Adventures in forensics

[Ian]

I've been working on a system to recover fired, cast bullets for examination, and am making some progress with a method of recovering high speed rifle bullets with minimal damage so that the results of the firing can be determined, and problems diagnosed.

The trap is a box constructed of 2x12 dimensional lumber, 9-3/4" square on the inside, eight feet long, with a hinged lid. The ends are solid wood with the exception of a hole on the entry side, blocked with a piece of soft, urethane foam rubber material to retain the media. The stopping media which has worked the best so far with rifle bullets travelling in excess of 2,000 fps is a lightly-oiled, fine, lightweight, pine sawdust floor sweeping compound.

Below are two photos of some ACE 140-grain, powder coated, lubricated, gas-checked bullets fired from my 20" Savage 1899 at just under 2200 fps. The alloy is straight, air-cooled wheel weights, air-cooled after powder coating. They check out at 13 bhn after a week. I was trying to make a determination of why the gas checks are heavily coated with lead on the bases, in the manner of powdered metal spraying. The lead has to come from somewhere and my assumption was weak alloy in the 10" twist barrel wearing out on the lands, causing trailing edge leaks. I was partly correct, there is some drive-side wear on the land engraves and some missing coating, but not much in the way of gas leaks is apparent. Perhaps the dusted checks are due to simple abrasion?

 

[Ian]

This photo shows the same bullets loaded down to 1850 fps or so. The abrasion of the lands is still apparent, though the noses of the bullets are considerably less expanded into the groove dimension just ahead of the forward driving band. The expansion is also a lot less dramatic, which is what I would expect from the alloy and the bullet shape. I only recovered one gas check from this firing and it was still coated with rough lead deposits.

There are some torched places on the bullets from the first photos which I attribute to suppressor baffle clips and the high-pressure cross-jetting the clip design induces.

 

[Ian]

The two bullets on the right are some NOE 310-160(?) that we call the XCB bullet. They were cast from a very hard 6 Sb/4 Sn, water-quenched alloy and test at 24 bhn. Sized .3098", gas-checked, and fired with the same, reduced load of powder as the second round of the ACE 140-grain bullets.

Here I thought it was interesting to note how terribly crooked these bullets were becoming in the bore when fired. I also thought it was interesting how much more the hard, tough alloy flowed up and expanded the noses when using the same charge of powder that barely affected the other bullets cast of straight, air-cooled wheel weights. 22 grains of Reloder 7 was the propellant for both.

This close-up attempts to show how eccentric the noses are with the bore. The seating punch line on the ogive makes a clear reference point for the length and depth of land engraves, thus showing how much metal flowed up the nose and the differences from one side to the other of the nose. On one bullet, only about half the nose length expanded to groove diameter while the other side has full-depth land marks cutting completely through the curve in the ogive. I did not shoot these XCB bullets for groups so I have no idea at this point how much the crooked start actually affects the groups, but we can assume that at anything above the speed they were actually fired, groups would begin to deteriorate very rapidly.

 

[waco]

I would have guessed the opposite to be true. Why do you figure the 6/4 alloy acted that way? The high Sn content?

 

[Ian]

It seems to me that dropping 350 fps off of the speed had virtually no effect on the drive-side wear, which makes me think the loss of paint and wear damage due to burrs in the freshly-chambered throat rather than excessive force of the 10" twist on the relatively weak alloy. A bore scope may be in my future.....or maybe some gentle fire-lapping.

 

[Ian]

I would have guessed the opposite to be true. Why do you figure the 6/4 alloy acted that way? The high Sn content?

I have a correction....after measuring a sized XCB bullet and the fired ones, the noses did not actually expand. The fired bullets measure .3082" down to the point where the un-fired ones start to drop off to .307", and follow the taper of the unfired ones forward. Sorry for the false conclusions, I'm still new to "forensics". One thing this has taught me so far is several lessons in "assumption" until a micrometer and some magnified inspection has taken place.

The XCB bullets absolutely did get crooked in the bore, though, badly so.

 

[waco]

The XCB bullets absolutely did get crooked in the bore, though, badly so.

What seating die are you using?

 

[waco]

How much run out are you getting?

