Does barrel leading occur with cast bullets only above a certain velocity, or can it occur at any velocity? Are there factors other than velocity coming into play in barrel leading? I am thinking primarily of shooting cast lead in the .45-70 using Unique for reduced/cowboy loads in the 500 to 1500 feet per second range.

Does barrel leading occur with cast bullets only above a certain velocity, or can it occur at any velocity? Are there factors other than velocity coming into play in barrel leading? I am thinking primarily of shooting cast lead in the .45-70 using Unique for reduced/cowboy loads in the 500 to 1500 feet per second range.

It depends. Leading can occur in the bore for a number of reasons. I suspect bullets from lead that is too soft or driven too fast are the most likely causes. The condition of the rifling can also be a factor. A fairly hard cast bullet driven at the velocities cited should not be a problem. All the best...
Gil

Are several types of leading. IF a bullet is smaller than the bore of the rifle, and it's being used in a low pressure loading that can't "bump it up", then the gas can erode around the sides...vaproize lead as it erodes...and deposit that lead. This kind of leading is usaully at the BREECH end of the rifle.

If you use a soft alloy and try to push it too fast, then will usually get leading more towards the MUZZLE end of the barrel....bulelt going faster towards the muzzle end, so is more lilely to strip.

If the lube isn't doing it''s job or the alloy is very very soft and being pushed way too hard, can get leading that just apears through-out the barrel...usually as streaks, but sometimes it looks more like lead strips.

So...velocity isn't the only leading factor...bullet size, bullet alloy, pressure of the load, condition of the barrel...all play a part.

First off, if you're going to load the large 45-70 cartridge with light loads, I'd suggest using Trailboss instead of Unique.

Most often leading is caused by gas blowing past the bullet in the bore...shooting too small of a bullet diameter. The hot combustion gases melt the lead and cause it to stick to the bore. Leading can also be caused by a rough bore as in one that is not polished smooth enough. Velocity will also play a roll but generally, you won't start seeing vecolity related fouling until you get above 2000 fps. Besides bullets that are not sized properly to the bore, the next leading cause of leading is either lack of lubricant or lack of quality in the lube.

You should slug the bore to ensure you use the proper size bullet, in cast you want the bullet to be .001" to .002" larger than the bore to ensure proper sealing wihtout creating pressure problems. While slugging the bore, check for tight/rough spots which will need to be removed by lapping then re-slugging the bore to check size again.

Once you have the right bullet diameter determined, you need to ensure your mold is throwing right size bullet. I've seen people complain about leading on XYZ bullet only to find that their mold is casting a bullet smaller than the sizer die they were using...they never verified the actual diameter of the bullets.

If you get a mold that is casting a little too small, you can lapp the mold as well.

I never messed with smokeless in the 45-70, I use blackpowder only in mine. You need a soft alloy for BP and I would suggest not getting too hard an alloy with light smokeless loads either simply because of the lower pressures you'll be working with. By cowboy shooting I assume you mean CAS comp which will be relatively close range stuff meaning that you can probably get enough accuracy from a lighter bullet in the 300-350gr range to suffice your needs. Putting distance on the 45-70, you definitely want a large bullet in the 405-550gr range using BP anyway.

You want a good quality lube too. I make my own lubes, one for smokeless loads and one for BP loads. Most commercial lubes have gone to the hard side making for much cleaner loading but not performing all that well. Most will suffice to a point but none have tickled my fancy enough to mention. In addition to the bullet lube itself, you need to get all the petroleum oil out of the bore before shooting cast bullets. I also have my own all natural bore lube blend I use which came about from my muzzleloader shooting but has proven high effective with cast bullets too.

As for alloy, I use a lot of strait wheel weight alloy even pushing velocities to 2200 fps w/o leading problems but I use either gas checks in BN cases or wads in strait wall cases when going over 1600 fps.

at the Lyman site they say lube can be the biggest cause of leading, I shoot 45acp at 800fps and 9mm 125gr at 1000 fps and have used alox tumble type and Lyman orange and have never had a problem, I will shoot 125 to 150 rounds in a match

I am not completely familiar with all aspects of Cowboy Action shooting, but from what I know, you don't use a .45-70 at very close ranges.
The type (or weight) of bullet that you select will depend on the type of shooting and distance that you are expecting to be faced with. Generally, a very short and light bullet would not be the choice if you are shooting at a couple of hundred metres or further.
Also, you really don't want to be thinking about shooting this rifle at velocities down at the 500 fps level! There will be enough problems getting consistent ignition in the large case with light bullets, and the potential for sticking a bullet at that low speed. It is also, in my opinion, far too slow for longer range shooting.
You could use Unique to make good loads that shoot at around 1100 fps, using any of the 300-405 grain cast bullets that are available as bullet moulds or from a variety of commercial casting sources. Any loading manual, or the Lyman Cast Bullet Handbook, can give you charges for Unique. A 405 grain cast bullet at 1100 fps is a pleasant and accurate load, at least at the 50 and 100 metre distances that I have tried them at.
Consider using a card wad at the base of the bullet, if you are using a plain base one. Not pushed down on the powder, but right under the base of the bullet to protect it a bit from the powder.
Good luck.