I too am working on developing CB loads for my .375 Big Bore. I'd like to share a few thoughts about what I've found out so far...
I don't know you or how much shooting you've done with lever guns so if my comments seem elementry, please forgive me.
I've found that shooting lever guns off the bench for accuracy requires different techniques than shooting bolt guns. For starters - I've had best luck when using a sand bag front rest that is not packed to the point that it is hard. The guns are usually light weight and recoil off the harder rests in an inconsistent way for me. The softer bag really helps.
I've also found that lever guns tend to respond better to having the front rest placed as close to the receiver as possible. This position seems to be much better than further out on the forend or on the barrel.
Guns that recoil seem to respond better to being held onto tightly and pulled back into my shoulder firmly rather than to use a loose hold.
For accuracy shooting - using a bead sight is a serious challenge. I have experimented with a variety of different target shapes to try and find something that allows me to achieve a consistant sight picture. Obviously - your gun can only shoot as well as you can consistantly hold on the same point of aim. To date - I have not found many targets that allow me to achieve a consistant hold. The best so far is a large triangle oriented point down. The point is positioned above a very wide horizontal bar. The bar needs to be about an inch thinck at 50 yds. to be seen well. It should also extend a couple inches either side of the widest part of the triangle. The effect of this target in sighting is that the triangle allows you to locate a consistant hold from side to side. The horizontal bar is used to find a consistant vertical aim point.
All of the above is just some of what I've learnded about how to get the most out of my own shooting performance. I'm not hoppy with my results yet but I'm getting on the order of 1.25"H x 1.5"V at 50 yds. for 5 shot groups averaging around 1750fps. This with a 300gr. cast bullet, a tang mounted peep sight and the bead front.
As for CB's in the gun and leading - I am still trying to overcome this problem.
First some background. My bore sluggs .377" I have a custom bullet mould that runs .379", 300 gr. and is a gas check design. I made some measurements and found that there is considerable freebore for this chambering. My bullet needs to travel .270" before it reaches the rifling. Way too far to seat it out and even be able to chamber it let alone get it to feed through the magazine. I choose to load to max OAL which is also where the crimp groove is. The alloy I'm using is air cooled WW + 2% Sn. Lube is a mix of LBT blue and Darr formula. I use this lube in other calibers and find it sufficient to much higher velocities than what I'll be able to get out of the .375 Win. so am confident it is not the problem. Regarding hardness of the alloy - mine runs about 12.5BHN. This has performed well in my .45-70's at velocities to 1900fps and in my .348 Win. to velocities of 2200fps with no leading.
Initially I was getting extremely heavy leading. I used one of the lead remover kits for handguns where you pull a brass screen through the bore that is fitted to an expandible rubber mandrell... huge slivers of lead after only 5 shots.
At the recommendation of others on this forum and on the CB-L forum I then cleaned the bore - REALLY CLEAN. It helped some but I still got significant leading.
Some other gentlemen suggested that the bore for a CB gun actually needs to become seasoned - somewhat in the same way that a cast iron pan gets seasoned with use. A product was suggested that could be used to pre-condition the bore before shooting and between shots. The elixer is an automotive rust preventative material sold by the Eastwood company (www.eastwoodco.com
) Part# 16017ZP. I got some and tried it. It has definitely helped. Now rather than needing to use a brass screen pulled through the bore. The leading is reduced and pushes out with a tight fitting patch.
I am still working to eliminate the problem completely. After giving the leading problem some further thought I have a suspicion that I may need to move to harder alloy. The reason I think so is because of that long freebore combined with a very fast 1:12 rifling twist. My thought is that the bullet already gets a good running start (.270") under a good deal of pressure before it first encounters rifling. When it does get there - the rifling twist is quite tight. I'm thinking it may be too much stress on the alloy and that it may be slightly stripping as it first enters the rifling. I have the heaviest leading in the area where the rifling starts and 4" to 6" in front of it. This would usually be associated with gas cutting at the bullets base but having a good bullet fit and a gas check - this simply should not be the case. That's why I'm thinking that my alloy may not be hard enough.
Once I run the next set of experiments with the harder alloy - I'll enter a post to let you know how it's going.
Good luck in your efforts - keep us posted on the results!