 

[Ian]

Forster, for all those shown. No appreciable runout on any of them before firing. The only difference was the alloy and nose shape. This rifle has only a bevel from chamber neck to rifling for a "throat", typical modern 30-30 chamber. The XCB bullet base is going fully against one side of the chamber neck when fired, as evidenced by the loss of obturation and light lead dusting on one side of the outside of the case necks. The loaded clearance with this turned brass is somewhere around three thousandths total, which is fairly snug but insufficient to keep this straight-tapered nose from turning at a bad angle prior to entry into the barrel. The ACE bullet has a very slightly tapered, bore-riding section which fits this rifle well and happens to be just enough to keep the bullets relatively straight when fired, at least based on the uniform engraving depths of most of them.

I thought this was an interesting lesson in bullet fit and what can go wrong at the launch if you don't have the necessary mechanisms in place to ensure the bullet is supported where it needs to be. It WILL go sideways if you fail to support the bullet in the correct way, even if a tough alloy is used.

 

[Rick]

I highly recommend the bore scope for what your doing. Yeah I know, they're expensive but so much can be learned from 25x close up visual inspection. Lyman has a much less expensive video camera out but I haven't seen it myself and don't know what the clarity might be. With the Lyman pictures can be saved to your computer.

One reason for recommending this is I can't tell from your pictures but it is possible that all of the rifling doesn't start evenly, TC is famous for that, I've seen some as much as a full inch up barrel on one side where the rifling finally starts. Would be a definite crooked start with the rifling on one side pushing the nose to the other side. Just a SWAG but might be worth looking at.

Lyman Borecam 20" Digital Borescope with Monitor
Hawkeye Shooting Edition Slim Rigid Borescope Kit

Check these out and you can easily see the appeal of the Lyman. Just a matter of clarity.

 

[Will]

It looks to me like in the second group of pics the XCB bullet has a twist in the engraving marks. Am I seeing that correctly?
Looks as if the nose was trying to twist during initial engraving and the base was fighting it.

 

[popper]

how terribly crooked these bullets were becoming in the bore when fired How can you tell?

 

[Ian]

It looks to me like in the second group of pics the XCB bullet has a twist in the engraving marks. Am I seeing that correctly?
Looks as if the nose was trying to twist during initial engraving and the base was fighting it.

Possibly a skid or twist. Could it be a side-effect of the base being forced back to center as it goes into the throat, while at the same time the nose is trying to bite the lands, sort of a simultaneous roll/yaw motion?

Popper, the tapered part of the nose nose touched the grooves clear up to the ogive break on one side and didn't even scuff at the same point on the other. The lands cut full-depth off the edge of the ogive on one side and didn't even scratch the surface on the other near the ogive. You should be able to see all that in the photos.

 

[Intheshop]

How many miles(round count) on the test vehicle?

Even with a prefect leade,and dead nuts straight ammo,there are reasons for the discrepancy at the starting of land's inprint.But will save that( theory that isn't germaine,at this time),test by comparing a moderate to hard jam vs short by .020" or so.

 

[Ian]

About 60 or so rounds through the new barrel, almost all have been powder coated, gas-checked bullets. Zero copper jax. I reamed it with a virgin reamer and started shooting, no polishing the chamber, no break-in, nothing. I'm pretty well convinced there are some burrs on the land origins still, and am pretty surprised the bullets aren't getting torn up any more than they are.

 

[popper]

Those bases look weird even sans GC.
edit: the base on the 140s look like a sudden LG there, the rear band is longer (as I would expect) , front band and groove are collapsed, expected. XCB kinda looks like the same effect to a lesser extent. Possible softening of the alloy with some stretch from GC friction? Or the GC going through the throat/bore. Thought I saw a pic of one with a bulge at the GC edge but don't see it now. I shot my BO carbine, PB @ 2100 and got a circular pattern on the target, thinking the bases weren't exactly true. Then I thought port cutting (AR) - not sure which now.

 

[RicinYakima]

"Could it be a side-effect of the base being forced back to center as it goes into the throat"

Just a thought, I don't know, but reconsider this idea. If the base is not square with the bore, why would it try to square up? The more cocked it is, the more pressure is exerted against the base. Pressure is equal in all directions in a gas, so the more crooked it is, the more it wants to get crooked'er.

 

[Ian]

Ric, I'm really glad you weighed in on this. The reason for putting this out there is to get input from others who know more about it than I do.

