Jack O'Connell's character was originally written to have come from Lancashire, UK, but at Jack's request was rewritten so the character came from Derby, Derbyshire, UK, the real birth home of Jack O'Connell.
In the initial chase scene where Gary runs from the shooter he is fired at 32 times (including the first kill shot) from what seems like just one man's gun, we don't see the younger boy fire his gun at all. We also don't see any reloading as they are running at breakneck speed. This would be impossible from a small 1960's era 9mm Semi-automatic pistol which have at most 13 rounds in the magazine.
The movie shows the British soldiers holding the L1A1 Self Loading Rifle (SLR) with black plastic butt stock and fore grip. These were not trailed by the British Army until 1974 and were then phased in over the next 8 years. The rifles should all have had wooden butt stocks and fore grips.