Stripping the rifling on a cast bullet

[Idz]

After working out the rpm limit of a spinning bullet (http://castboolits.gunloads.com/showthread.php?226044-Maximum-velocity-%28rpm%29-for-lead-bullets) I worked out how a bullet can strip off its rifling. I think the mechanism is mostly shear and is unlikely to occur for a normal load with reasonable lubrication. I've attached a .pdf file of my notes for those interested and would appreciate your feedback.

thanks

100943

[Land Owner]

I think a lot can be empirically learned from the Sticky above entitled "If you think so, try this." Read it to its end.

[geargnasher]

Interesting about the Lee "stripped" boolit, I have long thought that false conclusions were drawn from that. Firing into a swimming pool (IIRC how they recovered that boolit) is certainly not a proven way to capture a boolit in the same condition it left the muzzle. High-speed photography or even firing into a snow bank is far more effective.

I appreciate the detailed calculations regarding rotational torque values achieved at the various force peaks (particularly the acceleration max, which is more important than Pmax), this was long-sought after by some of us and was discussed most recently in the "Accuracy and trailing edge failure" thread. Unfortunately, some trolls ran off the Ph, D. physicist who was working on a formula to calculate the compressive stress on the edge of the lands in an attempt to correlate that stress into a meaningful prediction of accuracy loss at given velocity with given alloy, twist rate, and acceleration curve.

Gear

[popper]

I think the mechanism is mostly shear and is unlikely to occur for a normal load with reasonable lubrication.
It's all shear and has NOTHING to do with compression except compression & shear strengths are related to the material (tensile) strength. Actually all you really need is a pressure curve to do the calculations. QL is not needed.

[Idz]

Popper,
You can use a pressure curve except that it doesn't account for any losses due to friction, etc. What creates the torque is the bullet acceleration which requires some interior ballastics program. You can see from my tables that the pressure no-loss assumption yields accelerations about 10-20% higher than QuickLoad.

[longbow]

I never had an issue until I started reloading for my .303 Lee Enfields with 1:10" twist.

I was having accuracy problems so was trying different powders and boolits to see what worked. None of the loads were hot by any means.

Using 22 grs. IMR4227 under a 200 gr. boolit things were not going too well so I did some digging to recover boolits and found that the grooves in the boolits were considerably wider than the lands, where the lands and grooves in the barrel are equal width.

My conclusion was that the heavy boolit and fast twist resulted in a rotational acceleration lag which swaged the grooves in the boolits wider. I decided to try oven heat treating and the problem went away.

Now these boolits were not totally stripping, they were resisting rotational acceleration long enough to wind up with wide grooves in them and that allows gas leakage so leading and poor accuracy.

Not sure what it would take to totally strip a boolit but a bit of swaging making wide grooves pretty much ruins a boolits day so that is too much for me.

From past experience, I will hazard a bit of a guess that if the load is reasonably hot and the boolit skids a bit there can be enough gas cutting to remove most or all of the boolit's surface along with the rifling leaving a more or less smooth boolit. I base this on boolits recovered from snow after being shot from my .308. These were GC boolits shot barefoot over a Lyman recommended load of IMR3031 under a Lyman 31141. Boolits were keyholing by 25 yards and recovered boolits were virtually smooth. Lightening the load some and adding gas checks solved that.

Again, not mechanical shearing/stripping (or at least not totally) but same net result ~ no rifling on boolit = no accuracy (and leading!).

Longbow

[John Boy]

longbow - might want to try the Powley Computer for Handloaders for your issues
http://kwk.us/powley.html