Need to pay attention to the experts here

[Hick]

I've got a nice 225646GC mold I use with my 223 Remington (CZ-527) that I've been getting great accuracy with. I decided to try a little hotter load, and when I tested it I couldn't get a consistent group-- not two shots in a row on an 8" target at 100 yards. I couldn't even get on paper consistently to zero the load. After much puzzling I finally remembered to calculate the RPM-- and discovered I was Waaaaaay over the RPM threshhold for cast. I forgot to pay attention to the advice from the experts here on Castboolits. Live and learn.

[Ateam]

For a nooob, how do you go about calculating the rpm threshold?

[jeepyj]

Marked to monitor responses

[runfiverun]

when did we get a threshold?
sonafabitch don't be telling my rifles.

[Texas by God]

I'm scared now. Math is coming.

[mjkonopka]

Do you mean your barrel is too fast of a twist for the boolit you're shooting?

[Wayne Smith]

Too fast a twist for the alloy at the speed he's shooting it. Boolit design also factors in.

[Bzcraig]

.........This might get interesting

[wmitty]

Why do we describe linear velocity using fps and angular velocity using rpm? Why not revolutions / second? Doesn't a rifle with a 12" twist impart an angular velocity of 2000 rev/sec if the boolit is launched at 2000 fps?

[Hannibal]

The Man Who Kicked The Hornet's Nest.

Get out while the gettin's good !!!!

[Hick]

There was an old post from a man who isn't here anymore (he has his own site, I think) who observed that with relatively hard cast bullets (like Lyman #2), accuracy went to hell when RPM got up to about 140,000. I've called that a threshhold, but it's really a little bit approximate. Calculating RPM is easy--Velocity times 720 divided by twist. So-- for a 9" twist in my 223 Rem, when I was pushing my cast at 2050 fps, it works out to be 2050 x 720 divided by 9, or about 164,000 RPM-- and only about every third bullet could hit a 18 inch square target board. I realize that 140,000 number is a little imprecise-- but I wasn't even close.

[Peregrine]

Hey Hick, which formula did you use to calculate your RMP?

[44man]

RPM = MV x 720 divided by twist. Imagine a 7" twist WOW.
Or MV x (12/twist) x 60, same result.
Wmitty, true but RPM's seems standard measurement. However your assumption changes with twist. The lower formula only works with a 12" twist if you remove the X 60. 12 divided by 12 = 1, any other twist changes that.

[JohnH]

Don't mean to be a stick in the mud but here goes. There is a reason the old timers who laid the foundations of smokeless powder cartridges driving bullets in excess of 2000-2200 fps put copper condoms on them. Some say anything is possible. Some say not so much. Man believed flight was impossible for millennia. And even when the Wrights put that to rest I don't think they fully understood how they had done it and were certainly at the beginnings of aerodynamic physics. Maybe that's where we are, the beginnings of Mach 2.5-3.5 cast boolits. But I doubt it. I'm certain that under the right conditions, that getting above 2000 fps with great groups on demand is possible, I just don't think the ordinary off the shelf rifle, with ordinary COWW alloy and all the other ordinary things we use are going to get there. If they were, everyone would be congratulating themselves on how they beat out their buddies laser wand target specials with Jim Bob's Savage 110. There is a reason I bought a CZ 527 in 7.62 x 39 instead the Ruger American in 300 BO that was right beside it for $50 less... twist rate. No matter how you slice it, the 300 BO twist rate was conceived with 200-250 grain jacketed bullets in mind and while men are shootzen plain lead in their BO rifles with good results no less, I simply didn't think that adding a higher twist rate to the equation of getting good accuracy results was worth it. I've been shootzen 10 twist 30 caliber barrels a long time at velocities of 1600 to 2000 fps with ease often doing little more than water dropping my castings to make 'em a bit harder for those loads I want to shoot fast. But fact of the matter is I shoot to enjoy and get as much satisfaction from 1600-1800 as any velocity and the overwhelming bulk of my cast rifle shootzen these days is done at 1600 fps and it will ring a piece of steel as well as I can hold a gun with iron sights to hit it. It don't pound me to death and I can shoot it all day. That said many here do enjoy the search for higher velocity and my hat's off to 'em. Me? If it's more complicated than making some Felix Lube every few months or I have to drag out my calculator to decide if my twist and velocity are compatible it's gone from enjoyment to work and I ain't interested.

