The Science of "Leading"

[JWFilips]

I know we have some knowledgeable folks here that have a very good understanding of what takes place in milliseconds after the primer is struck on a cartridge holding a cast bullet.
I'm interested in understanding what takes place when successive cast bullets produce leading in a bore and what it is that causes accuracy to go down the drain quickly.

As a for instance: I was experimenting with down sizing some Lyman 311467 bullets ( loverin style) that were heavily coated in alox beforehand and the sized down to .291" which is the size I have been shooting 7 mm NOE bullets in my well worn 1916 Spanish Mauser. (Those bullets were gas checked & good for 1 1/2 to 2 " groups at 50 yds) The Sized down 30 cal Loverine's were plain base and and shot at about 1100 fps.

I knew deep in my mind that there wasn't enough lube left in the, now very shallow lube grooves after sizing down, but decided to experiment. The first 5 shot very well into about an 1 1/4" & then it happened ( I knew what it was because I have seen this before) Accuracy drop off due to leading!

To complete the experiment I continued to shoot groups of 5 shots and watch the deterioration..... By the 10th shot the group had opened up to about 5 inches, by the 15 shot It was up to 12", by 20 rounds I was now starting to fall off my 16" wide backer board & by 25 shots I was now about at a 2ft 50yd group! When I got the rifle home Just as I had guessed the rifle bore was fully plated with lead starting at 1/3 way up the barrel to the muzzle. The muzzle end looked like it was tinned with a soldering iron and the groove shown a bubbly & thick lead coating.
(Yes after 3 days I'm still scrubbing the lead out! but it was an interesting experiment.)

Just wondering if the "scientific minds" here can shed some light on ( not why the leading happens)
but why the progressive leading build up causes such large impact shifts?
Thanks
Jim

 

[quicksylver]

You are loosing your rifling..lead builds up and the rifling becomes shallower to the point you have no rifling..
If you want the reverse..shoot bullets sized .308 out of say a used 1903 Springfield...or soft (not hard ball) bullets out of a 30-30..or think marlin 30-30

 

[300BLK]

As the fire is lit, pressure builds, the bullet moves and case expands. If the throat isn't sealed, there will be blowby which will lead the bore. Whatever part of the bullet is below the neck isn't supported, so subject to powder gasses. The deteriorated base (and bands) aren't going to seal well, so will make the problem worse.

My suggestion for a worn bore: have a tapered design cut so as to have the base of the bullet IN the neck and the ogive out in contact with the bore. Look at the Pope designed 30-06 bullet as an example. The base should be as large as will chamber. If the throat is grossly oversize in comparison to the fired case neck ID you COULD turn necks to allow larger diameter bullets, but getting to be too much hassle for me.

 

[Glen]

It's a rough, non-uniform surface. In essence, you are turning your barrel into a rasp and roughly rasping your cast bullets down undersized. So, they don't seal properly, they don't align properly, they wobble, and there is excessive blow-by (meaning the lube gets blown out the barrel, instead of staying on the bullet), so it gets worse with every shot.

 

[popper]

Two problems with your experiment. Loverin bullets and lubed before sizing(removes lube from bands). Lead in barrel tears off bullet lead and puts more in barrel. Like my 40Sw experiment - 190gr TC single groove with BLL and near pure alloy. Rectangular holes from 7 ft. Big (relative) chunks of lead stuck in the barrel. Interesting, the chamber end was 'normal' leading, muzzle had the chunks. I thought maybe they were undersized but pulled a dummy and brass didn't size, maybe crimp did. PCd some I hope to shoot tomorrow to see if tumbling goes away. Attempting to determine if it's base deformation or 'smooth' bore causing the tumbling. just a tad harder alloy and the holes are round.

 

[JWFilips]

Glen,
Thanks for the explanation....this is what I was looking for ( and in very understandable terms). It explains why the shot impacts continue to become worse with each successive shot.
Additionally wouldn't the increase in lead build up actually increase the pressure also....adding to the erratic behavior?
Jim

 

[Brad]

The lead build up would likely increase pressure and alter velocity too. I doubt those factors cause as big a change in group size as the unbalanced bullet.
I bet that if you fired enough of them you would get to the point of tipping or full keyholes.

So, how to prevent the inital lead laydown?

 

[JWFilips]

Well Brad in my case I knew there wasn't enough lube ( or for that matter lube grooves left after that drastic resizing)
This rifle did not lead with any of the NOE bullets I had shot but then again they were Cast at the proper size for the rifle and lubed with Ben's Red and BLL overcoat.
I was just experimenting with the sized down 30 calibers because I ran out of the proper NOE .292 7 mm bullets that I had.
As soon as the 30's were resized down I knew I would have a leading issue but I wanted to see how it would affect the point of impact if I shot all 25 of them that I had and kept notes
Jim

 

[Brad]

Got any of those bullets left? I wonder if a BLL over coat would reduce, or slow, the leading?
I am more and more of the opinion that lubes don't lubricate near as much as they help seal the bore. The bullet seals most of the bore but on the trip down the barrel small gaps appear hear and there. The lube helps fill those gaps, like along the trailing edge of the lands to prevent gas cutting and blow by.

