Accuracy and the trailing edge - base erosion?

[PWS]

DrB's thread on accuracy and the trailing edge got me thinking...

Regarding the three bullets below, "H", "I", and "J", they are RCBS30-150, waterdropped range scrap at 20bhn, sized .310" in a Lyman sizer, and fired in a RugerMkII M77 .30-06.

Starting at the right, "J" was loaded over 11gr Herco/CCI200. As you can see in the photo, the edge of the base has been eroded but not to the point where it has cut across the width of the drive band. Immediately after firing, the bore appears to have a "dust" or light leading but interestingly enough, the dust quickly dissipates, leaving the bore shiney clean. The fired bullet measures .309 so I don't think the erosion is a case of being undersized. Perhaps it's a matter of hardness and that's the next item to investigate (both harder and softer). 5, 5 shot groups went into 1-2" at 50yards - not so good.

Bullet "I" was fired over 15grSR4759/CCI200 and shows a similar degree of erosion at the edge of the base but a noticable amount of land stripping. One five shot group was tried and it went 1 1/2" at 50 and the bore remained silky clean and NO "dust" evident. Not sure why?

Bullet "H" was inspired by Molly's suggestion that one way to completely seal a bullet from powder gas was by loading over granulated filler held under the bullet base with a tuft of fiber. This was loaded over 25gr SR4759/CCI200 and five rounds went into 2" at 50 with only very light leading at the muzzle. As you can see, the base is virtually pristine while the body of the slug is severly stripped - so much so that it's acutally .301" diameter and only the driving side of the land remains evident!!



http://castboolits.gunloads.com/imagehosting/166284f4d326687e55.jpg

http://castboolits.gunloads.com/imagehosting/166284f4d3266c9909.jpg

[PWS]

Now here's where the "base erosion" question gets strange. The bullets below are from the MOST accurate cast bullet load I've come up with. It's a 200gr LBT cut for the rifle (Ruger '06) loaded over 10gr Herco/CCI200, NO gascheck and only lubed with Lyman Moly on the check shank. This load is very clean and will print into 1/4" at 50yards when cast of linotype (and I'm in top form!)

When cast of water dropped range scrap at 20bhn, accuracy drops to 1/2-3/4" at 50yds and SOME of the bullets show erosion.

It's difficult to see in the picture but bullets A, B & C have "washout" on the very base but NONE on the check shank or bottom driving band. The base of bullet D is completely unmolested.



http://castboolits.gunloads.com/imagehosting/166284f4d32250beb8.jpg

[PWS]

And now, here's where the "base erosion" question gets REALLY strange. The bullets below are the same as above but include one 31141 (bullet F). Bullet's E&F show erosion channels on the gas check SHANK and up into the "trailing edge" of the first base band? Bullet G shows no erosion on the check shank and the trailing edge of the first base band remains intact but the surface of the band has eroded between engraving!

http://castboolits.gunloads.com/imagehosting/166284f4d32255eade.jpg

[PWS]

And finally, gas checks don't always check gas!! (20bhn LBT30-200 over 35gr H4895/CCI200).

http://castboolits.gunloads.com/imagehosting/166284f4d3225bcab2.jpg

[popper]

What is the fps of the RCBS309-150 CB? Nose flattened and bands stripped, must be much higher fps. Drive then fast without GC and they lose accuracy. Your filler did it's job but the fps was too great for the alloy. I looks like it has 2 adjacent grooves. How they do that? You size butt first?

[Bret4207]

Okay, in post #2 the bases that appeared washed out to you, take a close look at a virgin base under a glass and poke at it a bit with a scribe or something. I bet you find a porous area under the sprue. I've seen the same "washing" and always felt it was the porous area collapsing under the pressure. It's never uniform, just like the porous area under the sprue.

Just my theory.

