This pops up periodically......but here is 'pure' breech seating:
Breech seating is best done in a rifle chamber specifically done as a beech seat type. This has a groove dia. or slightly larger cylindrical throat with a cast bullet friendly leade to hold the breech seated bullet ahead of the case mouth. A variation on the cyl. 'throat' is a tapered one which a matching tapered cast bullet fits into (this is considered the best by more than a few).
The bullet is about .0005" smaller for the than the 'throat' for the cyl. version to facilitate easy beech seating with one of various tools.
As you can visualize, the bullet is held perfectly inline and concentric with the bore. When fired there is no transition of bullet from a case mouth through an unsupported void which can cause tipping or gas getting around the base band and causing cutting and leading.
The case is charged and often 'capped' with a card wad at the mouth to avoid spilling the powder, then inserted behind the bullet with anywhere from almost no clearance to 1/32 or even 1/16".
Often one one case is used, just decapped, recapped and charged, then card wad, load and go.
Now, with all that behind us, consider this as an alternative (and which I have done):
If your rifle has a real 'ball seat' as many modern cartridges do, or a straight or very minor taper at the chamber front as in possibly a 45C, etc. you may be able to breech seat a soft lead (it gets difficult with hard alloys) bullet of full throat dia. or .0005 less full up to the origin of rifling (note this is not the same dia you may be loading into the cases).
You most likely cannot get the case to chamber up behind this bullet unless that chamber is awfully long. But you can determine the max length and trim a case back to just clear the bullet base or up to 1/16" behind. Charge it, card or wax wad it (a sheet of wax about 1/8" thick can be pressed over the case mouth to 'seal her up'), load and shoot as above.
This is all a bit futzy, but is a leisurely and pleasant way to spend a nice afternoon.
Give it a go and report back
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