RPM Threshold barrel twist/velocity chart

[DR Owl Creek]

Larry,

Thank you for all your work, and for putting all that info out there for the rest of us. I've read the first two parts over twice, and studied the graphs. Its going to take a while just to digest that part. Please keep the info coming. Its really interesting!

Thanks again,

Dave

[44man]

Can't argue larry, there sure must be a difference in in what cast can take. I admit my work was with jacketed in varmint rifles where I shot worse at 100 then at 350. I firmly believe a boolit or bullet can go to "sleep."
It drives me nuts to shoot small groups at 100 with cast in my BPCR and can not hit a ram at 500 no matter what. I watch boolits go way off course. I can bust every pig but after so far the boolit just goes on it's own.
To find a boolit that is linear, 1" at 100, 2" at 200, 3" at 300 is a pipe dream. But it is true that a bullet that does 1" at 100 can shoot smaller at 400.
But no way a 14-1/2" group will be reduced.
Pilgrim stated lighter bullets with slow twists and is correct. Drive faster so spin is up but to mention Greenhill in any context is off.

[Terrence Clarke]

Good work,I often wonder what effect if any that different number of grooves in a barrel would have on the same tests that you have done

[Pilgrim]

I didn't mention # of grooves in my earlier post, but again the HBR folk are plowing new ground (I think, anyway). I dunno what the current flavor is, but 6 or 8 years ago it was 17 twist 3 groove barrels. I'm not entirely sure what the number of grooves has to do with accuracy. Remember the 2 groove military barrels, and the current Marlin microgroove barrels? Both can be made to shoot extremely well. The military 2 groove barrels appear to me the minimum, while the microgroove barrels are at the other extreme. I don't think you could make a single groove barrel work any too well due to bullet imbalance, but any number of grooves 2 or more can be made to work IMO. FWIW Pilgrim

[L Ross]

Impressive amount of work Larry, thanks for sharing.

[dtknowles]

Larry, nice work. I am still not clear on why accuracy deteriorates above the RPM Threshold. I agree you have shown it does but the BC data does not show bullet damage or instability. I can think of a couple suspects but have no way to confirm them. If you have a idea would you care to speculate? One kind of damage that would not show up in the PC data might be if the bullet became unbalanced with the center of mass not on the centerline of the bullet. Not real sure how that could happen but higher RPM would make that damage's affect on accuracy more obvious. Something else that might not show in the BC data but could maybe be seen on the muzzle of the barrel would be if because of skid there was uneven blow by the bullet when exiting the barrel. I know that you said you did not see any lube failures or excessive leading but did you check the muzzle for the lube star and was it even? It might also be possible that it was some other bullet damage that would not show up in the BC data. I don't have a clue what fins or shreds would do to BC data.


Just some stray thoughts, thanks again for sharing.


Tim

[JeffG]

Thank you Larry

[Larry Gibson]

.......... I know that you said you did not see any lube failures or excessive leading but did you check the muzzle for the lube star and was it even? It might also be possible that it was some other bullet damage that would not show up in the BC data. I don't have a clue what fins or shreds would do to BC data.

Tim

Here are the muzzles after a HV testing of lubes with the 311466 bullet. The photo is of all 3 test muzzles after 10 shots each with Javelina lube at 2600 fps using the same load in each. Accuracy really sucked with the left 10" twist barrel, just sucked with the 12" twist middle barrel and was very good with the 14" twist right barrel.

Lube star looks good to me considering the psi at exit from the muzzle.

Larry Gibson

Attachment 108203

[dtknowles]

Here are the muzzles after a HV testing of lubes with the 311466 bullet. The photo is of all 3 test muzzles after 10 shots each with Javelina lube at 2600 fps using the same load in each. Accuracy really sucked with the left 10" twist barrel, just sucked with the 12" twist middle barrel and was very good with the 14" twist right barrel.

Lube star looks good to me considering the psi at exit from the muzzle.

Larry Gibson

Attachment 108203

Yeah, I would say those look fine, no problem there, so uneven blow by is not the problem.

Tim

[leeggen]

Larry, great info. and you have given a lot of time and effort collecting it for all of us to see.
Bet ya a dollar Felix was watching and he would have been happy with the test results also.
You both give us alot to think about and asorb.
CD

[Larry Gibson]

Wouldn't surprise me in the least. I learned a lot from Felix over the years. He is missed.

Larry Gibson

[TXGunNut]

Thanks, Larry. Well done, well written.

[44man]

Lead is a problem, it is a proven fact that even a tiny wobble with jacketed is no good, why the Juenke tool was developed. Tests showed much increased accuracy when bullets were sorted from the tool.
Molds can't make perfect boolits no matter what. Usually a little out of round and forced through a bore just displaces lead. Even a piece of lube staying in a GG can upset things.
I still appreciate Larry's work.

[Char-Gar]

Larry...You get a giant attaboy! You have given a data and scientific basis to what cast bullet shooters have known and experienced for generations, but could not quantify beyond a few basic things. Good going, truly good going.

Charles...

[popper]

Wonderful data - thanks Larry. I think many who DON'T believe the RPM threshold don't understand 200-300,000 RPM. Stick a 1/32" dremel drill in the tool and crank it up, watch the tip. That's only 35k rpm and a tool steel precision bit. Now imagine that type action on a boolit in flight. It's not RPM but rotational mass (inertia) that does the stabilizing. Can't prove it but those long skinny boolits do flex in flight.

[MBTcustom]

I knew a guy once who didn't believe in gravity. He finally found a cliff that convinced him permanently.

Excellent write up Larry.
You're willingness to provide pictures where most of the holes are in a group and not "called flyers" is commendable. It takes a lot of time money and patience to zero in on specifics like this (as well as being a stone cold, darn good shot), and no matter what our membership takes from your article, I hope they glean the importance of keeping the variables to a minimum so that you can learn something.
I'm still trying to rid myself of the temptation to just throw a bunch of lead into the berm and try to learn something from it later. LOL!

[Centaur 1]

Thanks Larry, the time and effort that you put into this is appreciated. I'm curious about if you ever found the RPM threshold to move downward as bullet diameter increases? I'm thinking about this from a machinist point of view and how you run a larger diameter cutter at a slower spindle speed than a smaller cutter. Even though the cutter is turning slower, edge of the tool is moving at the same S.F.M. (surface feet per minute) as the smaller cutter. Transferring this knowledge to cast bullets, if there is a small imperfection in a cast bullet it would greater effect bullet stability the further it was from the center of rotation.

[btroj]

I knew a guy once who didn't believe in gravity. He finally found a cliff that convinced him permanently.

Excellent write up Larry.
You're willingness to provide pictures where most of the holes are in a group and not "called flyers" is commendable. It takes a lot of time money and patience to zero in on specifics like this (as well as being a stone cold, darn good shot), and no matter what our membership takes from your article, I hope they glean the importance of keeping the variables to a minimum so that you can learn something.
I'm still trying to rid myself of the temptation to just throw a bunch of lead into the berm and try to learn something from it later. LOL!

No Tim, keeping variables to a minimum in a SINGLE test is important.

Multiple variables are to be tested in multiple tests, with one thing being varied at a time.

[swheeler]

Excellent write up Larry and thanks for sharing your work, good job as usual! Imagine one of those 30xcb's chambered in a 15 or 16 twist barrel the way it was meant to be done, smokin.

[Digital Dan]

Nice job Larry, thanks!

Uh...I'm going to sleep now. No spin, I win.