George - Here is my perspective as a digger and as a collector of flintlock conversions for the past 15 years or so.
I think that there is a great deal of evidence that minie balls do not have to equal a rifled arm shooting them. The first wave of US military conversions after the "Great Sorting Out" of the 1840's were intended to change existing flintlocks to the percussion system - period. The result was the ever-familiar "belgian" conversion.
When the US adopted the concept of rifled arms for general use with the model 1855 series of longarms - they turned to these Belgian Conversions to see if they could also be adapted to a rifling system.
Sadly, these experiments were a failure - the combination of the removal of barrel metal for the rifling (even though it was pretty shallow) and the weak point created by the upseting of the metal to make the nipple platform created dangerous breach pressures. The resulting gun was a danger to the user as well as the receiver of the shots. The result is that the rifled Belgians were pretty uncommon - although I think Greenwood of Ohio did do some in the early days of the civil war - and were again largely a failure.
The result was that 1816's would need a patent replacement breach in the future if they were to be rifled. I think the best known examples are the Frankford/Remington conversions that used the Maynard tape primer. The newly made one piece replacement breach could stand the pressures. The most commonly seen replacement breaches are the Leman style that infest gun shows to this day.
The model 1842 was able to stand the pressure with it's different cone seat and over 55 thousand were rifled - (but many never were sighted - so go check your 42 and see)
As a digger - 69 minies are found in considerable overabundance to the number of rifled 69's in the field. The obvious conclusion is that they were fired and used in smoothbores that were not rifled. The sealing action of the design should have given greater muzzle velocity and the nose-heavy bullet might have been marginally more accurate than any round ball.
Without the loss of the barrel thickness coming with the rifing cuts - the breach may well have been within safe(er)limits.
As an example, we know that the 37th Ill Infantry was armed in the flank company's with colt revolving rifles - and the rest with belgian conversions - smoothbore. Their camps have colt RR bullets and 69 minies - roundballs are a real oddity in their camps. The only conclusion is that they were firing minies out of their smoothbores. Finally, in the Prairie Grove battle / Cane Hill Battle - there is an abundance of fired 69's that show no trace of rifling marks and some few others that are very clearly fired from rifled guns. However, the huge preponderance is towards the 69's that were fired in those battles having no rifling marks visible - at least in my years in the field digging both battlefields.
Hope this is of interest - Gene