[PWS]

Okay, in post #2 the bases that appeared washed out to you, take a close look at a virgin base under a glass and poke at it a bit with a scribe or something. I bet you find a porous area under the sprue. I've seen the same "washing" and always felt it was the porous area collapsing under the pressure. It's never uniform, just like the porous area under the sprue.

Just my theory.

Hmmm... I have had a lot of trouble in the past with pin-hole bubbles (too heavy of a bottom pour stream/poor venting) but never thought of a porous area. I'll snoop around a bit with a probe.

The weather was decent yesterday (cold but calm) so I got in another session and tested rounds with 12 and 13 grains of Herco under the "H", "I", & "J" plain based bullets. The "dust" in the bore was reduced with each increase and at 13 grains, the bore was just about mirror bright. Accuracy didn't seem to improve much however. Velocity is getting upwards of 1500fps with the 13gr charge and that was just enough more umph to scatter the bullets beyond recovery in the snow so I don't know how the base edge fared.

If it were a pistol and the bullets showed that base edge erosion, I'da said they were too hard and weren't upsetting. Perhaps this is true in the rifle throat? The decreasing fouling with increased powder charge points in that direction but the as-sized diameter of .310" makes me wonder about lack of a seal.

I'm still thoroughly baffled by the cause of erosion on the check shank and on the driving band between lands (bullets E,F&G). Erosion around the base edge or up an engraving mark make sense - cutting by gas flow - but it would seem that the gap on the check shank would dissipate gas flow a little and especially at the base? Some bullets have dark bases and the ones with "washout" are shiney. Still working on it...

[Bret4207]

One of the GC shanks had "errosion" under where the GC would be. Something else in play here.

[runfiverun]

the gas cutting [imo] is happening in the throat.
the gas pressure is about at it's peak [especially with a faster powder] and there is an open area there before the boolit has fully entered the bbl.
your case necks could be shorter than the chambered [neck cut] area also.[even if you don't have a larger throat]
we measure everything and sweat,and ponder loads and alloys and squareness and then cut the cases to the minimum book length or to the shortest of the batch instead of to the chamber.
we then hit it with 40k + worth of pressure right at it's most vulnerable moment.

[JeffinNZ]

I can't help but think your alloy is unnecessarily hard for the load applications. 10 BHN would cope with those loads and seal the bore better. Also, maybe .310 is too small for your throat. Try .311 sizing.

[beagle]

Gas errosion happens to all bullets. It's just a matter of keeping it to a minimum with proper bullet fit and hardness and seating depth.

At the instant a bullet leaves the case, there is "blow by" of gas pressure for a few micro seconds before the bullet takes the lands, pauses and bumps up or seals. This is one reason that us casters seat our bullets out to "kiss the lands". THis minimizes this process by a few microseconds.

Again, I have very strong feelings that a super hard bullet is not the way to go. I'd rather use them a little soft and get a good bump and seal.

This errosion is not common to cast bullets but exists in our jacketed brothers as well. I have picked up freshly fired .50 slugs from the old M2 downrange and they all showed signs of errosion around the body in front of the boat tail heel.

I once fired a bunch of #316 Saecos (Gc'd) at 100 yard steel plates. When cleaning op targets, I recovered a bunch of fired GCs from the plates. These had expanded to the size of a dime and as this was a Marlin, the micro-groove marks were clearly visible as was a blackened area where the corner of the base of the bullet was so it was happening there as well but the gas check minimized the affects on the bullet base.

Now, this is with rifles. Revolvers are a different matter and take a beating twice. Once when bumping up to fit the chamber and again on the jump between the chamber mouth and forcing cone. Again, I prefer softer bullets for this application instead of hard bullets.

You're not going to get away from this phenomonen. You can minimize it with a GC or a filler or by using the right size and alloy bullet that will form to seal the bore and minimize bullet jump to the lands but it will still be with you./beagle

[1Shirt]

Yep, as Beagle said, "It will still be with you". This is why I believe in a filler (dac) for cast rifle loads, and seating very close, but not into lands with both GC and PB blts. If it happens with jacketed which are made with high pressure, and very accurate machinery, it will definately happen with cast blts being made by God only know how many casters. To many variables!
1Shirt!:coffee:

[Larry Gibson]

What kind of GCs were on those?