My thought about the odd engraves on the XCB bullets was that at the end of the day, the back third of the bullet has to get into the barrel one way or the other, so if it starts out being forced hard into the slack space of one side of the chamber neck, one of two things happen: Either the side of the bullet gets raked off, or the whole bullet is guided by the throat entrance funnel back into the approximate center of the bore (path of least resistance). If the throat funnel is guiding the mis-aligned rear section of the bullet back to the center of the barrel while the nose which is presumably mis-aligned in the opposite direction) is being engaged by the rifling, the whole bullet might be twisting and yawing all the while moving forward, which might explain the odd looking engrave marks which are wider at the back than the front and appear to not perfectly match the angle of the rifling twist as the engraves on the ACE bullets do. This is just my speculation, I don't really know either, just pondering the possibilities.

The original reason for building a bullet trap that could recover HV rifle bullets relatively un-damaged was to diagnose alloy, lube, and fitment failures. I figured there would be more questions than answers arising at first, and it certainly appears to be the case; right now it's difficult to resist going off in ten different directions at once here. Since the only failure I can really find with the powder-coated ACE bullet is on the driving side of the engraves, my efforts will be focused on that for now, in this one rifle. There doesn't seem to be any error in the way the bullet fits and finds its way into the bore, although the nose damage at near 2200 fps (6' from the muzzle) may indicate a need for some tin in the alloy. The engrave damage is pointing me to the barrel itself, not excess abrasion, lube failure, or anything else really. The XCB bullet was an expedient for seeing how a different alloy would change that drive-side damage, but since the whole dynamic fit of the nose failed in this rifle's "throat", that opened up another can of worms.

Ian needs a bore scope.

 

[pokute]

I see a lot of problems here. The first is that your bullet mold has no orientation marker, so your bullets are starting out with random orientations without regard to eccentricity, and you have no reference for comparing the lands.

Another issue is that there is bulk deformation of the bullet proportional to the nose deformation that resolves as shear at a ~45 degree angle, which taints conclusions about alignment.

Also, consider that the engraving that you see is the net result of 20 inches of travel. There's no way to know where along the way the final appearance was forged, though the safest bet would be somewhere well along the way (Julian Hatcher made some amusing observations re the National Match Springfield in this respect - He discovered that the barrel was significantly constricted by the front sight band - Applied after the "star-gaging").

Have you pushed a ball down the bore and felt the lay of things? A borescope won't tell you anything about that. In fact, in my very limited experience, a borescope will mislead you more than it will help. It's about as useful as "star-gaging". Harry Pope didn't have a borescope. I made a borescope. It showed me that a badly pitted bore with badly worn rifling could shoot better than a bore as pretty as a new minted penny.

I'm wondering about the 6 foot test range. I guess you are doing this in the basement or something?

 

[M3845708Bama]

The two bullets on the right are some NOE 310-160(?) that we call the XCB bullet. They were cast from a very hard 6 Sb/4 Sn, water-quenched alloy and test at 24 bhn. Sized .3098", gas-checked, and fired with the same, reduced load of powder as the second round of the ACE 140-grain bullets.

Here I thought it was interesting to note how terribly crooked these bullets were becoming in the bore when fired. I also thought it was interesting how much more the hard, tough alloy flowed up and expanded the noses when using the same charge of powder that barely affected the other bullets cast of straight, air-cooled wheel weights. 22 grains of Reloder 7 was the propellant for both.

This close-up attempts to show how eccentric the noses are with the bore. The seating punch line on the ogive makes a clear reference point for the length and depth of land engraves, thus showing how much metal flowed up the nose and the differences from one side to the other of the nose. On one bullet, only about half the nose length expanded to groove diameter while the other side has full-depth land marks cutting completely through the curve in the ogive. I did not shoot these XCB bullets for groups so I have no idea at this point how much the crooked start actually affects the groups, but we can assume that at anything above the speed they were actually fired, groups would begin to deteriorate very rapidly.

View attachment 5840

What indication did you see that would suggest that you had alloy flow from base to tip area of bullet rather than the whole bullet being shortened? Also do you think this is the best target media or have you seen any drawbacks? Several of us have talked about milk jugs set at angle to match bullet arc.