You can only carry a limited bulk of items; the heavier or more oddly-shaped the item, the greater its bulk. For example, a coffee mug would have a bulk of 1; a book would have a bulk of 1 to 4; a cinder block would have a bulk of 15; some very small items have a bulk of zero. You may carry a maximum of 12 bulk of items. Any more than that would hamper your movement. Your space suit and jetpack are already figured into the bulk equation.
BULK OF BULLETS TABLE
Handgun bullets: 15 bullets = 1 bulk
Rifle bullets: 10 bullets = 1 bulk
Shotgun shells: 5 shells = 1 bulk
Note that once you have chosen to fire a gun at the enemy, you must fire at least one bullet, even if you realize afterwards that you have no hope of hitting the enemy!