. - I recently found and bought HANDLOADER Ammunition Reloading Journal No. 306 Dec.2016 (from a North Canterbury bookshop.)
In an article by Dave Scovil headed '.455 Eley Conversion To .45 Colt' he writes the following:
" Some folks believe a bullet sized to .454 or .455 inch that is fired in a .451 inch barrel will generate excessive pressure. Then too Colt .357 Magnum barrels measure .354 inch and folks have been shooting .357 to .358 jacketed bullets in them, which are much harder than the hardest cast bullet, since 1935.
I prefer cast bullets with a one-in-16 (tin-lead) mix, the same as Elmer Keith's, for a Brinell hardness number (BHN) of 8 for the .45 Colt, so a modestly hard bullet that is up to .004 inch over barrel groove diameter has been standard on my Colts for nearly 40 years. Additionally, Winchester, Remington, and Federal offer .45 inch Colt factory loads with .455 inch jacketed or swaged lead bullets, so they apparantly don't believe over-sized bullets are an issue either."
But the indications certainly are that there is some degree of tolerance in "fit" of bullets to barrels - but all shooters need to be very aware at all times that we are dealing with very high destructive pressures and velocities in firearms that can be described in mechanical terms as 'Heat Engines'.
Lead bullets can be a few thou oversize without any adverse effects. The slightly higher pressure lasts only for the length of the bullet as it enters the rifling. After that is the correct size and no different to normal loads. The same rationale applies to jacketed bullets as well but the initial pressure would be slightly higher. Years ago it was not uncommon to hear of people (I knew a few personally) who made a regular habit of firing jacketed factory .44/40 loads through .410 Kea pistols with no ill effects! I don’t recommend that practice!