Jeez, I guess the .45/70 is too "light" for larger deer in the UK. Who knew? Like most of the large bore black powder rounds, if you can estimate range well and know the trajectory of your load, these guns are still deadly at longer ranges. But even a slight error in range estimation will result in a miss.
Here are the rules - I don' make 'em, I just have to abide by them.
Game Licence is no longer needed to kill or take deer, anywhere in the UK.
Firearms and ammunition
The stalker must only use a rifle and ammunition which are legal for the species of deer being shot. In addition to compliance with the law the stalker should be guided by knowledge, experience and personal preference in their choice of a rifle and ammunition. The legal requirements are laid down in the several Deer Acts and Orders. For example:
England and Wales
For Muntjac and Chinese Water deer only - a rifle with a minimum calibre of not less than .220 inches and muzzle energy of not less than 1000 foot pounds and a bullet weight of not less than 50 grains may be used.
For all deer of any species - a minimum calibre of .240 and minimum muzzle energy of 1,700 foot pounds is the legal requirement.
Northern Ireland
For Muntjac and Chinese Water deer only- a rifle with a minimum calibre of not less than .220 inches and muzzle energy of not less than 1000 foot pounds and a bullet weight of not less than 50 grains may be used.
For all deer of any species - a minimum calibre of .236 inches, a minimum bullet weight of 100 grains and minimum muzzle energy of 1,700 foot pounds is the legal requirement.
Scotland
For roe deer, where the bullet must weigh at least 50 grains AND have a minimum muzzle velocity of 2,450 feet per second AND a minimum muzzle energy of 1,000 foot pounds may be used.
For all deer of any species - the bullet must weigh at least 100 grains AND have a minimum muzzle velocity of 2,450 feet per second AND a minimum muzzle energy of 1,750 foot pounds.
It must be stressed that all these figures are the minimum legal requirement.
For all deer stalking the bullet must be of a type designed to expand/deform on impact.
I guess that a modern .45-70 JSP might make the figures in the rest of UK, but certainly NOT in Scotland. BTW, I've never heard of anybody taking game with a single-shot BP rifled gun here in UK.
tac