 

[quicksylver]

JW..do I see paper patching in your future?

 

[JWFilips]

Dan,
I would rather go broke and buy a new NOE 293 170 mould then try paper patching again! ( did that with my JP Sauer Mauser; No sir Didn't like it!)

Brad, Actually after sizing I did roll them lightly in BLL however because the were virtually smooth All the BLL came off on seating in the case. There was actually a ring of dried lube around the case neck when the bullet was fully seated. However they are probably a candidate for the new fangled "Powder" thing!

 

[popper]

OK, shot the PCd soft (just Pb/Cu/Zn) that I tried before. No tumbling, no leading, round holes @ 25 yds. Flat Base was NOT coated so I assume no base distortion/gas cutting. I have one of those plastic right angle light pipes I checked for leading after each of 10 shots. So far my conclusion is the leading causes tumbling, size may cause the leading. I will agree that undersized will cause tipping. Details in the alloy section.
edit: When I started PC I found that PC would not scrub out leading. A single 'tooth' rasp of lead just makes it worse. Yes, I have shot undersized (like 20:1 alloy) PCd that didn't cause leading - no accuracy. Yes I read the posts on using soft lead to remove leading - don't agree.

 

[JSH]

Some place I ran across an article a gent shot sized and as cast "neckid" bullets. Results were interesting as I recall. There was a happy medium with alloy.
I forget what speed and caliber. He did get to a point though where there was zero issues with leading. Accuracy was fair to good.

My experiences with leading, once it starts it only gets worse.
My results were on par with what Glen explained about a rasp. It may only have one tooth on that rasp but what a mess it makes. Lube or not.
Shooting the lead out, is akin to pulling on a knot in a rope, only makes it worse from my findings.
Jeff

 

[JSH]

Forgot, one other thing of interest maybe.

7-30 Waters TC barrel. It shoots full length gas check projectiles MOA easily. I tried a few CB's in it. Shot a match with it, ok but not up to my standards. Cleaned it with wipe out thinking that would help, I had not used it in this barrel up until that time. Took it to the range, fired a few at 50, about the same. Went to 100 and it got worse. Leading.
Home again and cleaned. Not quite a sewer pipe but it had issues. Did some measuring and went easy with the lapping. Shot CB's again, still leading.
It had some pits or inclusions about half way down the bore.
Ran several hundred jacketed through it.
No more wipe out, just a patch and some JB bore paste.
Slicked it up nice and went back to cast. Accuracy came around to acceptable standards and won a couple of matches with cast.

I guess the gilding metal acted as a "pot hole filler". Have not shot it for a few years. If that gilding metal has curled it will be game over.

All I cleaned it with was a quick lite swipe of Kroil, then a dry patch and a swipe or two of JB and then a clean patch. I didn't want the Kroil to get under that gilding metal.
May be worth a try.

 

[fiver]

take 2 of your ingots and rub the corner of one across the other.
do it with a couple of different alloy mixes.

when you fire the bullet you change it's alloy.
when you have lead laid down in the barrel you get that same rubbing resistance you got from rubbing the two ingots together, only it's being rubbed at about 100 times the force you apply by hand.
you lose the diameter, and of course the rifling isn't gripping the bullet any more.
you lost accuracy because you lost stability.

 

[KHornet]

Jim, I would be inclined to forget going down from 308, and be more inclined to go
up from .285 by beagleing. You have nothing to loose by trying same, and you will
in all probability be able to get up to .291 or better, with ample lube capacity. Beagling
works well in some rifles.

Paul

 

[JWFilips]

Thanks Paul for your intuitive thoughts. I'm sure you are correct. But the Old Girl has spent a week of De-leading and now is put away until a time when I can get a proper mould!

 

[popper]

ring of dried lube around the case neck Something else wrong, I BLL only, 2x on slick sided rifle boolits and no problem. You turned the louvrin into a Lee TL, should have been enough drive band area to hold the grooves. I'm thinking my 40sw experiment is like yours - gas cutting and lube gets blown off. I assume a relatively soft alloy for 1100 fps. Sounds like a perfect case for PC TL.

 

[fiver]

that's why I started adding bees wax to my 45/45/10 lube.

 

[Pistolero]

I believe (have not proven it, so just a theory) that if you fill the lube grooves fully
with lube in their .30 cal size and THEN size down, you will keep your lube groove depth
good because of the hydraulic effect forcing the grooves to maintain their volumn,
which would force the bottom of the groove to size down the same as the outside of
the groove.

Not responsive to your question, but possibly helpful for the underlying problem of coming
up with .291 diam bullets.

Bill