It will go bang either way. Since I don't do Breech Seating I will let the experts here explain it.

Hepburns are extremely versatile and user friendly. Have breech seated, muzzle loaded and fired fixed ammo in one of mine.   

       Joe.

After all this is a schutzen website; This is my favorite breech-seater, it's a concentric seater that fits into the chamber and uses a button on the side of the barrel for leverage. Essentially it's a plunger that presses the bullet into the rifling just ahead of the chamber. This one happens to be for a Ballard.

Correct, Bob.  Almost forgot the ASSRA and it's forum have a strong base in Schuetzen competition.

When requiring the utmost in accuracy for SS BR matches, breech seating the plain base bullet, is the only way to have a chance.  Muzzle loading a close second and fixed a distant third. 

Hunting and many BP matches require fixed ammo. Used to shoot with Jerry Tinker, he breech seated bullets while in the prone position with a Greener Martini 45-70 in BP midrange matches. If you can manage it and have time, breech seating is always superior.  Muzzle loading is fun but try to find someone loading that style. Have been chided by competitors for muzzle loading in matches.  They're jealous, lol.

           Joe. 

Remember, that breech seating does not work with jacketed bullets.   Ledball

About the only type of shooting sports that used/uses breech seating (other than naval and other very heavy artillery) is the American "schuetzen" shooting. 
The European shooters did not seem to do it. 
Even in America it fell out of favor in America when "schuetzen" matches died out-for a number of reasons.   
      It was resurrected when ASSRA was formed in the late 40's and a number of  old schuetzen-style techniques were reintroduced and institutionalized in the Association rules to create a "modern" shooting match venue for single shot rifles.
As mentioned it adds a significant extra margin of accuracy that is especially advantageous in bench rest shooting.  However it also adds a time/handling factor.  It works in ASSRA matches where we have pretty much unrestricted time limits.  In other match shooting sports the relay time limits might present a problem (in addition to any rules requiring "fixed ammo").
   When shooting offhand breech-seating can add extra rifle handling motions adding to the fatigue-factor in long 50 or 100 shot matches.
  One also has to consider the probability that most offhand shooters have inherent "angle of dispersion" problems in their shooting "form" that will more than cancel out the mechanical accuracy advantages gained by breech seating.  Most of the top-scoring offhand shooters appear to be shooting fixed ammo.  {at least from what I have seen at the Etna Green matches|

QuestionableMaynard8130 wrote on Feb 21^st, 2014 at 10:25am:

Thank you for adding that, as I was going to inquire about any potentials for offhand use. When Paul throated my CPA he set it for fixed ammo, as I hadn't chosen a bullet yet, and told him I was building it for offhand/vernier sight use. 

Now that I have a good bullet, Paul will re-throat it to that bullet for me. But Paul also opines that a throated rifle will also shoot fixed ammo just fine. Do you all agree with that?.

I shoot offhand exclusively (less load dev) and have a load shooting sub-MOA @ 100Y (4 thru a ragged hole and a flyer outside ... , load was bench tested with a 20X ext adj scope). As long as throating it wouldn't hurt that 'fixed' potential, I'd sure like to try my hand at breech seating. Thoughts?