The use of wads and fillers to hold the powder back against the primer has resulted in the ringing of numerous chambers, especially those of straight walled cases such as the 45-70. Ringing is a radial enlargement of the chamber, usually occurring at the base of the bullet in a cylindrical portion of the chamber, i.e., in the body of the chamber or in the neck of the chamber but, as far as I know, not in the transition of the two. It can happen either suddenly (one shot) or gradually over a series of shots.
This phenomena was first recognized (to my knowledge) in the early seventies by serious cast bullet shooters. The word has been long getting out but, although infrequent, the ringing has ruined many a chamber. The NRA and the Cast Bullet Association have for quite some time recommended that no fillers or wads be used in any loads where they are positioned against the powder so as to leave an air space between the wad/filler and the bullet base.
The reason for ringing has not been established, but some hold that the wad or filler is propelled forward and when it strikes the base of the bullet, the bullet acts as a secondary projectile and rings the chamber (in the same manner that a bulged barrel is likely to occur if a bullet is lodged in the barrel and another is fired behind it).
Three methods are commonly used to avoid the above discussed problems of powder position sensitivity and chamber ringing.
Wads that stay up against the bullet base do not pose the ringing hazard described. Polyethylene (HDPE and LDPE) wads are commonly used in place of gas checks to protect and seal bullet bases. Wax wads of all sorts are used to do the same.
I have found that there are ways to avoid the need for fillers, or use them in ways that are not risky. The old cornmeal fill (filling the case to bullet base with appropriately reduced powder charges, if warranted) seems to work ok. So has super grex, used in nearly the same way, and is my preferred method as it adds practically no weight to the ejecta while at the same time completely filling the case. I add enough super grex to fill the case to its mouth as it compresses easily when the bullet is seated. A CAUTION HERE - super grex is not to be used with ultra-lite loads. I have had it melt into a plug and stick in the barrel of my Lee-Enfield .303. The bullets exited ok but the plug of super grex was difficult to remove. With moderate pressure or better loads it works very well.
Another approach is to fill the case with some of the very slow surplus powders that have sold for about $2.00/lb. I use MR- 8600 and MR-8700 this way, sometimes with an igniter load under it to make for cleaner burning depending upon the cartridge and the gun in which it is fired. Used alone, these powders can leave some unburned powder in the barrel, but has not caused any sort of problem. Accuracy does not change if I blow the barrel clean between shots. One of the better aspects of using the very slow powders is that they seem to be gentle to cast bullet bases. My use of gas checks and wads, even in the higher velocity loads, has dropped dramatically since I started using the very slow powders with cast bullets.
I have used this approach for about seven different cartridges for inexpensive shooting fun. A .308 becomes a .30-30, but tin cans, paper, and such do not know the difference - and recoil is low.
A rec.guns reader wrote:
-- query, is the dacron tuft I place above the powder working the same as your 'super grex'. The tuft is large enough that the bullet presses it into the case on seating, and when the case is shaken, the powder does not appear to be moving. I have used this practice for 25 years without incident.The THEORY is that the filler acts as the primary projectile, hence the bullet becomes the secondary projectile. To the best of my knowledge, no chamber ringing has occurred with fillers that do not allow an air space between powder and the filler (such as corn meal or super grex filled cases). If the space between your powder and bullet base is small enough that the Dacron is tight against the powder, perhaps it is not needed - try this load without a filler.
If protection of the bullet base is needed, you might try either the wax wads (CFVentures makes hard wax sheets) or HDPE or LDPE wads that can be made out of cottage cheese containers and lids, milk cartons, etc. Make sure that the wad is cut to such a diameter that it will not move away from the bullet base during handling or the situation might be worse than your 25 year old practice of using Dacron. Sometimes two wads work better than one. Incidentally, I too have used thousands of Dacron tufts without incident but have taken the CBA and NRA warnings as a word to the wise - especially in straight walled cases which seem to be easier to ring.
One more question -- if you use a duplex load, is migration of one powder into another a problem. If not, then the use of a very slow powder as the filler might be the best solution.Powder migration is prevented by using slightly to moderately compressed loads for duplex loading. It is possible that mixing two powders could make, in effect, a faster powder that results in dangerous pressures. Since a case full of the very slow powders will result in low pressures when used in most cases from .223 to .308 to .45-70, achieving a compressed load with an igniter load next to the primer is not a problem.
In summary - Chamber ringing is a real problem and it would not seem prudent to continue over-powder wads or fillers that leave an airspace between wad/filler and the bullet base. It is possi- ble that just one round will ruin the rifle, even if you have been able to get away with it for 25 years. As I said, the NRA and CBA no longer recommend this practice because of the number of ruined chambers being reported.