revolutions per minute debate

[royinidaho]

Well, hmmmmmm, doesn't all of that change when the bullet crosses the equator? /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/grin.gif

It had best not rain tomorrow...... /ubbthreads/images/graemlins/frown.gif

 

[Buffalobob]

Its raining right now. A funny thing -while I was getting my Browning gold ready to go tomorrow turkey hunting I looked at the choke tube I had in it and noticed that I had killed the last two geese back in February with a rifled slug choke tube instead of extra full tube. I was shooting 3 1/2 inch hevi shot #2s. I wonder what rpm those
# 2 shot were turning when they hit the geese.

 

[4ked Horn]

[ QUOTE ]
doesn't all of that change when the bullet crosses the equator?

[/ QUOTE ]

Yep, The bullet flips around so it is pointing backwards and starts spinning the other way and actually speeds back up.

It is due to the quantified molecular montum seeking an equalibrium impeadance with the bullets digitized calculum sarconium. But that is sort of a "no brainer".

 

[Bob S.]

You guys lost me but I bet GG is LHAO right now with what he started. I guess if I shoot across the equator I need to duck before the bullet reveres spin and direction and gets me in the eye. /ubbthreads/images/graemlins/confused.gif

 

[4ked Horn]

I see you reading this GG. You better put this thing to rest.

 

[goodgrouper]

Sorry guys. I meant to come back to this sooner, but I have been loading ammo like a madman for my new standard 6br I just got back Wednesday.

Thanks for all your posts. Unfortunately, there are conflicting opinions on this subject so I have not told the fellows in the debate about this site as of yet. We need to come to an agreement on the subject before I start the argument again with these guys.

The reason I brought this whole thing up was because I was shooting a match Wednesday at 100 yards for group with my fast twist 6br improved which is really better suited for 1000 yard shooting. I made the mistake of mentioning that the 105 amax that I was shooting needs more distance to settle down and shoot it's best and the guys there (who are not long range shooters, just 100 yard br shooters) just about laughed me off the range! Of course, Old Bear had just left so I was all alone trying to tell these guys about the world that exists past the 100 yard target frames. Then to make matters worse, one guy mentioned how he just built a 220 swift on an 8 twist barrel and was shooting 60 grain bullets in the thing. I told him that the rpm's involved in that scenario may not let that bullet shoot well, and told him to try a 75 grain Amax or 80 grain Sierra MK and they will handle the rpms better because they will be have a slower velocity, thus reducing the rpm's. Then we kinda got confused on our terminology, and one thing led to another and the debate of whether the 60 grain or the 80 grain bullet has more revs per distance. I told him that they both have the SAME rpd's. Each bullet revolves 1 time in 8 inches regardless of speed.
Then it got into an argument of if each bullet is still revolving 1 time in 8" clear out to the end of it's traj.

Now, there are differing views on this as mentioned in earlier posts by you fine gentlemen. And in fact, I did tons of research on this until about 3:00 AM and actually found differing scientific info! This is not good. /ubbthreads/images/graemlins/frown.gif

It seems that there are several reputable sites that give conflicting info on this subject. I believe that it is all theory and no one has actually devised a way to "capture" a bullet at long range on slow mo video to see if it is still spinning the same rpd's as it was when it left the muzzle!

I found this:
www.steyrscout.org/extbal.htm
Scroll down to where it says, "some caveats"


What I believe happens is that if there are no marrs in the jacket, and no voids in the core, that 80 grain bullet and the 60 grain bullet are still spinning just fractions under 1 time in 8" at 1k. I believe that if it weren't true, the bullet engineers would rate the needed twist to spin the vld's in an additional boundary. For example, they would list it on the box that it needs an 8 twist in the barrel out to say 600 yards. Then from there out to 1k it needs a 7 twist. Then it needs a 4 twist for 1500 yards and so on. Since that 80 grainer can and does get shot in all ranges without deviating from the 1-8" twist, I would tend to think that it is still spinning at the same rpd clear out until it hits the ground and as long as the rpm's are held above the loss of stability point, it should continue to do so. Of course, at one point, the velocity will dip below the speed of sound and consequently the rpms may not be sufficient to keep those long bullets stabilized.

Anyhow, I have researched until I can't research no more. And still, I find conflicting answers to the questions.

Oh well, I have loading to do anyways!
Catch you guys later. GG

 

[4ked Horn]

OK I am back to your original questions. I don't know what conflicting info you found but the correct answers seem clear and generally understood by most people responding.

[ QUOTE ]
If a bullet leaves a barrel that has a 1-8" twist, is it still spinning 1 time in every 8" all the way through it trajectory say out to 1000 yards?