Larry Gibson

[PWS]

First off, thanks everyone for the input and bearing with me. Second, sorry for throwing so many factors into these posts.


Larry, your question regarding gas checks is at the heart of the conundrum. Obviously, the plain based bullets "H", "I", & "J" did not have gas checks but bullets "E", "F" & "G" also did NOT have gas checks installed. The bullet is setup for a check but instead of a copper cup, they were fitted with a "wax check" on the shank and lubricated nowhere else.

It seems strange that there's erosion up the side of the check shank and not beyond into the "trailing edge" at the base of the first driving band (where the gas appears to be fully "checked"). Apparently, there's considerable gas flow around the check shank and not just relatively static pressure. Also, on the base of bullets "A", "B", & "C", it appears that the very base itself has areas of gas "wash out", again, as if there's a considerable amount of gas flow across the base and not just perpendicular to it. Meanwhile, bullet "D" sneaks through with the very same load and ends up with narry a tell tale mark of erosion?!

The photos of bullets with erosion at the leading edge of gas checked bullets give difinitive proof that even a copper check does not always prevent erosion so it's not unexpected that "wax check" is also, strictly speaking, a misnomer. What is strange is where the erosion is occuring.

It makes sense that when it happens, erosion occurs in the throat with the transitional gaps. It also makes sense that the rate of conflagration of a fast powder probably offers no self buffering like a larger charge of slow powder. It also makes sense that a harder bullet would be more likely to allow blowby than a softer slug that upsets and seals.

I may be wrong but what I think is that the best accuracy, with a plain based bullets in particular, is found with the load that best limits bullet distortion whether by gas erosion and/or plastic deformation. While a slower powder may be easier on the slug with lower pressures and a self buffering effect, the burning characteristics of slow powders are not particularly well suited for the reduced loads needed to match a typical cast bullet's strength. Quick powders behave much better in lighter loads but may be too erosive on the trailing edge - even if it's not the actual trailing edge as evidenced by the check shank erosion. Finding a balance between a bullet's strengths and the forces kicking on it's way is not always easy or even possible...?

[Larry Gibson]

And finally, gas checks don't always check gas!! (20bhn LBT30-200 over 35gr H4895/CCI200).

http://castboolits.gunloads.com/imagehosting/166284f4d3225bcab2.jpg

Larry, your question regarding gas checks is at the heart of the conundrum. Obviously, the plain based bullets "H", "I", & "J" did not have gas checks but bullets "E", "F" & "G" also did NOT have gas checks installed. The bullet is setup for a check but instead of a copper cup, they were fitted with a "wax check" on the shank and lubricated nowhere else.

I mistook your post #4 to indicat that some kind of regular GCs were used, my mistake:cry:.

Larry Gibson

[uscra112]

How much of that damage is happening in the first 1/2" or so after the boolit leaves the muzzle?

[PWS]

How much of that damage is happening in the first 1/2" or so after the boolit leaves the muzzle?

Tough to say. I doubt much bullet erosion occurs out of the barrel as these are light loads of pistol powder.

I have noticed, however, there is one particular groove that seems to lead the first and most of any if it occurs. It's in the end toward the muzzle, about 1/3 of the way in. Might be a loose spot???

[PWS]

http://castboolits.gunloads.com/imagehosting/166284f58fa5fccbed.jpg


Softer bullets didn't help the basal erosion. These slugs were 8bhn and with the same load show even more erosion. Accuracy levels remained the same however, right about 1" at 50yds.

Will try a small wax wafer under the slug next.

