Oh, sorry, right. I was actually just now editing the inititems file..
If you've done those two things--you're done!
Wait. Did you add a normal item define, or a InitGun() call? Or both?
The former is seldom used anymore because we mostly all use Scheffnow's weapons mod.
if you've not added an InitGun() line, find the big block of InitGun() calls in inititems.c:
they work like this:
<!--c1--><div class='codetop'>CODE</div><div class='codemain'><!--ec1--> //----------------------------------------------------------------------------------------------------
// GUNS: ItemIndex picIndex Price Accuracy
// | gunID modelID | rare | minDamage | Charges
// | | | picTexture | | MinLevel | | ma

amage | | Reload time
//--------------|---|----------|----------|---|--|-----|-----|-----|-----|------|--|---|-----------------
n = InitGun(n, "pistol1", "pistol1", 6, 1, 0.50, 3, 1500, 15.0, 30.0, 30, 1, 8); // Short Pistol<!--c2--></div><!--ec2-->
Add a new n = InitGun() line after Alan Smithee's, and replace the names and stats with those you desire.
To give it to the player at start: open up questsboth_reaction.c
find (if you're running PB's modpack)
<!--c1--><div class='codetop'>CODE</div><div class='codemain'><!--ec1-->/*
GiveItem2Character(Pchar, "giveitm"); // PB: Gives you any item you might want to test when you start a new game
// Replace "giveitm" with the item you want to be given (find names in initItems.c) */<!--c2--></div><!--ec2-->
Remove the /* and the */, and replace "giveitm" with the ID of your gun.