[/ QUOTE ]

It will be spinning at a faster "Rotation Per Distance". If your MOAG puts a bullet at 2k in 3 seconds there will be almost 0 spin decay. The RPMs we are talking about here are rediculous fast. I have seen video of a steel ball bearing spinning at one million RPM in a magnetic bearing. It is indestinguishable from a stationary steel ball. If left to settle on its own it would take days to stop. If the bullet is going 360,00 RPM at the muzzle it is probably going 359,999.9 RPM at 2k. However your bullet is slowing its FPS. This means the bullet RPD will be increasing to 1:7 then 1:6 then 1:5 and so on. If your 2k velocity is half of your MV then the twist ratio is (really really) close to double (1:4).

[ QUOTE ]
If you were to shoot a 55 grain bullet out of an 8 twist barrel at 4000 fps, and then shoot a 80 grain bullet out of the same barrel at 3000 fps, they have different rpm's, but do they both still spin 1 time every 8 inches?

[/ QUOTE ]

If the barrel they are leaving is wearing a 1:8 rifling how CAN'T they. But he moment they leave contact with the muzzle they adopt their own velocity decay and in turn the paragraph you just read applies to them at different magnitudes.

Clear as a bell. /ubbthreads/images/graemlins/laugh.gif

 

[goodgrouper]

Yes. I see your point.
Can anyone find some more data on this and provide a link?

 

[Richard338]

GG, I'm surprised to hear that you found conflicting info. Classical physics is well understood. The best book I've found is McCoy's book on Modern exterior ballistics. I work in science, not in this area, so this doesn't make me an expert, but I was able to read and understand McCoys book as well as the next guy.
Your original question was "will the bullet spin more or less than 4 1/2 times in the 786th yard". The answer is MORE. I think it is easy to imagine the translational velocity decaying more quickly than the rotational. The reason that they call 6 DOF calculations six degrees of freedom is because there ARE 6 degrees of freedom. This means that one component of the velocity doesn't have to be fixed by one component of the rotational freedom.

 

[Buffalobob]

RPM

We will shoot a 50 gr bullet with a BC of .251 from a 220 Swift with a 1:8 twist 36 inch barrel at two different velocities 4000 fps and 3000 fps. These bullet will be sprinkled with magic faery dust to prevent them from going subsonic and getting 4fkdHrn upset with us.

At time T=-0.??? and X=-1 the bullet is at rest in the chamber. The trigger is pulled and the bullet begins to accelerate down the barrel. At Time T= 0.0 and X=0.0 the bullet reaches the end of the barrel where it is now traveling at V=4000fps and has an RPM of 360,000 and a RPD of 1.5 Revs per foot

Quickly we do this again and shoot the downloaded round and at Time T=0 and X=0 it exits the barrel with V= 3000 fps and a RPM of 270,000and a RPD of 1.5Revs per foot.

We quickly run down range and observe the rounds as they go by at 100 yards

The first round now has a velocity of 3532 fps, Rpm is 360,000 = 6000RPsec, RPD =6000RPS /3532 fps= 1.698 Rev per foot
The second round has a velocity of 2615 fps, RPM is 270,000 = 4500RPS, RPD =4500/2615 = 1.72 Revs per foot.

Having recorded our data we turn and sprint down to 200 yards and observe the two bullets going by

First bullet has a velocity of 3112fps, RPM is 6000 RPS RPD = 6000RPS/3112 FPS = 1.92 Revs per ft

Second bullet has a velocity of 2247fps, RPM is 4500 RPS, RPD = 2.00RpFt

Like roadrunner with Wiley Coyote in pursuit we swiftly relocate ourselves at 300yds and record our data.

First bullet V= 2726fps, RPS = 6000 and thus RPD = 6000RPS/ 2726 fps = 2.2RpFt
Second bullet V= 1910fps RPS = 4500 and thus RPD = 4500Rps/ 1910 fps = 2.356revs per foot

Hustling on down to 400 yds we observe
First bullet V = 2349 fps RPS = 6000RPS and RPD = 6000rps/2349fps = 2.55 revs per ft
Second bullet V = 1589 fps, RPS = 4500RPS and RPD = 4500rps/ 1589fps = 2.83rev per ft

From here we head for the next observation point but have to make a slight detour around the rattlesnake.

500 yards

First bullet V= 2003fps, RPM = 6000rps and RPD = 6000rps/2003fps = 2.99 Rev per ft
Second bullet V= 1313fps, RPM = 4500 rps, and RPD = 4500rps/1313fps = 3.43 Rev per ft

Just like superman, we are faster than a speeding bullet so we head on down range

At 600 yards

First bullet V= 1679fps, RPM = 6000rps,and RPD = 6000rps/1679fps = 3.57Rev per ft
Second bullet V =1114fps, RPm = 4500rps and RPD = 4500rps/1114fps = 4.04Rev per ft

Now then because we have been drinking a lot of beer, we stop and pee on a bush and the bullets get to a 1000 yards before we can get zipped up and on down range.

