Breech seating (notice, two "e"s ) is loading a cartridge rifle kinda like loading an artillery piece..
First you stuff the bullet into the chamber.
Then you put in a cartridge case with powder and primer in it.
Then you shoot it.

I'm sure those that shoot breech seated ammo can elaborate some more...
I haven't tried it yet. 


Paul F.

Ziggy,  Breech seating is when you use a plugged case ar a breech seating tool to push a lead bullet into the rifling ahead of the chamber. Then the loaded case is inserted and fire'd.  The base of the bullet can be almost touching the case mouth or can be as far ahead as half an inch or more depending on the lead in.  This is the most accurate way to shoot a lead bullet out of a schutzen rifle.   Hope this help's. 
 
                                                                      Joe.

Ziggy they are open questions.

On plugging if you mean the plug in the case used to seat the bullet brass is the most common material. However hard wood works fine too as would some harder metal. Most people like something soft so the chamber cannot be scratched.  You can also seat with a push type tool as well as a lever operated mechanical seater 

If you mean by plug the wad that some people use to hold the powder in place when breech seating it's a much discussed and debated topic.  Two schools here no wad at all, or wad, then what the wad is made of, then how it's positioned over the powder.

How much space between cartridge case and bullet ?  I let the bullet fit in the rifles leade tell me, It needs to be pushed into the rifling firmly but not so firm as to damage the bullet.  The cartridge case follows the bullet and space can vary considerably one rifle/bullet combination to another. Many seaters are adjustable and people will changed spaces looking for the best combination.

Boats

It depends.  If starting fresh most people will have the leade cut for breech seating.  It's probably impossible to tell visually the barrel is different than any other chamber. It's just a little taper to the leade.

Bullets are another matter and it's not likely a standard mold will throw something than slips into the rifling well.  I would try some bullets from a proven .25 mold and see how they work. the 25's are less common but someone on this board will probably send you a couple dozen to try. If they work you could either buy the same mold or go for a custom.

I would bet you can get the rifle to shoot without alteration,

Boats

We went round and round on the air space when breech seating issue last year or so.  Everybody who breech seats Schuetzen rifles with Black powder leaves air space between the bullet and wad.

It's a often told tale that it's dangerous.  Probably true when the firearm is an old musket or shotgun. Not relivent to modern steel reproduction Black Powder cartridge actions.

The most dangerous thing when breech seating any of our Schuetzen rifles is double charging, Impossable with a case full of Black powder, has happend more than once with double charges of smokeless.

Ringing is another hotly debated topic.  Personaly I don't use any sort of wad at all if I don't have to.  It just introduces another variable in the mix. When loading a full case of Black I have to put a wad over the powder to keep it from spilling when inserting the case.   Everybody who breech seats black has to do the same thing.   

I have never seen a barrel rung with black, I have heard of rung barrels using small charges of fast smokeless powder and some sort of sticky wad materal. Dacron etc.  There is no other option I know of to load black B/Seated, wads are the only way.

Boats

FWIW, it's not the air gap by itself causing problems - unless you have way more air than powder... - IMO, it is an urban myth that black won't allow any air gap between bullet and powder:  Been there, done that, with original rifles.  What you don't want to do is use small charges of fast-burning powder in big cases (like half a case of FFFg in a 577/450), that is asking for trouble...
And this was with fixed ammo, BTW, but the situation would be identical to breechseating - air is air.

A nominal 1/16 inch space between the bullet and case has been used since Will Hayes brought the first breech/muzzleloading rifle to a match.
I have never seen an old load data that did not call for at least a 1/16 gap for breech seating. 

40 Rod