Tell me why undersized cast bullets tumble.

[Ian]

We know it happens, whether through sizing too small, having a barrel restriction, allowing too much case neck tension, or even washing out the base bands of a plain-based bullet with too much/too fast of a peak pressure in the load, but what mechanism actually causes the bullet to yaw and even tumble (if maximum yaw damping capacity is exceeded)??

 

[KeithB]

Ian, I always assumed (note that word) that bullets undersized enough to let gas blow by would tumble from unequal gas pressure around the base of the bullet as it exits the barrel. We know a damaged crown, nicked bullet base, or slanted bullet base can cause this. But I don't know this for sure.

 

[Brad]

Based on some personal experience in handguns I figured it is a lack of sufficient rotational motion. I think in some cases, made worse by severe leading, that the bullet doesn't engage the rifling sufficiently to have adequate spin imparted to the bullet. Couple inadequate spin with a bullet probably made lopsided and we have a recipe for disaster.
I have fired some commercial cast bullets in my GP100. Supposed to be .358, actually .356. Cast from hardball they lead badly. First 10-20 are reasonable for accuracy but by 40-50 they keyhole a bunch. The leading is horrendous. As more shots are fired more lead is laid down. More lead in grooves means the bullet isn't riding the lands and grooves but rather a mass of lead.

 

[Ian]

I think stripping can and does occur, but what got me thinking more on this is two of my current projects which opened my eyes. One was my .458 Socom before I fixed the sizing die, the cases were scrunching my soft bullets down and also the base bands were getting washed out. But recovered bullets which were known to have gone through the target at 90° at 25 yards showed good rifling on three of the four bands, just jagged cuts in the third and the fourth (base) band looked like it had been chewed by a squirrel with virtually no engagement of the barrel. The other is my Radical .300 Blackout upper, which is a little generous in the throat and land/groove dimensions compared to my other Blackouts, and this one will yaw with .3095" powder-coated bullets (very consistent yaw, and not enough to affect accuracy much) but will shoot a different POI and not yaw with .310" bullets. I haven't recovered any of those for inspection but assume they are starting out very crooked in the throat, so yaw is more of an effect of bent/crooked bullets than it is a stability issue. The Socom bullets that went sideways had a stability factor of five (if I remember correctly) so needless to say I was a little mind-blown. Simply fixing the case swaging issue cured the problem completely. Also, the Socom bullets were nose-sized for an exact fit in the bore, virtually no way for the bullet to get sideways, and recovered bullets showed nose engraves that were even, again indicating the bullet went up the pipe pretty much straight. Even after passing the suppressor with no baffle strikes, the bullets turned sideways inside 25 yards, very scary. What Keith said about un-even gas pressure has been my theory as well, lacking a better one.

 

[Brad]

If only high speed cameras weren't so expensive. Something in the 50K frames per second would answer so many of these questions.

Even more important in Ian's example is the knowledge that PC isn't a tool to overcome poor fit issues. It doesn't eliminate gas erosion in cases where the fit is all wrong.

 

[KHornet]

Good thread so far!

Paul

 

[Ian]

I'm going to end up with a high-speed camera, eventually. There's just too much that keeps me up at night regarding the first six inches of bullet travel past the muzzle to not try and put it to rest.

Powder coating a bullet lets me shoot a little softer, a little faster, with a little better accuracy than plain base, and with a gas check really improves things to the point I can forget I'm even shooting cast bullets or worry about lube issues at all. But paint isn't the magic cure for anything, you still have to balance alloy with pressure curve and cast GOOD bullets to begin with, it just stretches some of the operating parameters a little bit by strengthening the bullet's surface and reducing abrasion.

 

[Rally Hess]

In my mind it is both gas cutting and poor fit that would cause tumbling at the 25 yds you mentioned. Wouldn't surprise me if it had tumbled several times by then. Seems to me if it enters the bore crooked to start with the bullet would not be concentric from the start and bear harder on one side than the other of the bore. Makes sense to me mechanically that a larger meplat would also aid in tumbling sooner if the bullet left the bore that way also.

 

[Pistolero]

So, vicious gas cutting, it would seem, then big side push as it exits the bbl. Gas cut
groove has gas pressure acting on it while the base is still in the bore, the rest of the bullet is sealed to the bore, so that
side has ambient air pressure. BIG side thrust.

Even with only 5,000 psi at the muzzle, over a .75 x.1 area, that is 375 lb side force!

That would turn me sideways.

