Is the 45 Colt less accurate than other pistol carbines when using slow powders?

[Lee Robinson]

Speaking to someone that "should know" I was told the 45 Colt lever action guns are less accurate than is the 357 Mag OR the 44 Mag lever action guns because SAMMI specs have oversized chambers in the 45 Colt...and that this "issue" isn't specific to any single manufacturer, but is simply a product of SAMMI chamber specs being excessively over sized when compared to the round itself. I was informed that this causes one to get "blow by" when using slow burning powders like H110 or 2400 as one doesn't get the brass to properly "seal" the chamber early enough without an early pressure spike...and therefore we get more variation in pressures with slow burning powders in the 45 colt than is achieved in other pistol rounds that have tighter SAMMI specs.

Their claim made sense, but is this true?

[dpe.ahoy]

Using the slow powders means you must use Ruger/T/C loads to seal the brass case to the chamber walls.  I have no blow-by issues running 300+ gr bullets using H-110 or LilGun powder in my rifles or Ruger revolvers.  If you want light bullet loads or plinking loads, use faster powders to get the results you are looking for.  If you want full power loads, use the slower powders like the two I mentioned above as well as Accurate 9, IMR 4227, W 296 (same as H-110), 2400 and a few other powders.  If your firearm cannot handle the pressures these loads need to work effectively, DON'T GO THERE!!!!  DP

[tacklebury]

I use Reloader7 in .45 Colt when shooting them in my carbines.  I use a tougher bullet, 300 gr. XTP Magnums to take the extra velocity and 21 gr.(starting load) of RL7.    Shoots critters dead from a 20" barrel like they were struck by lightning. 
 
Started with data from this article if interested...    http://www.leverguns.com/articles/paco/45coltlevergun.htm