Well here is a reality check, I think that statement is completely incorrect.
When you load very fast powders in extremely large cases originally loaded with Black Powder it's very possible that powder might not ignite correctly because it's position sensitive. Even if it's not position sensitive there isn't much of it. After all, there isn't much powder in a charge of Bullseye when put in a 45 Colt case. A slower powder would fill the case more and possible burn better because of it. (I said slower, not slow powder) I get the best results with AA#5 and HS-6.
Ah, I love to read your posts Arch because of your persistance
in consulting your logical mind instead of throughly reading
the postings and checking data you help make my point.
The OP has Bullseye, not your HS-6 and AA#5. Your logical
mind implies a direct relationship between powder bulkiness
and position sensitivity, do you actually believe this? Can
you really think that at 11,000 psi level Unique or HS-6,
which Speer uses a magnum primer with, will be less position
sensitive than Bullseye? Have you ever tested various
powders for position sensitivity like I have? The most ardent
fans of the .45 Colt recognize that the large amount of air
space in it's large case makes it the worst offender for
inconsistancy with light loads. Some very popular gun
writers are great fans of the .45 Colt and have stated
repeatedly that best consistancy comes with fast burners.
But who cares anyway? The point is that the OP has Bullseye
and it works as well or better than most other powders for
his purpose of modest loads with cast bullets and to alarm
him and insist that he must buy something else is just wrong.