Do combat difficulty skill modifiers stack?

Example: A Space Marine (BS 68) with a Bolter fires at an enormous alien.

Combat difficulty skill modifiers:

Point Blank Range +30

Size Modifier (Enormous) +20

Firing Semi-Auto +10

Would this be a combined skill test chance of 128% or do you just take the highest modifier, which is +30 in this case, and add that for a 98% chance?

Combat difficulty skill modifiers like you've listed above stack just like normal skill modifiers. Up to the normal +/-60 modifier cap.

So with the skills you've listed your marine would have a +60 to his roll and his test would be at 128. But even though his test is at 128 a 96+ is still failure, and a jam. If he hits he'd determine the number of hits from semi-auto based off of 128.

Thanks! Where is the +/-60 modifier cap in the rule book?

Page 244 has the max bonus/penalty thing.

Remember that a jam result misses completely even with a master crafted weapon that doesn't jam and the number for a jam changes with standard, semi, and full auto.

Perfect! Thanks again.