have a considerable effect on an elongated projection and I suspect a lesser effect on a round ball.
1596861772719.png
 

TNGhost

40 Cal.
MLF Supporter
Joined
Jun 4, 2014
Messages
1,872
Reaction score
1,536
Good question, Larry. The bullet won’t spin faster or slower until/ unless an external force acts upon it. This usually happens when it impacts something. If you could fire a round ball in outer space, it would keep spinning and spinning and spinning at the same rate as when it left the muzzle.
The "something" it impacts is the air the split second it leaves the barrel,, and it is also affected immediately by gravity.

The rate at which it accelerates in the barrel is also retarded by a degree by the air column in the barrel which has to be moved for the ball to escape.

Insofar as the rotation, it will be slowing as well, from friction, and the rate of slowing will be determined by the density of the air. While not as great as the resistance on forward velocity, it is still there.
 

hawkeye2

58 Cal.
Joined
Apr 12, 2004
Messages
3,793
Reaction score
809
Location
Winchester, VA
"Insofar as the rotation, it will be slowing as well, from friction, and the rate of slowing will be determined by the density of the air. While not as great as the resistance on forward velocity, it is still there."

That is very true but it is my opinion that the amount of slowing before the ball struck the ground would be almost negligible considering it was spinning at something like 20,000 RPM. Air resistance increases as the square of the velocity and the velocity we would be looking at here is the surface speed (rpm times circumference) of the projectile in the direction of rotation.

In my previous post (#16) please substitute projectile for projection in this sentence:
"a considerable effect on an elongated projection and I suspect a lesser effect on a round ball."
 

TNGhost

40 Cal.
MLF Supporter
Joined
Jun 4, 2014
Messages
1,872
Reaction score
1,536
I agree, negligible in relation to forward velocity, but measurable, and significant in comparison to traveling in a vacuum.
 

hawkeye2

58 Cal.
Joined
Apr 12, 2004
Messages
3,793
Reaction score
809
Location
Winchester, VA
Yes measurable. Ballistics in space free of an atmosphere and gravity can be quite an interesting subject.
 
Status
Not open for further replies.
Top