I strongly advise against coning any barrel based on my experience in dealing with barrels that had been coned. Even if it's done correctly, the mechanics of the barrel cannot overcome the mechanics of the consumables when shooting. When the patch/ball combo enters the cone on the way out, the most common problem is caused by the high pressure gas leaking past the patch. Even if the gas leakage is concentric (same all the way around the patch), the gas is moving at a considerably higher velocity than the projectile causing considerable disruption of the surrounding air, that disruption more often than not results in disruption of the flight path and/or stability of the projectile. A small amount of excess patch hanging on one side of the ball can get pushed around the front of the ball causing the ball to be deflected. Excentric gas blow-by (more on one side than the other) is all too common with a coned muzzle and results in the ball being pushed away from the area of gas leakage. Any one or combinations of those conditions result in loss of accuracy and because the conditions can vary greatly from shot to shot, obviously one can not maintain any reasonable expectation of consistency.
The three most common coned muzzle complaints clients come into my shop with are:
1- Occasional fliers.
2- Loss of accuracy at longer ranges.
3- Reduction in maximum load capability.
As stated above, inconsistencies created as the projectile passes through the cone on the way out of the bore often result in the occasional flier which can be way off the POA (point of aim) even at short ranges. Minor inconsistencie result in larger groups as range increases. The maximum load capability is often reduced as well because the higher the pressure the load is running at, the more effect even small amounts of excentric gas leakage will have on the projo. For example; a .50 or .54 rifle that would shoot good groups at 75yds with say a 90gr main chanrge will often top-out with a much lower main charge such as 6-gr.