It's also going to take a serious amount of pumping from empty
As Bod says it's likely that you don't have enough pressure to push everything shut and seal, so starting with the gun attached will put that in the way of the leak
Were you effectively just pumping straight through the bottle and out?
Once you've got a fill to a decent pressure then top up with the stirrup pump as often as possible, avoid shooting off too much air as it just means you will be pumping for much of your life
In practicality it's still the same overall amount of air pumped and shot, but more pumping sessions of less air at a time aren't as soul destroying as pumping from low pressure all of the time