{A} They're destructive
{B} I'll be forced to control them.
{C} How far is a typical shot?
{D} What's the best firearm for the job.
{A} You got that right...and then some. There are places where I am afraid to take my tractor over concern for its axles and tires. My PU truck does not like the soft earth left in ther passings either.
{B} Don't kid yourself. If you have one, you probably have 50. Once they become established in an area, and it does not take them long to take control, with two to three litters of from 7 to 11 piglets per sow per litter; there is no getting rid of them - EVER!
{C} As Arier stated...sometimes 10 yards or less...sometimes 500 yards or more, if your hunting area will tolerate that range and you can hold on the game for a shot that long. My typical shots are from 220 yards down to 25 yards.
{D} GB discourages me from this, but I have killed a lot of hogs with a Ruger Model K77/22RP (22 LR) from a steady rest, 14-foot elevated position, unobstructed path, 25 yards or less, optimum conditions, in the ear or the "X" between the ears and the eyes (where the brain is just below the skull), DRT. I do not advocate this caliber for others, but I use it to my advantage from a rifle that spits out the cheapest ammunition into the tightest grouped holes at 50 yards that I have ever had the pleasure of shooting. As S_J_KENNELS pointed out in his post (caliber-wise), I also use a Marlin Model 336 in 30-30 caliber, a Winchester Model 70 Classic Featherweight in 270 caliber, and a Ruger New Model Super Blackhawk in 45 LC caliber.
Bullets of choice are Speer 150 grain Flat Nose for the 30-30 tubular magazine ; 150 grain Nosler Partition for 270; 230 grain hard cast semi-wad cutter lead for the 45 LC; and 36 grain hollow points for the 22 LR.
I have reloaded and shot the 150 grain Speer Grand Slam and Nosler 130 and 150 grain Ballistic Tip in 270; 170 grain Winchester Factory loads and 130 grain Speer FN's in 30-30; but I have found the others to shoot straighter, pound harder, disrupt less meat, and cause more DRT hunts, which IMO makes for better tasting game.