My understanding is that manufacturers use O rings in the barrel as the most practical way to achieve breech sealing. Agreed, I prefer to have an O ring on the bolt probe, but I believe this can cause issues with "dragging" while cycling the bolt through the magazine.
If the magazine has fairly tight tolerances (a good thing), then an O ring on the bolt probe has to pass through it, while still sealing in the barrel - which is, of course, just about the same diameter.
Hopefully
@BenjeneerJohn will be able to contribute and provide a professional opinion on what I have just said