Some of the disagreement here seems to arise from people thinking that shotgun pellets landing over a boundary are equivalent to bullets crossing a boundary. It is obvious that they are not comparable. Falling shot has no potential to cause damage to property or person, whereas bullets are extremely dangerous.
I think this is a red herring. 'Spent shot'
could still injure an eye, for example. I think the thing here is one of the fact of where projectiles go after the shooter has lawfully discharged them while on ground where he has permission to be shooting, rather than the amount of damage they have the potential to cause. The fact that projectiles crossing a boundary is not criminal, but rather a potential civil trespass, does not seem to be in doubt. This is presumably why new law was made to make airgun slugs crossing boundaries a criminal offence: a law that wouldn't otherwise have been needed.
Therefore I'd suggest that any argument of the type ...'if it's not an offence with a shotgun, then it must be ok with a rifle' is dodgy.
First, 'not a criminal offence' and 'OK' are not, in my view opposites. We know it's a criminal offence with an airgun. We seem to know that there's no law that makes it so with firearms ot shotguns, and we've seen a circumstance described where a bullet crossed a boundary, but which summarised as 'no offences committed'.
Fwiw, my GUESS is as follows:
I suspect bullets passing over a boundary are likely to be a criminal offence in a situation where that flight is detected - i.e. it causes a nuisance or damage. It may be a civil offence if nobody minds.
I think if it were a criminal offence it would be just that, detected or not. In fact, I think it is the civil offence of trespass - and I think for that to be complete some nuisance or damage must be proven by the plaintif.
I doubt that the criminal offence would arise from contravening FAC conditions but suspect it is covered elsewhere by the Firearms Act or game law. It must be an offence under the text of the Act without having to rely on an offence being created by the contravention of conditions which are discretionary and which the HO guidance indicates shouldn't exist.
So where is the hypothetical offence described, I wonder?
Whatever the case is, there's clearly something seriously wrong with the thought processes of anyone who thinks it's a good idea to have their bullets flying over or into property they don't have permission to shoot over.
Why? Would it be criminal (or irresponsible) to take a perfectly safe shot at a beast on one's own ground just because the backstop is a bank a few yards over the march?
I don't think anyone is suggesting that shooting randomly over and onto other folk's ground is a good idea - but nor is shooting randomly over you own ground. A safe shot is a safe shot; and I think a shot (other than taken with an airgun, or course) taken lawfully on ground where you have authority to shoot is lawful notwithstanding that bullet or shot may end up harmlessly on a neighbour's ground.