I've found that these hollow plastic practice golf balls:
http://www.walmart.com/ip/Club-Champ-K40321/19487474 work wonderful for target practice in a large yard setting. They are slightly heavier and more aerodynamic than ping pong balls, provide a little sting with a good hit, especially on the skin, but by no means is there enough mass behind one of these to cause any type of injury. The only con is that they are highly affected by wind. I was able to get on average 60ft range with no wind resistance, but if you have a good 15mph gust of wind behind you, these things will literally fly 200-250ft (tested, unfortunately.) You will need a heavier pouch to get these going far (woven paracord or seatbelt/duck tape pouch)
Another consideration are these foam practice golf balls (I am strongly considering spending 10$ for a pack of 50 from this amazon seller:
http://www.amazon.com/gp/product/B015GSLCWA/ref=s9_newrz_hd_bw_g200_i2?pf_rd_m=A...They look to be a good deal heavier I would imagine, with way less effects by wind, but these might hurt a little, especially a head shot could cause a concussion I'm guessing? Better yet, soak them in water and we're talking serious ammo! Maybe these with a helmet of some sort and we're on track. I'm guessing you could send one of these 100-120ft dry, and 300-400ft if soaked well?
Both of the above types of golf practice ammo are durable, they're meant for being wacked at with metal wedges (golf sticks) so having these fall apart on you like a sock ball or something wont be an issue.