RPM theshold discussion

[303Guy]

I've been wondering about this. The problem I see with the RPM debate is that there are several factors at play simultaneously. Chamber pressure, muzzle pressure, alloy resistance to skid, alloy ability to obturate both in the throat and further down the bore, lube, nose slump, base deformation and maybe centrifugal boolit upset in flight to name a few. How can one say it's RPM that's causing the problem?

Here's recent example of a higher RPM boolit. It suffered no problems with obturation but a careful look shows it did skid slightly - not all the way through the land - but obturated after the partial skid. But take a look at the base - there's no way that base is perfectly even on its circumference. It like is not even square any more. There's no way of telling what nose slump there might have been. All I can say is anything standing in its path at close range would have had a bad day. It could have been pushed faster with a slower powder but it was fired to check for cast in a so-so slightly rusted bore which had been cleaned of rust. It failed!

[NVScouter]

if a boolit skids its my opinion the pressure curve was too high at some point. Also as some have put it the RPM is set after leaving the barrel.

I can get 125g Speer TNTs to explode about 20' after leaving the barrel in my 300WBY. I know because I ignored Speer and loaded them to 130SP data. My first round didnt hit and the second one didnt hit. Then on shot 3 my wife asked me why my rifle was "doing that". I checked the bore and brass...looks good. So I had her squeeze off 1. Sure as can be around 20' from the barrel a little shimmer/poof was seen. We watched the other few rounds do the same. However its not RPM pulling them apart as its the HP being expanded and jacket opening mid air.

[303Guy]

Apparently it is the RPM blowing up those bullets. They start to expand slowly at first and as they get large so the rate accelerates until they explode. I believe this has been put to the test with rifles of different twist rates. I'm sure the hollow nose would contribute to the expansion. However, the drag coefficient reduces above transonic speeds.

My apparent skidding could have been at throat entry or at peak pressure but it could also be a mistaken observation. I'll keep that boolit until I can get a camera with real close-up capability then I'll be able to view it magnified.

[Chicken Thief]

[HangFireW8]

Well I need to read this again. I do see lots of lost variables in most theories.A couple points:

I have seen many long range rounds do funny things but most are due to ground heat, cross breezes, and chaoitc air. Example shooting over a small hill or across a canyon. You have 2-3 air sources usualy and they will play with your shot. However if you get blown 2" right and 3" left then 4" low every time you get a similar group. Nothing helical about it. At gunnery with 120mm rounds the ballistic computer takes in: Distance, air temp, target speed, round used (instert BC/FPS) with certain constants taken from original built and MRD. Aim and shoot effectively to 4,000 meters in combat. Those are precision machined rounds with a sabot and stabilization fins @1600fps MV.

T

More like m/S.

[303Guy]

1600 m/s would be a bit fast! Is that possible? I mean that it's 1600 m/s and not 1600 fps. That would be 5,250 fps!

[HangFireW8]

1600 m/s would be a bit fast! Is that possible? I mean that it's 1600 m/s and not 1600 fps. That would be 5,250 fps!

All day every day. I used to clock them myself.

[finstr]

Here's an insightful link that may prove worthwhile to some. I personally haven't read thru all of it but I'm certain there's some theoretical anomoly's that may spark debate.

http://www.rathcoombe.net/sci-tech/b.../wounding.html

Apologies if it's been posted here already, I've read thru 10 or so of the 16 pages and my eyes are on FIRE!

[Chicken Thief]

[NVScouter]

All day every day. I used to clock them myself.

Yes that is correct. The entire case burns and all that is left at the breech is the aftcap.

[303Guy]

HOLY COW!

I'm going to have a look at that link later when there's more time. It should be interesting.

[Hamish]

why commercial J bullets are more accurate than milsurp J bullets? Why are match J bullets even more accurate? The answer is the more accurate bullets are held to tighter tolerances in manufacture, i.e. the jackets are more uniform in thickness and weight, the cores are more uniform in weight and the dimensions are more uniform. This all adds up to a more balanced bullet and the jacket keeps them as close to balanced as possible during acceleration. Larry Gibson

First off, let me be very clear that I am not stepping into the ring on this discussion, it's just that the above struck me a little odd when I read it.

Looking at it from a reverse angle, does that mean that the train of thought says that the cast boolit, as a rule, is not cylindrical enough, or of a homogenous enough makeup, that it cannot be as rotationally stable as a jacketed?