[PWS]

http://castboolits.gunloads.com/imagehosting/166284f5d00051e05f.jpg

A wax wad helped with basal erosion but did not eliminate it. Accuracy levels acutally went down, probably due to the uneven release of the wax wad from the bullet.

[35remington]

Try the 4759 load with dacron that fill the space from powder to bullet and report back with pictures. If concerned about erosion, this will help to a considerable degree.

Before and after photos are quite illuminating, per my own results.

Naked bullets, as beagle mentioned, will suffer some degree of base erosion no matter what. Being soft or hard doesn't cure anything, really, save having a soft bullet with a quick charge of fast powder. Too much pressure with the same fast powder will start gascutting again with the same soft bullet. As will low pressure loads with some slower powders that are not so voluminous that a buffering effect is present.

Very good discussion though.

I too have wondered if the link between accuracy and gas cutting starts to go south when the gas cutting is of such a degree that the bullet becomes nearly boattailed. Advanced gas cutting of the bullet base is almost always uneven, and that would seem to allow tipping from uneven escape of powder gasses as the bullet exits.

I am sure that is not the whole reason, but maybe part of it.

[303Guy]

Might I suggest trying a fibrous filler like wheat bran or wheat germ. It seems to form a sealing plug behind the boolit protecting it from gas cutting. Only one way to find out. [smilie=1:

[PWS]

Been doing some more shootin' and thinkin'...

There's no way the erosion is the result of abrasion/galling/obturation as the bases are always smaller than the bore.

Tried the suggestion to use a granular filler to eliminate base erosion and it works as suggested. Except for minor surface impactions of the filler, bullet bases are pristine - just like they should be for top accuracy. However, accuracy wasn't all that good. Perhaps the filler blows away unevenly at the muzzle?

So far, one particular load has stepped ahead of all the rest and prints an 1/4-12"@50yards if I do my part whereas most run in the 1/2-1". My guess is that the load is "balanced" and the trailing edge does not erode. (Hard to confirm without finding those particular slugs and the snowbank is getting pretty saturated with spend projectiles).

After giving it some thought, it occurs to me that

1. A charge of ~10grs pistol powder in an 06' case must not only evolve rather quickly into gas but with all the space and possible arrangements of powder, air, and primer flash, it probably does so with much random turbulence. This would explain the across the surface of the base erosion (such as bullets A, B, and C - post on 2/28) and might also help to explain the erosion up the sides of the check shanks (bullets E&F - also 2/28).

2. In addition to the sudden and turbulent swell of pressure, perhaps some of the primer flash is also hitting the bullet base?

3. While the bullet may not be exposed to the hot powder gasses for very long, a light charge of quick powder could be the worst scenario for potential melting of bullet base because there is no self buffering effect of a large charge of slow powder and the bullet is exposed to the flame for a much longer period of time than a full velocity load. Also, the typical location of erosion is the very corner and geometrically, the easiest place to melt first. So perhaps erosion in these loads is not only the result of gas particle flow erosion but also plain old overheating and melt flow?

Bore condition after the shot is typically clean but with a mid bore "dust" like a condensed vapor and nothing like a scaly leading. Perhaps these are unadhered lead droplets that once comprised the material at the areas of erosion?

4. As has been suggested, peak erosion probably occurs as the base transitions from case mouth to bore where the base is unsupported and subject to gas flow from any and all sides and is still at a high temperature and pressure.

Perhaps this is where the proper balance of gas pressure and heat, and bullet size and hardness must achieved to maintain an intact trailing edge... I guess?!?

[303Guy]

In all honesty I do not see any possibility of any flame induced melting of any part of the boolit. Gas erosion is not flame melting or cutting, it is high velocity gas molecules striking the metal surface and taking bits of it with it. Those bits are now molten. Cotton filler comes out the muzzle not even scorched most of the time (the disclaimer here is that cotton can be ignited and smoulder but is actually quite hard to do). Even polyester often comes out with no sign of melting and that has a low melting point. High velocity gas molecules are achieved when hot gas under high pressure is forced into tiny gaps between boolit and bore. Remember that hot gas has high velocity molecules in it. Ad that to high pressure and they are very fast. Another source of skin heating of a bullet is friction. A jacket can discolour from heat. Lead does not seem to get heated by friction much. Well it does but that manifests as leading or 'galling' of the boolit.