[oldblinddog]

The Wright brothers knew very well how they had done it.

[runfiverun]

yeah there ain't no way my 7 twist 300 is going over 2ooo.
my 8 twist 223 ain't making 2800 either.
and my 10 twist 308 certainly isn't breaking 2400.
unfortunately they all managed to hit an accuracy node right at those speeds and that was as far as I went.

look.
if you take an off the shelf long parallel sided boolit mold and pour some ww's and tin in the cavity's your gonna have problems when you try shooting them in your deer rifle.

if you take the time and think about things you can easily take steps to go past the threshold.
measure your rifle.
start the boolit straight.
work on the design features to fill in those air gaps.
fix the launch.
have a good barrel.
those few things alone will gain you another 400 fps.

[Scharfschuetze]

I've been shooting 30 calibre military rifles with cast bullets for well over 40 years now. In that time, I've come to standardize cast bullet weights between 190 grains to 210 grains and I push them between 1,800 and 1,900 for what works out to be the best average weight and velocity for accuracy in the 7.7 Jap, the 303 British, the 30/40 Krag, the 7.62 NATO and the 30/06. Invariably, almost all of these military barrels (Russian, Japanese, British and American) have twist rates right at 1 in 10" or very close to that figure.

If you do the math (1,900 X 720 / 10 = 136,800 fps) my preferred and most accurate cast bullet loads in numerous rifles fall right in line with the theory of the RPM threshold. Attempts to go faster (easy to do with the 30/06) even with sorted and weighed Linotype bullets that are loaded in match-in-line dies with neck sized cases and are virtually breech seated into the lands invariably fail as bullets start to wonder right off of the targets, even at closer ranges.

Why? Cast lead is only going to stand up to so much abuse. Jacketed bullets are stronger and can take it, although a poor quality jacketed bullet will certainly fail in accuracy when spun too fast. When we adopted the M16A2 rifle, we still shot a lot of the M16A1's M196 55 grain ball ammo until those stocks were expended. Now and then we got a specific lot of M196 that was extremely inaccurate in our 1 in 7" twist A2s, but shot very well in the 1 in 12" twist M16A1s and G33s still used by our foreign allies.

After reviewing my post, it occurred to me that I was only recently aware of the concept of RPM threshold after reading several posts here. While I had been ignorant of the concept for nigh on 40 years, I had certainly been subject to it. Without realizing why, I had come to regard a 200 grain bullet at 1,850 fps as about as good as it gets for shooting in 30 and 31 calibre rifles with their 1 in 10" twist. The velocity is correct and the weight and ballistic coefficients of bullets like the 311299 Lyman design hold well to ranges further than most people think is possible even with factory ammo.

[JohnH]

yeah there ain't no way my 7 twist 300 is going over 2ooo.
my 8 twist 223 ain't making 2800 either.
and my 10 twist 308 certainly isn't breaking 2400.
unfortunately they all managed to hit an accuracy node right at those speeds and that was as far as I went.

look.
if you take an off the shelf long parallel sided boolit mold and pour some ww's and tin in the cavity's your gonna have problems when you try shooting them in your deer rifle.

if you take the time and think about things you can easily take steps to go past the threshold.
measure your rifle.
start the boolit straight.
work on the design features to fill in those air gaps.
fix the launch.
have a good barrel.
those few things alone will gain you another 400 fps.

Respectfully, everything you just described is everything I don't want to do. I spend my work life keeping track of 3-5 guys building handrails from architectural prints/drawing and my own drawings based on field measurements, all day everyday all week. I spend half my day with a calculator in one hand, drawings on the table and setting up jigs or frames to be welded out. I follow the product from raw material to fabrication, to cleaning, painting and installation. I'm in the middle. People love to think management has it made. Not my experience. I get it from both ends. End of day, I don't want to have to think about anything. End of week all I want to do is go to the range and be left alone. The last thing I want is for my hobby to be a reflection of the attention to detail and hassle my work life is. Maybe that will change in in seven years when I won't have anything to stress over but getting another 100 fps out of my rifle or shrinking it's groups by another half inch, but right now, I don't care. I don't want to care. I don't want the hassle. I'm not even going to do something as simple as slug the bore. I'm gonna stuff a boolit that should be .003-.004 over groove diameter in the case and fire it. If ten rounds makes a two and a half inch group with five or six going into half that, I'm happy, all of 'em will slap an eight inch gong. If I don't get that, I'm gonna play a little with loads or change boolits or stop shootzen that rifle with cast or blame it on bad eyes and iron sights. I don't want the hassle. And I don't want the battering. The recoil of a two hundred grain boolit at 1600 fps from a seven to eight pound rifle over the course of a hundred or so rounds is all the battering I'm gonna put up with. And I like COWW or something near that (led and lino at two to one for example) It works good in both my handguns and rifles without having to do anything special for either. Hassle free. I like that.

