I need some advice on choosing next Glock. I have a 1 month old G19 (gen3) and love it! Now i am already looking at 10mm and 45acp as a possible new sibling to my 19. My issue is full size (20 or 21) or compact (29 or 30). Another thought is ammo; availability and price difference. And my biggest concern-can barrels be changed/converted to other calibers in the compact models? Any advice us much appreciated.