[runfiverun]

nice explanation there peter.
gas erosion is the issue.
you got lands and such there, making cuts and pathways.
lube is the seal that stops this from happening.

jumping the gap [from the case to the bbl] is a vulnerable area for the base too.
the boolit itself is the seal in the throat gap.

having a base that's banged by powder is unavoidable, the gas check is the solution.
muzzle pressure needs to be low to release the boolit gently.

[303Guy]

Thanks. My question is how to get the boolit into motion with a gentle launch and a gentle muzzle exit at the same time. A long barrel is one way but the ideal powder is what I'm looking for. Tall order I think. Has anyone tried venting a barrel some distance from the muzzle? Gentle launch and gentle exit. Mmmm.... I might have to go out and buy another Brit to try this out with (I have a candidate but the bore is counter tapered from the middle). Think there might be merit in that idea?

I have a carbine that I would like to suppress without lengthening the gun. This could be the way. No filler, slower ball powder and vented barrel into suppressor body with about an inch overhang. Thoughts?

For you folks with suppressor registering hassles, a simple shroud or gas deflector directing the gasses forward would work.

[PWS]

Cotton filler comes out the muzzle not even scorched most of the time (the disclaimer here is that cotton can be ignited and smoulder but is actually quite hard to do). Even polyester often comes out with no sign of melting and that has a low melting point.

You're right. :oops:

I've seen this lack of filler burn many times myself. Dunno what I was thinking...

It is amazing how much erosion can occur though. Come to think of it, a pierce primer will etch a bolt face and that's a LOT harder and higher melting temp than lead.

[runfiverun]

it will but think of the size of the hole plus the primer material hitting it.
i have seen pierced primers but rarely all of the material that come from the hole.

if i could remember where i read it, bbls have been vented almost all the way to the breach to drop gas pressure at the muzzle.
something like a muzzle break on a 50 cal, and a bit of false bbl after could turn the trick.
in my 2 308's i am not sure if it's bbl length or the break of this style on the end of it, that makes the exact same loads shoot better in the one or if it's the whole rifle.
but muzzle pressure definately plays a part.
you could port the bbl into the can and baffle around the bbl without adding too much length.
a couple of inches would be necessary though.
you could also use a slower powder and not get it up to optimum burn pressure which will change the sharp crack into more of a boom as the unburnt powder sucks in oxygen in the open air.

[geargnasher]

Good thread, PWS, I subscribed to it so I wouldn't forget to check back and read everything in detail when I had time. Excellent pics of your recovered boolits BTW, a good contribution to the discussion.

I get confused by the term "trailing edge" here, in my mind I think of the trailing edge of the rifling engrave failing due to slight skidding and opening up a gas channel that leaks and leads the bore. This seems to occur at high velocity an has its own implications. Base edge failure (also a trailing edge) has been battled since Job was a pup. I think gas checks work more as an erosion-proof piece of armor than a true gas seal and assist accuracy by providing a clean, square muzzle exit. Try shooting a gas-checked boolit at 2K fps with no lube and you'll see just how much of a sealing job your lube is doing, and how little the check is doing. Annealing gas checks can improve lower-velocity accuracy, I think by conforming to the rifling better and perhaps reducing gas erosion somewhat. Gas checks also make up for the collapse of hidden base voids, or prevent it unless they are huge.

