There are few things more frustrating than getting to the range, setting up and then then your bullets don't 'print' properly on the target.
Ideally the hole left in a paper / card target when using a .577 minie bullet should be round with a constant black / dark grey ring around it. This is not always the case due to a variety of reasons.
Below are two typical examples of the print that is left when problems are encountered.
There are a number of reasons why bullet flight can be affected, particularly in reference to a .577 Enfield minie bullet.
The purer the lead quality the better. The minie bullet is designed so that the base expands into the lands of the rifling. Impure lead will be harder and as such the skirt of the bullet will not expand properly and will not be spun correctly, if at all in the rifling.
If you know where your lead has come from and it as pure as it can be then there is a strong argument to add a little tin which will enable more accurate casting.
If the lead is too hot during casting the lead may become too hard giving the same results as above.
Black powder is a dirty propellant if you have been using the rifle a lot then it may be an idea to clean it through before recommencing firing. Or when you get home clean it through with a 12 bore (gauge) bronze brush and cleaning fluid, water (or whatever your preferred method is) and ensure that the rifling is clean and clear.
It would make sense that with a rifle with worn rifling this will occur more quickly than one which is new i.e. a reproduction such as a Parker Hale
With the photos below I have used a mere 35grns of Swiss 2 from a 3 groove Enfield with a bullet with a thick skirt. It is probable that not enough powder has been used to expand the skirt into the grooves and as such the bullets have become unstable in flight...testing is the key here. Too little or too much powder and the bullet will become unstable.
Poorly moulded bullets which are either not correctly formed or have air bubbles causing weaknesses within the body of the bullet will cause variations in flight pattern.