RPM in a nutshell......

[NSP64]

I have tried to read the RPM debate thread and my mind shuts down Could anyone break it down into plain english what the RPM threshhold is??? I mean I think it talks about RPM of boolits and the range they should be shot in to give the best chance for accuracy?

[runfiverun]

the whole thing was about the best accuracy occuring at or below the rpm's for certain twist rates.
example a 10 twist bbl has a potential velocity ceiling of 1950 [about]
a 12 twist of 2100 [example]
a 14 twist of 2250 [ example also]

and lets not start this again guys...
some say the best ACCURACY happens at or below these velocities.
others say it is hogwash..
it don't matter what the twist rate is, you just gotta be able to put a package together that works at the higher velocities.
don't matter if it is only for 4 shots.

[Bass Ackward]

Very simply.

The RPM theory basically says that "best" accuracy will be established at the first velocity level that rotation is adequate to stabilize that slug and that accuracy will deteriorate from that point on. It supports the old cast bullet adage to use the slowest twist rate that you can to stabilize any projectile. And there is clearly a basis in fact for this theory.

It's the conclusions drawn by both sides and the numbers established that cause the argument that you read.

[crabo]

How do you determine that it has "stabilized"?

[BABore]

How do you determine that it has "stabilized"?

The boolit's wittle butt precisely follows behind its nose at the maximum range you intend to shoot it.

[blackthorn]

At the risk of throwing gas on the fire----I have tried to follow those threads relating to the RPM "threshold" and it appears to me that the (sometimes) heated arguments revolving around the testing (that is STILL in progress) boils down to trying to determine whether it is possible to achieve equal (or better) accuracy above the certain velocity range that seems to be attainable by most shooters. In MY opinion the dispute is unlikely to ever get resolved to the satisfaction of everyone of us because there are just too many variables that form part(s) of the answer! At the end of the day, what is important is not the end of the journey but rather the thought stimulation, ideas and knowledge found along the way. Having said that, it also appears to me that those threads sort of got hung up on a dispute over "BEST" Vs "ACCEPTABLE" accuracy (likely an over-simplification on my part). Again likely an unwinnable argument given the variables involved coupled with the needs, wants and individual goals of each shooter. For myself, I will continue to read, consider and learn. Have a great day!!

[crabo]

The boolit's wittle butt precisely follows behind its nose at the maximum range you intend to shoot it.

And you determine when that has happened by..... doing what?

[45 2.1]

And you determine when that has happened by..... doing what?

Nice round holes grouped very tightly on a target are a very good indicater.

[BABore]

Nice round holes grouped very tightly on a target are a very good indicater.

Yep, uh huh, that's how I'd tell.

[Marlin Junky]

I lost interest in those threads long before the last page but that doesn't mean the topic is not interesting to me. During the testing, I didn't see an effort to isolate the damage to the boolit from the rifling, versus the effect of imparting high RPM to the boolit during flight. I mentioned high speed photography at the muzzle and that fell on deaf ears. Did anyone make an attempt to "catch" the boolits at long range in water to inspect the difference in rifling marks on the boolits... under a microscope? I realize this isn't CSI Miami, or whatever crime show is popular these days; however, it sounds like nobody here really has the resources to prove this theory one way or the other. Am I wrong in assuming this? The thing that keeps me leaning toward believing the RPM theory as stated here (I think it goes something like this: flaws in the casting [boolit] produce larger groups at high RPM) is bunk has to do with the fact that putting a hard jacket around the Pb core results in immunity (to a point) from RPM. Did anyone test this theory with swagged bullets... that should eliminate the "casting void" variable to a certain extent. Maybe all my questions were answered in the volumes of rhetoric that were placed here, I just don't have time to sift through it.

MJ

[NSP64]

O.K. but what is the equation and the rpm max??

[Shiloh]

the whole thing was about the best accuracy occuring at or below the rpm's for certain twist rates.
example a 10 twist bbl has a potential velocity ceiling of 1950 [about]
a 12 twist of 2100 [example]
a 14 twist of 2250 [ example also]

and lets not start this again guys...
some say the best ACCURACY happens at or below these velocities.
others say it is hogwash..
it don't matter what the twist rate is, you just gotta be able to put a package together that works at the higher velocities.
don't matter if it is only for 4 shots.