At 1000 yards we observe the following

The first bullet has a velocity of 934fps and RPM is still 6000RPS thus the RPD = 6000RPS/943fps = 6.36Revs per foot

After waiting awhile the second bullet finally comes by and it has a velocity of 796 fps and a RPM of 4500RPS thus the RPD = 4500RPS / 796fps = 5.65revs per foot traveled.

We are very perturbed because we note that somewhere between 600 yards and 1000 yards the faster bullet is now turning more revs per foot than the slower bullet. We suspect that GoodGrouper has set us up with a trick question. Somewhere in the vicinity of 800 yards the two bullets will actually have the same revs .

We are also mad because we are a long way away from our beer.


Now then we are hot and tired from all that running so we drag ourselves back to our 429SCJ mustang and get some more beer. It occurs to us that maybe the spin decays with time and distance so we can make a few crude calculations to see what might happen. Being as we are now semi drunk we will assume spin decay is related to the time bullet spins. So at 1000yards
First bullet time of travel = 1.5 seconds and it has turned 1.5 X6000Rps = 9000 revs

Second bullet time of travel = 2.13sec and has turned 2.13sX 4500 rps = 9585 revs.

So we see the slower bullet would have more decay by the time it reached 1000 yards and would therefore be turning even fewer revs per foot than the faster bullet assuming friction acts on each rev the same for both bullets.

Having solved this weighty problem we decide to call it a day when we notice the dark puddle under the engine and bending down we perceive we have knocked a hole in our oil pan by driving off road with our mustang. Taking out our cell phone we see that there is no reception. Going to the cooler we find we are out of beer and the dam ole rattle snake has crawled under it and bites us on the leg. We decide to take a leak and get ourselves hung in the zipper. Some times it just doesn't pay to get out of bed.

 

[ricka0]

GG writes: [ QUOTE ]
It seems that there are several reputable sites that give conflicting info on this subject. I believe that it is all theory and no one has actually devised a way to "capture" a bullet at long range on slow mo video to see if it is still spinning the same rpd's as it was when it left the muzzle!
I found this:
RPD increases
Scroll down to where it says, "some caveats"


[/ QUOTE ]

My background is MS in applied math specializing in numerical solution of PDEs. I used this in CFD (computational fluid dynamics) as an applied mathematician at Boeing. I can't say I'm an expert on supersonic calculations as all the work I did was commercial airliner (sub-sonic). I could check with my pdh pals who do super-sonic/boundy layeer/turblent flow work, but I don't see where there is any difference in opinion. The ref you site looks very accurate to me
[ QUOTE ]
Now consider a bullet chronographed at about 3000 f/s muzzle velocity fired from a rifle with say a 10" twist. It is rotating at around 3600 revolutions per second (216,000 rpm). Let the flight velocity decay to 2000 f/s. Now what is the bullet rotational speed? It doesn't fall off much because the only things slowing it down are inertia and skin friction drag which is pretty low, so the rotational velocity is only slightly slower than 3600 rps

[/ QUOTE ]

The statement could be made more precise.
For axial symmetric projectiles with angular laminar flow (smooth bullets), the ratio of rotational drag to translational drag is > 10^n ( I don't know, maybe 100,000) in the supersonic range.

Even with pulled bullets with surface marks (that have fairly high turbulent skin friction, the ratio is still very high)

If you provide references to conflicting sites I would be happy to review them.

 

[LB]

The only thing I see that has not been nailed down is the imprinted rifling marks on the bullet, necessary to impart the spin in the first place, but they appear to be a mechanical brake, of sorts; in free air. I would think that the muzzle RPS would actually degrade enough at a thousand yards, so that, perhaps.....

[ QUOTE ]
It doesn't fall off much because the only things slowing it down are inertia and skin friction drag which is pretty low, so the rotational velocity is only slightly slower than 3600 rps


[/ QUOTE ]

....the above may not be perfectly true?

Am I way off base? Skin friction drag, wise?

Good hunting. LB

 

[4ked Horn]

We all seem to know how much I love supersonic bullets. /ubbthreads/images/graemlins/wink.gif

During the time the bullet is supersonic the shock wave coming off the bullets point will cause an area of reduced air density near a bullet body ( which is why a boat tail gives very little benefit to bullets in SS flight) so the drag of the rifling groves would still be quite small (nearly 0) untill the fairy dust was all blown away and the bullet came back from the "spirit world" and flew subsonic again.

 

[ricka0]

Maybe that's what changes the RPF. I thought buffalobob was pulling our legs with his data, so a quick check with Shoot V3 and excell shows he's not. I posted the excell data at Buf Bob's 220 Swift RPF

Sorry about the inconvient format, this was just a 15 minute Excel upload to one of my sites. RPF tab (bottom) gives Rev Per Foot vs. Velocity and is what we would expect. Sheet1 is just the boring data you can review (differs somewhat from BB's) Sheet2 shows the counter-intuitive transtions to lower RPF - probably due to going sub-sonic

If somebody will post Kirbys 7mm with a good (projected) wildCatCullets.com pill I'll post that data to 1000 yards.