Contrary to popular opinion, I don’t think obturationplays the role with cast bullets that many shooters suppose. In at least some cases (with hard cast bullets),obturation apparently occurs neither in the throat norwith the peak pressure, even with very powerful loadsintended to produce such base upset at any cost. Imake this statement on the basis of .30-06 bullets (bothlinotype and wheelweight alloys) recovered and inspected as the powder charge was increased until the shooting became wild.

Now let me disavow any notion that the informationI’m about to relate came from any massive scientific program like Col. Harrison’s exploration of bullet lubes.Some portions of it actually were designed ladder studies to carefully investigate some unexpected observations, but some of it was simply accumulated over yearsof shooting, and pieced together as other informationbecame available to link earlier observations. But it hasall come together to form what I think is a pretty goodexplanation for what is going on with high pressure hardcast bullet rifle loads. A lot of people are going to disagree, but I’d like to hear any hard evidence to the contrary.

Yes, I’m aware that the base of some bullets like a.22 long rifle and .32 Colt were designed to expand. Inmy investigations, I did some work with them. (Firing a.22 rifle or revolver underwater is a hoot, and so is ‘hunting’ bluegills!) Recovered bullets left no doubt at all thatrounds like these DO expand at the base. But these areloaded with the softest bullets it is possible to manufacture.

I don’t know how soft a bullet has to be to upset, butI know that it has to be pretty darn soft: Even pure lead.32 Colt bullets will often fail to expand and obturateunless the bullet is of hollow-base design. This is thesame approach used by Keith in his 358431 design, andinnumerable swaged wadcutters: Soft bullets, often witha hollow-base design assist.

While I recognize that obturation CAN occur undersuch circumstances, soft pistol bullets and hollow basesare not the subject of this particular ‘ramble’. I intend toaddress what I’ve found happens to hard cast rifle bullets at higher than ‘normal’ velocities, and the followingcomments should be understood in that context.

Bullet recovery during these tests was achievedmostly by firing into deep pools of water (at anglesexpected to preclude ricochets) and swimming down togather what I could of the bases. (The nose portionsfrom powerful loads were usually shattered or badly distorted, but it was seldom that the lower half of the bulletcouldn’t be recovered.) Some were recovered by happenstance from stacks of boards or Styrofoam, wetnewspapers and even sand banks. While most of thesewere much more destructive to the nose of the bullet, Iwas still able to retrieve some pretty clean bases inmany cases.

Granted, these (generally) mushroom shapedrecovered bullets were seldom in a condition thatencouraged me to rely on diameter measurements.However, I am not blind, and it was easy to see that thegas check shank was still substantially smaller than thediameter of the gas check or the bore riding body of thebullet. Expanded as the base may have been by impact,the step was still there: A blind man could see it, or atleast feel it with his finger nails. If the base had expanded, it would have filled the neck and throat portion of thechamber. And when it was swaged back down by passage into the bore proper, it would have obliterated thegas shank, and any upset lube grooves.

This did not happen. The gas check shanks andlube grooves remained intact. The depth of the ‘stepdown’ from the full bullet diameter to the gas check didnot change. The depth and width of lube grooves did notchange noticeably. No evidence of base upset (obturation) or other damage that could be attributed to propellant pressures was observed. In fact, it was observedthat small imperfections in the base of the recovered bullets (sprue marks) had been telegraphed through thegas check, and were quite visible on the bottom of thebullet. Any conversion of the bullet to a putty-like consistency under the pressure of firing (as postulated byDr. Mann) would have eliminated both sprue imperfections and gas check shanks. The upsetting lead shouldhave conformed to the shape of its surroundings likewater in a bucket. This did not occur.

Relatively mild loads provided recovered bullets thatappeared to be mirror images of the bore through whichthey passed. As the powder charge increased, the firstevidence of damage was not base upset, but etching ofthe lead alloy immediately in front of the gas check. Thisetching was initially limited to a small spot in front of theedge of the gas check, right where it was engraved bythe rifling lands. The remainder of the gas check shankshowed no initial damage or upset.

As the powder charge was increased further, theetching became more extensive. It grew mostly alongthe line of the engraving until it eventually began toencroach on the bottom of the base driving band.Etching marks also enlarged horizontally until it encircledthe base of the bullet.

Initially noted only on the lead gas check shank atthe edge of the engraving of the gas check, the etching shifted its location when it extended forward to the basedriving band. The etching on the base driving band (andon all forward body bands) invariably began on only oneside of the land engraving and was later far more severeon that side than the other.

Investigation and study revealed that the side of theland engraving which received little or no etching wasthe side pushing or rotating the bullet. At the same time,the trailing edge of the rifling was extensively etched inrough proportion to the powder charge: More powder,more etching, and the further the etching ranged towardthe front driving band. In all cases, etching was farworse toward the bottom of the bullet.

Most interestingly, the width of the rifling alsoincreased far beyond the width of the rifling that produced it. This could be observed very dramatically byplacing recovered bullets from light, moderate and heavyloads side by side. It was also observed that in allinstances where this land enlargement occurred, thevisual severity of the etching was heaviest on the side ofthe rifling that was opened up. As the land forced thebullet to rotate, it apparently wore against the side of thebullet against which it pushed. Rotational forces kept thedriving side of the land tight against the (worn) side ofthe bullet, and the wear was evidenced by an increasinggap on the trailing side of the land. This gap was notclosed by obturation at any point. Instead, it provided achannel of escape for propellant gas to etch the bullet.As the width of the engraving enlargement graduallyincreased, the etching of the trailing edge of the landengraving continued to increase in width and depth,while the more recently exposed area of the enlargedengraving on the bullet was etched less due to shorterexposure.