I get better accuracy from the Krag and '03 at the lower velocities. I thought 1625 to 1650 was the cats pajamas. I lowered to around an estimated(haven't clocked 'em yet) 1550-1575. This has cut down on flyers and tightens thing up. Both barrels are 1:10 twist. Off-hand shooting is quite mild.

Shiloh

[onceabull]

If we can keep this thread going,perhaps "Tiger" will entertain us once again ! Onceabull"

[sundog]

Try this. Find a powder that seems to work well for a particular load. Without changing the lot of boolits in use, run from a start load on up until groups shrink and then start to open up. Back off to the 'best group' load and that is your threshold for that load and gun combination. At that point, shoot that load a lot in all kinds of environmental conditions and note the changes, especially ambient air temperature extremes which can affect pressure, which affects velocity, which affects rpm.

I dropped out of that conversation long before it was over, because I don't care if it's pressure or velocity or rpm or whether one in an indicator of another. It just doesn't make any difference to me. If it does to someone, then have at it. If someone doesn't like the way I do it, so be it.

[NSP64]

I was just trying to come up with a way to figure best velocity for given boolit weight. Say X boolit shoots good @ 1500fps . If i use a boolit liter than X do i add velocity to make it shoot good?

[45 2.1]

Try this. Find a powder that seems to work well for a particular load. Without changing the lot of boolits in use, run from a start load on up until groups shrink and then start to open up. Back off to the 'best group' load and that is your threshold for that load and gun combination.

Corky makes an excellent point. With some powders and cartridges, you can run through three cycles like this before you hit maximum for the powder. The cycle starts with a vertically stringing group which will tighten up as charge goes up to a bughole group, then starts stringing horizontally as the charge increases, then the cycle will repeat itself. All through the wonder of barrel harmonics. These seem to repeat at about 300 fps cycles in the shotgun speed powders. Try for a 1500fps load and see what happens.

[runfiverun]

try with slower powders also.
i have a load with h-4831 in my 7mau, that is abysimally boring at 50 yds,
and just falls apart at 100 yds due to not enough velocity. but comes around again
and will shoot to 300 yds well enough to hit bowling pins.
one of the points discussed in the thread also, was what effect twist had on forward
velocity, and whether velocity or rpm's fell out in long range shooting.
[ the lineal target test.]

[Marlin Junky]

runfiverun,

That's pretty amazing. How about some load details? What size groups does it shoot at 100 yards? Do you own/have access to more than one 7x57?

MJ

[runfiverun]

i had 3 but sold one to build another so i will have three again.
i started at 36 grains of 4831 and a rifle primer, using the time tested W.A.G. method
and had predicted about 1900 fps [ended up with 1685] so much for predictions.
it did leave some unburned powder in the bbl but shot very well at 50 yds.
when i ran it too 100 the groups were just awful.
but by taking it up to 38gr they started pulling back together again.

the funny thing about my wifes rifle is that it shoots jacketed and cast to about the same
accuracy level at 100 but the 50 yd load in cast is far more accurate.
what i have found in her rifle is that the powder doesn't make a big difference,as long as
you keep the velocities the same it don't care what powder you use to do it with.
2400,4895,4831,rl-19,h-322,aa-2230, whatever.
........but consistent ignition did make a difference in the group sizes.......
i have her h/v load right now at 46 gr rl-19 in win+p 257 bob brass
the same stuff at 51 gr gives her 2775mv in jacketed.

[Marlin Junky]

i had 3 but sold one to build another so i will have three again.
i started at 36 grains of 4831 and a rifle primer, using the time tested W.A.G. method
and had predicted about 1900 fps [ended up with 1685] so much for predictions.
it did leave some unburned powder in the bbl but shot very well at 50 yds.
when i ran it too 100 the groups were just awful.
but by taking it up to 38gr they started pulling back together again.

What load shot awful at 100 yards but was able to hit bowling pins at 300? Do all the rifles have the same twist?

MJ