I tried a wide variety of things in my worn 336 Marlin .30-30 to allow me to shoot un-checked plinking loads (12-1400 fps), but I kept getting bad throat leading. A gas check stopped the leading, IF it was annealed. What also worked was a PAPER gas check cut from a piece of common manilla file folder material and formed with my Lyman lubesizer, .311" die, and .30 caliber top punch while using the gas-check seating stop to block the ejector so it could be used as an anvil. I used spray adhesive to glue them on the boolits, base-first sized/lubed like normal and proceeded to push them to 1700 fps with good accuracy and NO leading. The accuracy began to drop off at about 1600 fps, but I kept going just to see how they would hold up. The muzzle blast was full of microscopic fibers, not a trace of the "check" was large enough to be found. I suspect the check turned inside out while passing through the throat and sealed the gases just long enough to get the boolit engraved.

The effective transition from case to barrel has long been the biggest challenge for me with cast boolits, and one of the biggest drawbacks of fixed ammunition. Breech seating makes all manner of things possible by eliminating this challenge. When I think of how I want to launch a cast boolit, I think of how in the world will I get it into the barrel without gas-cutting, slumping, riveting, skidding, wadding up, or getting scraped or pushed at an angle. All we can do really is just damage control, or try to make it so that what damage is inevitable is uniform. Compacting fillers, proper boolit/throat fit, seating to engrave, use of slow ball powders, and low brisance primers all have an effect as well as good chamber neck fit with the case, proper case trim length (just enough to be safe, about .005" short of the chamber at most with fireformed brass), inverted gas checks, proper chamber design with a step at the end of the brass so the boolit doesn't have to leap across the Grand Canyon before entering the bore, etc. are all ways of minimizing the trauma to the boolit at launch. Whether or not you have been successuful will be very evident on your targets.

Gear

[303Guy]

I get confused by the term "trailing edge" ...Good point Gear. Perhaps we should attemp to define 'trailing edge'. I think of it as the bit of boolit where the shank ends and the base begins.

http://i388.photobucket.com/albums/oo327/303Guy/FIRE-POLISHING002.jpg

In this one it's where the tiny rebate begins. But also shown is a gas erosion groove. Is this trailing edge failure? it begins at the trailing edge and forward. Distortion to the 'trailing edge' is to me the unevenness of the trailing edge as well as 'feathering' of the trailing edge.

http://i388.photobucket.com/albums/oo327/303Guy/256grImoldAUTOSOL003.jpg

This one shows the base being not square to the shank. That would be trailing edge failure to me too. There is more drag on one half of the boolit than the other resulting in more feathering on one side and 'dragging the base out of square.

[runfiverun]

to me it is where the boolit goes 90*.
trailing means following here.
so anything drug behind the 90 is trailing the edge.

i think in your second picture that that may not be gas cutting but a collapsed void.
allowing gas to flow into that area. and push it into the boolit.
the edge is clearly eroded but i am not sure it was compromised otherwise you would have seen little lead globules deposited somewhere in the bbl's first few inches.

[popper]

Postulate that a non-square base creates radial force on the CB and could cause work softening of the base. The GC, being harder, won't allow this work softening. Softer side of a CB distorts more and is easier to gas cut.

[303Guy]

Softer side of a CB distorts more and is easier to gas cut.Uneven boolit base hardness! Mmm ....
That hole is a surface inclusion and was there before firing. I do however have a few examples of collapsed voids. I had a void collapse in front of my eyes forming a dent. On firing the void was really collapsed. This alloy is fairly strong yet seems very ductile. I wonder what's in it.

That top boolit - would the dragging of the rifling impressions be an issue being as they are forward of the rebate base edge? That's the idea of the rebate to give the alloy somewhere to go without distorting the base.

[runfiverun]

well you can see it's distorting the squareness of the base.
i'd bet this is happening fairly soon in the pressure and the softened lead has to be getting worked over by it.
and not giving a good release from the bbl either.

that is breaking one of my big rules for accuracy.
protect the boolit at all times from casting to the muzzle.
that is the most important one b.t.w.
and it don't matter how you do it.