Interestingly, these loads seldom gave any evidenceof leading until the charges were increased to exaggerate effects. The first leading was in the form of traces offlashing at the muzzle which also began along the trailing edge of the land. This leading did not appear untilenlargement of the rifling began to become quite evident. This marked the end of the finest shooting, andaccuracy quickly deteriorated as the charge wasincreased thereafter.

I found it interesting that the engraving enlargementwas quite uniform: If the evidence of the etching wasignored, the result would have been the same as if thebullet had been fired through a bore with much wider lands.

As noted, this enlargement of the rifling engravingwas proportional to the power of the load, though I can’tput a number on the proportionality. And these testswere many years ago, but as I recall, shooting becamepoor when width of the bullet engraving was about 120%to 140% the width of the rifle’s lands. Higher power produced even greater enlargement until the shootingbecame wild.

Now what makes all this pertinent to the question ofbase upset or obturation is the fact that at no point wasany obturation or base upset observed, even when pressures (or at least powder charges) were well into jacketed bullet territory, and shooting was wild. The progression of symptoms / results was quite uniform, and therewas absolutely no evidence of any upset or obturationthat should have filled any enlargement or erosion.

Removing gas checks from the base of recoveredbullets didn’t show any marring or smashing of the spruemarks that all cast bullets have. There was no evidenceof base upset in the throat, where pressures were beginning to smack the base hard enough to get it moving intothe bore. There was also no evidence of any upset taking place further down the bore, where peak pressuresgenerally occur. I saw no evidence of anything resembling base upset at any pressure level that might haveoccurred at any point in the bore.

Now I grant that these tests – though encompassinga good deal of work on my part – were limited to relatively hard alloys and gas checked Ideal bullets – specifically Lyman 311291. I make no apology for that; it wassimply my area of interest at the time. Someone working with softer alloys and / or plain base bullets may wellhave different results, but I’d recommend care in his testsand conclusions: Dr. Mann produced some very extensively upset lead bullets – that couldn’t possibly haveoccurred within his barrels simply because the throat andbore diameters were too small. It is self-evident that theywere expanded outside the barrel. It is my personal conviction that their expansion resulted from muzzle blast,and as recently related, I’ve been able to correlateapparent muzzle pressure and maximum loads too.

But for what it’s worth, I drew several conclusionsfrom this work that I would like to present, along withwhat I think are some reasonable speculations:

1. Base upset – though condemned by Dr. Mann andmany other skilled and knowledgeable shooters – isapparently not the primary failure mechanism of hardgas checked cast bullets. That’s not to say that damaged or deformed bases will still provide good accuracy.They won’t. But hard alloy bullets apparently do notdeform (by upset) on firing.

2. Enlargement of the rifling by sliding wear of the bulleton the driving side of the engraving is apparently a primary failure mechanism of hard gas checked cast bullets. At the very least, it encourages both etching andleading, and may be entire cause of both of them.

3. When enlargement of the engraving is sufficientlyadvanced, it is indistinguishable (for all practical purposes) from ‘stripping the rifling’, a condition that was formerly blamed for gross inaccuracy.

4. A sufficiently enlarged engraving width will obliteratethe land engraving altogether, and should be comparable to a bullet with NO engraving. This would essentially convert the rifle into a smoothbore for all practical purposes. This would account nicely for the leading andloss of accuracy that has been typical of cast bullets atvery high (jacketed) power levels.

5. This enlargement mechanism also seems to accountfor the difficulty that many have reported withMicroGroove bores: The shallow rifling would producemuch less depth of engraving on the bullet, which wouldbe less resistant to wearing on one side, and cause morerapid enlargement of the bullet grooves.

6. Granular fillers like Cream of Wheat could be reasonably expected to ‘plug’ any channels opened by abrasive enlargement of the rifling and prevent gas blow-by.While I have no direct evidence that this is the case, itwould account nicely for the rather remarkable ability ofCream of Wheat to completely prevent leading with powerful cast bullet loads.

7. Paper patching is currently the premier technology forobtaining enhanced cast bullet performance at highpower. It (lubricated paper) is not subject to etching, andapparently will not wear to permit enlargement of therifling engraving. I know of no way to recover a shedpaper patch in a condition to confirm this directly, but theevidence from paper patched bullets in even magnumrifles without leading suggests strongly that it will notabrade in the bore. The enlargement of rifling engravingappears to require such abrasion.

8. The next step up in performance – jacketed bullets –are also immune to etching, and I have never noted acase of rifling enlargement with them – though it must beadmitted that I seldom use them, and have not lookedcarefully for any evidence of enlargement. Suchenlargement seems most improbable, given the muchgreater strength of the jacket over that of even the hardest lead alloy.

So there you have it: Within the limited scope ofhard, naked gas checked cast bullets, I believe that leading and loss of accuracy in high power cast bullet loadsis primarily due to etching and abrasive wear enlarge ment of rifling engraving on the bullet. I see no evidenceof base upset, much less any empirical evidence that anyobturation that may occur has any influence on bulletaccuracy or performance. I also believe that there isexcellent reason to expect these conclusions to hold trueover a much larger population of cast bullets, should anyone wish to explore further.