Magic RPM Number = 720

[Zeek]

The revolutions per minute exhibited by a PPCBoo, as it exits the barrel, can have a direct effect upon how difficult it is to get it to shoot accurately, even if the twist rate matches the projectile length, pursuant to the Greenhill Formula. That is one of the things that makes the 6.5x55 Swede such a challenge . . . its 1:7" twist rate, combined with its fairly high MV, make for very high RPM values. In my experience, any RPM over around 150,000 is going to give you a challenge both in the torque-up department and in the bullet-flight department (accuracy). Below that rate, things are usually easier.

Here is an easy way to determine your load's RPM, given the barrel's twist rate (in inches per turn) and the PPCBoo's MV (in fps). All you have to remember is the number 720. Multiply 720 by your MV and divide by your twist rate (# of inches per turn) and you get rpm.

Example for 1:7" twist and 2300 fps for a 6.5x55 load:

720 x 2300 / 7 = 236,571 rpm
Now THAT is REEEALLY a root'n-toot'n turn rate! No WONDER that baby is to tricky to load for with anything but JBoos!

Example for 1:12 twist and 2400 fps for a 308W load:

720 x 2400 / 12 = 144,000 fps
That is not goofing around, on the turn rate, but it is manageable, even though the MV was higher than in the 6.5x55 example.

Regards, Zeek

[303Guy]

Thank's Zeek. So my 303 Brit firing a 145gr CBoo has a spin of 130,000 RPM. That was an accurate load too. So theoretically, I should be able to shoot casr up to 20500 fps. That's not insignificant for a 220 gr boolit I intend shooting as plain cast smooth side in one of my Lee Enfields 9probably at around 1900 fps due to pressure constraints.

Now, how does that translate into PPCBoos? They are lighter and have a smaller diameter for the same length over and above the method of transporting down the bore.

[pdawg_shooter]

So a 30cal out of a 1/10 barrel at 3000fps wont shoot? News to me. If you support the bullet completely in the barrel so it does not distort under acceralition and use the correct alloy it seems to work fine.

[Larry Gibson]

Zeek

As you can see you are already getting the same misunderstanding of what you are saying as I did.

It always amazed me how "any RPM over around 150,000 is going to give you a challenge both in the torque-up department and in the bullet-flight department (accuracy). Below that rate, things are usually easier.", "tricky to load for " and "it is manageable" get instantly interpreted and argued as "wont shoot"! Wrapping a PP around a very hard cast bullet "support the bullet completely in the barrel so it does not distort under acceralition" can make them shoot quite well as pdawg shooter does. Others have a considerable amount of problems with it though. In the bore during accelleration a PP does basically the same as a jacket. Getting a regular cast bullet to shoot accurately at HV/high RPM is indeed "tricky" but yet it can be done. Shooting regular cast bullets at HV/high RPM is not anywhere as easy as some would have us believe. If one is looking for the best consistent accuracy possible with regular cast bullets then keeping the RPM under 140,000 is the place it is going to be done.

This is all I'm going to say regards to this on this thread.

Larry Gibson

[303Guy]

Well said, Larry. That makes it very clear where the challenges lie.

Something else I have found is that with softer boolits or cores, it is possible for the rear shank portion of the boolit to bump up un the neck/throat and on sizing down again, the base cups and sometimes unevenly. That to me is another challenge which is pressure curve/fit/alloy related and not rpm and muzzle velocity related. Just mentioning.

But the magical number remains 720.

[Zeek]

So a 30cal out of a 1/10 barrel at 3000fps wont shoot? News to me. If you support the bullet completely in the barrel so it does not distort under acceralition and use the correct alloy it seems to work fine.

That is not the meaning that I intended to convey. It is just that getting it to work becomes trickier the faster the bullet is turning. Your hotsy 30cal load is doing just over 200,000 rpm, which is not way radical (compared to a comfy 150,000 rpm), but it IS getting up the on the turn rate. If you do everything right, then no problem. My main goal is to define where "the easy meat" is, but doing the hard stuff is also interesting. From my viewpoint, even doing ANYTHING with the 6.5x55 puts me out of the easy-meat area, and my inclination about the WHY of that is the high rpms involved.

Consider, for example, a 47 Kpsi load with a PPCBoo made on an NEI 125gr 6.5 CBoo (cherry #24). It would give you 2900 fps, which, at the 1:7" twist, would be 300,000 rpm!! As you can see, that is getting to be on the far side of sane. However, if we could get it to work, then a dead-hard (~27 Bh) CBoo would be ultra-explosive on varmints. CBoos (including ones that have left their PPatch behind after exiting the muzzle) don't lose their rpm's much at practical distances. The centrifugal force would be really something, and the impact would let that loose.
Regards, Zeek

[303Guy]

Aah .... but! We are talking RPM v/s velocity and twist rate. If we start talking max RPM v/s caliber (and length is going play a roll here) then we need to add another constant. I suspect that such a constant is going to get complicated by the addition of length to diameter ratio. The Swede has a torpedo shaped boolit which increases the required spin but also causes a spin/length issue I should think? It's also possible that the Swede case causes its own set of challenges because the case/shoulder shape is going to restrict high pressure gas flow into the bore, flattening the pressure cure and at the same time, raising the muzzle pressure. Or not!