[earlmck]

Respectfully, everything you just described is everything I don't want to do.The recoil of a two hundred grain boolit at 1600 fps from a seven to eight pound rifle over the course of a hundred or so rounds is all the battering I'm gonna put up with. And I like COWW or something near that (led and lino at two to one for example) It works good in both my handguns and rifles without having to do anything special for either. Hassle free. I like that.

Yep I hear ya' John H; that describes my place in life also, except I'm an old retired guy and I really don't need to be pushed hard. I have taken a liking to the little 117 grain bullets that I make for the 32/20's. I just size 'em on down to whatever it takes to allow the cartridge to fit in the 30/30 or 300 Savage or 308 chamber OK, load them at 1600 to 1700 fps, and they ring the gong just fine also. And consume less of my lead stash and hardly kick at all. Life is good!
I might also admit that I cast up some nice 22 cals out of Linotype using the 6-cavity Lee that I shoot about 2400 fps out of 16" twist hornets. Very effective on sage rats!

[Larry Gibson]

The RPM Threshold is indeed alive and well. There was a very good sticky defining and explaining the RPM Threshold but it was deleted by the moderators when I was banned. Thing to remember is the threshold will most often fall between 120,000 and 140,000 RPM with regular lubed cast bullets. Where in there depends on a number of variable. The threshold can be pushed up w/o too much difficulty as I demonstrated in 10 and 12" twist .308Ws in a thread posted on the NOE forums; http://noebulletmolds.com/smf/index....2.html#msg3772. The same can be done in other cartridges.

However, if you are rebarreling why limit yourself to the faster twists? A 7" twist 5.56 is there to stabilize the long tracer bullet. The Match twist of 8" is to stabilize longer 75 - 80 gr match bullets. The M852 62 gr green tip bullet is easily stabilized in 9 and 10" twist ARs and other gas guns. If you're not going to be shooting those longer bullets and want to shoot a lot of cast then why not look at the 12 or even 14" twist barrels. That twist will stabilize a lot of cast bullet designs and will give accuracy well above the velocity level needed for reliable functioning. As to bolt actions or single shots I've long been shooting cast bullets in 12, 14 and 16" twist .22 calibers. They work fine as do all the 35 - 55 gr jacketed bullets made for them.

In .30 caliber you can easily control the RPM with slower twists. A 14" twist 26" barrel in .308W will stabilize a 200+ gr 311299 at 1900 fps and accuracy up through 2500 fps is quite possible w/o jumping through a lot of hoops. I shoot a 311041 at 2600 fps out of my 16" twist 30x60 XCB and maintain hunting accuracy (2 moa or less) out to 400 yards. However, best accuracy at high velocity is easily obtained with the slower twists using designs better for cast bullet HV shooting such as the 311466 and the NOE 30 XCB. Here's a typical 10 shot group shot at 100 yards with the 311466 out of the 14" twist .308W. The alloy is Lyman #2 and the bullets are WQ'd out of the mould. As you can see the velocity was 2600+ fps.

Attachment 192361

Here's a typical 10 shot group shot at 100 yards out of the 30x60 XCB 16" twist with the NOE 30 XCB bullet. The velocity was right at 2900 fps. The sighting diamond is 3/4" on a side BTW.

Attachment 192362

HV is not that hard to achieve with cast bullets w/o a lot of fuss. You just have to control the RPM is all. Yes the RPM Threshold is real. If you want to shoot cast bullets accurately in 10, 12 or faster twists you just have to be aware of it and accept, as most have, accuracy capabilities under the RPM Threshold. On the other hand, if you want to shoot HV successfully with accuracy w/o jumping through a lot of the hoops some think is necessary then simply control the RPM through the use of slower twists.

Larry Gibson