At 40 yards, the 00 buckshot pattern from a Military Armament MAC 1014 Marine 12ga (which is a semi-auto shotgun with an 18.5" barrel and likely a cylinder or improved cylinder choke) will generally spread significantly, and effectiveness becomes marginal for tight groupings.
Expected Pattern at 40 Yards with 00 Buck:
00 buckshot typically has 9 pellets (in standard 2¾" shells).
At 40 yards with a cylinder bore (likely the case for the MAC 1014 Marine), the shot spread can be:
30–40 inches across, depending on load and choke.
Pellets may disperse widely enough that not all 9 will hit a man-sized target.
Effective range for tight, lethal grouping with 00 buck is more in the 20–30 yard range with a cylinder bore.

Hi Edward and welcome. You will get the tightest pattern possible using the Cylinder Choke and this "Flite-Control" 00 Buch load: budsgunshop.com/product_info.php/products_id/36183/federal+pfc15400+premium+vital-shok+12+ga+2.75+9+pellets+00+buck+round+5+bx+50+cs ------------- Cheers, stay safe and enjoy!