 

[popper]

Few years back I decided to see if bad bases was the cause of inaccuracy. I nicked the base of some 40sw and shot, IIRC it was #2 alloy. No tumbling but a nice circle of POI. Recovered boolits a few years back showed no sign of skidding, same load, isocore alloy. Recently was tinkering with sprue cut and shot some 145gr PB in BO pushed hard (~45k psi). Got the nice circle of POI @ 100 , no leading in any of the tests, I assume non-perpendicular bases. Cut the sprue by hand and plate lifts a bit when the alloy gets hard - bases have a 'cut' look to them. No tumbling! Have been playing with a real soft alloy in 40sw. Tumbling and leading. First test was too small so I beagled the mould. Second test did the same. I checked size as I've gotten small sized boolits from Pb/Sn before - no spring back. Thought maybe the cases were sizing down the boolits - don't have any to pull and check, but they all tumbled and leaded. My normal practice is to use the 40sw TC die to insure they pass a plunk test - possibly swaging the alloy down. Next time I will just TC to remove the bell - shoot the ones that plunk OK. I'll also load a dummy and pull it to check. So far my conclusion is leading causes tumbling. Validating reason? First shot is OK from a clean barrel. The rest went sideways - of 20 shots. Gas cutting, stripping? Don't know. I did have to cut the load fro 5 to 4.5 231 in the tests, 170gr Lee mould dropped @ 190. 5gr is really stout. Oh, lube is 3x BLL so I test the alloy only. I will PC some to compare - insure no lube blow off - leading mostly at the chamber end. The alloy is PB/Cu/Zn. Previous testing in pistol & rifle indicates PC doesn't clean out leading but accuracy doesn't degrade seriously. Unfortunately, public range so none can be recovered.

 

[fiver]

it is cause they ain't spinning fast enough.
stuff happening before that is why.
I have de-stabilized bullets in a 7 twist 30 caliber barrel by not putting enough powder behind them.

 

[popper]

it is cause they ain't spinning fast enough. True but IMHO, leading is the real cause for proper loads (S.F.). I gave my SIL a BO upper that needs 310 and some of my ammo that is 308 (PCd). At 100, no tumbling, no leading! Accuracy was cr*p and it was fun watching him try to sight the R.D. Actually, he ran through the 150SPP I gave him and was using the cast when I realized his problem. Per miller formula, dang hard to get < SG=5 from a 40sw @ 200 fps. Same for my 30/30 with a 170gr.@ 800.

 

[freebullet]

Undersize & not spun up will do it almost every time, ask me how I know

 

[RBHarter]

So why did just a harder alloy fix a non-leading nominal middle weight tumble in my 40 ?

I suppose the 50/50 that works in 9mm/38/357 could have skidded enough to not reach nominal twist speed but it should have "plated the bore" if that were the case . My money is on base deformation in my case . The 40 is the only 1 that has done it ....... except very early on in my cast adventures and I basically did everything wrong with those .

 

[popper]

From the deformed base pics I've seen and comments on results, plus my experience, nope. Appear to affect accuracy but not tumbling. SIL shot some of my 130gr HiTek coated in his 9s. Keyhole every once in a while, isocore alloy AC. Just undersized doesn't appear to do it. My SWAG is gas cutting, poor lube, etc. cause leading and pre-existing leading causes/aids tumbling. Only way to know for sure is recovery of a known tumbler (not one that just tips).

 

[Ian]

I have some dim memory of reading about a test that showed a full-speed rifle bullet got the last 150 fps or so once it had cleared the muzzle. Suppressors can add to that even more sometimes, based on my experience. My Socom would turn bullets sideways and yet leave a clean bore.

This sort of goes along with the eternal question of what makes accuracy degrade with velocity? I can cast a batch of bullets, size/check/lube some of them, and size/paper patch some others with two wraps and a twist and no gas check, shoot them over the same jacketed-level load of powder and the mere addition of paper will make the one shoot as well or better than jax and the other will go all over the backstop but still not lead the bore. It ain't casting quality or RPM here, nor is it stripping, but somehow PC, copper, and paper jax makes a bullet shoot better at high rpm/velocity. Linotype is velocity challenged because it sloughs off material through abrasion at HV, even if they don't strip completely or leak at the gas check. Something else is happening and we don't know what, maybe never will. I bet it's the same thing that causes tumbling.

 

[freebullet]

Yaw vs. Tumble & how small changes can affect it is an interesting study.

only guarantee is if you cast, load, & shoot enough you'll see some odd things occur.

With certain powders I've noticed neck tension & crimp can make dramatic changes in tendency to yaw or tumble. Leading or not never seemed to make much difference for me far as tumbling. Leading did seem to have an effect on yaw. I dunno.

 

[freebullet]

Wait for it....

 

[freebullet]

There we have it...well ya can't really see it well but, the bullet is just ahead of the bright fire.

You can see the outline of the bullet base in the dim fire now.

 

[freebullet]