You should always draw back on needles on any injection, subcutaneous, IM, or IV, just to make sure you are not in a vein, since some drugs are not supposed to be given in the vein. Of course, that is how you tell if you are in a vein giving something IV, or if the IV is still working and not infiltrated. Most injections on chickens should be given into the breast muscle only 1/4 inch deep. Using the shortest needl possible is good, but it needs to be large enough gauge for the medicine (or thick medicine) to go through.
Ahh, yes, now I see what you meant.... excellent explanation that will help many, thanks.I meant that when giving an IM injection you should check for blood. IMs should not be given in veins.