[9.3X62AL]

I'll start the popcorn.......

[dnepr]

good thread , I am slowly figuring out some of the variables that might be causing me grief

[Zeek]

Aah .... butt!

Well! I'll have you know that I HAVE been called worse than that! Mah!

We are talking RPM v/s velocity and twist rate. If we start talking max RPM v/s caliber (and length is going play a roll here) then we need to add another constant. I suspect that such a constant is going to get complicated by the addition of length to diameter ratio. The Swede has a torpedo shaped boolit which increases the required spin but also causes a spin/length issue I should think? It's also possible that the Swede case causes its own set of challenges because the case/shoulder shape is going to restrict high pressure gas flow into the bore, flattening the pressure cure and at the same time, raising the muzzle pressure. Or not!

There are other things that are valid variables, but the twist rate sits all by itself and helps make things simple. Here's how: The MV is similar for most high power rifles, so that is really not a variable. . . . rather, it is a desired destination. Therefore, it reflects your degree of success (or, in the case of failure, your suck cess), so is not really a "variable." A fast twist rate limits your ability to GET into the desired MV range (while keeping accuracy) because, for any given MV, the twist controls the RPM and RPM can have a two-fold problem: 1) the torque-up phase is MUCH harder on the CBoo core; and 2) the free-flight (assuming that the RPM is high enough for stability) gets a hit because the higher the RPM, the higher the adverse effect of ANY unbalance, including micro-bubbles located in the CBoo's rear (steering) end.

My observation is that both of these ass-pecked-s begin to become seriously obvious ABOVE (correction) around 2000 fps and faster than ~1:10" twist. This sounds complicated, but it is far simpler if we realize that 2000 fps is not high enough and that we are shooting for around 2400-3000 fps on the MV, so the ONE variable that rolls all of this together is the RPM variable. That perspective simplifies what would otherwise be a complex multi-variate consideration.
Regards, Zeek

[runfiverun]

stayin outta this one maybe i can pick up larry on the way over to the popcorn smell.

[barrabruce]

Can you get me a large coke while your over there???

Cheers.

[303Guy]

Well, what I was getting at is that theoretically a fairly soft CBoo with reasonable length can deform as it leaves the muzzle if the muzzle pressure is higher than it's upset pressure. This type of effect can blurr or complicate the magic number. Frinstance, according to all the stability factor formulae, a 1-in-16 twist 22 hornet cannot stabilize a 60gr j-word. Yet it does and it shoots them with a high degree of accuracy right out as far as the trajectory allows us to see strikes. BUT! It does this only with a certain powder. MV is around 2650fps. Even that is not supposed to be do-able in the pressure limits of the hornet case. But it does. (And it makes for a hard hitting load). AND, case life is indefinate! Go figure. (55grainers shoot even flatter).

OK, the point is there other factors involved and my question would be how do those factors change or influence the 'standard baseline'? The shape of the pressure curve frigzample? The volume of muzzle blast gas? And so on.

[DIRT Farmer]

The peanut gallery is growing while you'al figger thisn out.

[barrabruce]

Got me stuffed already..but the mud may settle and I may see a bit of light through the murky water yet.

Munch munch....hey how much butter and salt did you put in this popcorn!!!!!..munch munch..

Gurgle...burp!!!
Buuurrrpp ....aghhhhh that's better.

Please continue.

[runfiverun]

it can deform in the bbl too.
muzzle blast will upset your boolit.
the ideal with our little speed demons is.
to start them out slowly,and straight.
an even long acceleration down the bbl.
then a drop off of pressure as close to the end of the bbl as possible without excess gas pressure to cause upset on one side of the boolit or another.
the twist rate in a bbl is another issue.
skipping and overspin are the biggest obstacles in say a 10 twist 30 caliber bbl.
you make your alloy harder,use fillers,match up your primer for good ignition, impact your throat twice.
change powders four or five times slower and slower,etc. and finally get it to shoot decently at 2100 fps.

then i show up with my 30-30 and a 12 twist bbl shooting at 2200 fps shooting 1" groups
and all i did was flare the case mouth, and am using up some powder i had on the shelf.
and figured the boolit would fit fairly well by pushing it in the chamber with an empty case.

thats where the rpm fight starts.

[303Guy]

then i show up with my 30-30 and a 12 twist bbl shooting at 2200 fps shooting 1" groups

Aha! Another important facter - case capacity! The objective as laid out by runfiverun would more easily be attainable with a smaller case and no doubt boolit sectional density optimization. Too high a SD and slower powder with high muzzle pressure results. Has the 30-30 got the perfect case capacity to bore to sectional density ratios? The twist rate seems optimum too. Mind you, the case shape is prolly just right as well.

[Nrut]

The .32 Special waxes the 30-30 three ways to sunday...
Just to add to the entrainment value, eh?...

[felix]

That's correct, Nrut! Twisting and Shouting don't do boolits justice. ... felix

[Larry Gibson]

Naw......the 16" twist of the 32WSPL couldn't be making a difference..... now could it?

Larry Gibson