I find that a little tough to accept.

[Bullshop]

Hamish
Too late your in now! You will soon begin to feel the gravitational effects of this black hole as your drawn in deeper and deeper to the center of the blackness of its dimention .

[Larry Gibson]

I've been wondering about this. The problem I see with the RPM debate is that there are several factors at play simultaneously. Chamber pressure, muzzle pressure, alloy resistance to skid, alloy ability to obturate both in the throat and further down the bore, lube, nose slump, base deformation and maybe centrifugal boolit upset in flight to name a few. How can one say it's RPM that's causing the problem?........

There in is the problem; like others you fail to differentiate what occurs to the bullet in the barrel (internal ballistics) from what occurs to the bullet in flight (external ballistics). Once the bullet leaves the barrel what has been done to it has been done; the unbalancing of the bullet from all the factors you mention. Once in flight it is the centrifugal force of the RPM that acts upon those imbalances creating inaccuracy, (a little or a lot). If the imbalance is sufficient then at a certain RPM the bullet will go into a helical spiral that increases in diamter as the range increases or it will fly off on a tangent to the line of flight. The group size of such will be non-linear as the range increases. Bullets imbalanced but not pushed over the RPM threshold will still shoot linear increasing size groups as the range increases. They may not be very "good" groups but the size dispersion will still be close to linear as the range increases.

Your example shows perfectly that the bullet is imbalanced and in flight will not be accurate. If you push it to a certain RPM it will begin a helical spiral or go off on a tangent and result in non-linear group expansion as the range increases.

The RPM threshold is about what happens at a certain level of RPM while the bullet is in flight. We can lesson the adverse affect and up the RPM threshold level by decreasing the amount of imbalance done to the bullet during casting, loading and in the internal ballistic phase.

So when accuracy really goes south it is because you've done terrible nasty things to the bullet. But consider if there were no adverse effect from RPM on any bullet your badly messed up bullets would be just as accurate as any other bullets. The RPM threshold is all about what happens to the bullet in flight.........

Larry Gibson

[swheeler]

The boolit will only be making one rotation per X number of inches, even at half the muzzle velocity in a 9" twist it's only going to make one revolution in 4-1/2", not exactly a tornado. The action of boolits buzz-sawing their way through a target, in my opinion, is highly overestimated.

Gear

? In a 1;9 twist the bullet is going to travel 9 inches per rotation whether it is going 2000 fps or 200 fps, what you talkin about willis?

[Hamish]

Oh ****, I guess I'm lost now too,,,,

"like others you fail to differentiate what occurs to the bullet in the barrel (internal ballistics) from what occurs to the bullet in flight (external ballistics)."

What about while the boolit is partially in and partially out of the muzzle? (really, I'm an honest to goodness idiot savant, I can be a genius at asking stupid questions)

(it only matters if it does?)

[geargnasher]

like others you fail to differentiate what occurs to the bullet in the barrel (internal ballistics) from what occurs to the bullet in flight (external ballistics).

Larry Gibson

? In a 1;9 twist the bullet is going to travel 9 inches per rotation whether it is going 2000 fps or 200 fps, what you talkin about willis?

Larry, would you mind 'splaining to Scot the difference between what happens to a boolit in the barrel vs. after it's out of the barrel? Something about rate of decay of rotational velocity vs. rate of decay of forward velocity?

Gear

[btroj]

Gear, that twist rate is only for inside the barrel. Once the bullet leaves the barrel it spins so fast it makes millions of revolutions per inch. It isn't a buzz saw, more like a centrifuge with razor blades!

Why isn't there a sarcasm font? My life would be so much easier.

[cbrick]

Gear, that twist rate is only for inside the barrel. Once the bullet leaves the barrel it spins so fast it makes millions of revolutions per inch. It isn't a buzz saw, more like a centrifuge with razor blades!

Huh? Your saying the boolit rotation increases after it leaves the muzzle?

I am very interested in hearing what would make it suddenly spin faster than the twist it was given in the barrel.

Rick

[btroj]

Sarcasm Rick, sarcasm.

In response to a previous post who doesn't understand that if the barrel twist is 1 in 9 that the bullet only makes one rotation for every 9 inches of forward motion. You know, the "buzz saw" effect in game.

Me? I know better.