Generally there is no rule stating whether to use a singular verb or plural verb after a collective noun. It depends on how you are referring to the collective noun. Are you thinking of it as a single unit or multiple items in a group?

weapons means more than one weapon
The weapons on the new fighter jet are lethal.

weaponry means weapons regarded collectively
The weaponry on the new fighter jet is extensive.

While investigating these nouns I found them to be part of what are called collective nouns. They are not as straightforward as I thought. I will be posting more on this topic but to read the details now, visit Grammar Girl.

scissors is both singular and plural

The tailor used scissors.
The tailor bought five new scissors.

The tailor used a pair of scissors.
The tailor bought five new pairs of scissors.

For more details, see Grammar Girl.

pair means two similar or identical things taken together
This is my favorite pair of shoes.

when referring to more than one, use pairs
Two complete pairs of glasses for $57.92.

Some words follow patterns for pluralization and some do not.

crisis becomes crises and hypothesis becomes hypotheses

appendix becomes appendices and matrix becomes matrices

child becomes children and ox becomes oxen

bacterium becomes bacteria and curriculum becomes curricula

antenna becomes antennae or antennas and formula becomes formulae or formulas

louse becomes lice and mouse becomes mice

knife becomes knives and self becomes selves

echo becomes echoes and tomato becomes tomatoes

deer becomes deer and offspring becomes offspring

foot becomes feet and goose becomes geese

I guess that for me it either looks correct or it doesn’t.

Here are some observations gathered from the Web to explain this chaos. I’ll cover the consistent ones in this post and the next. Then we’ll look at the inconsistent ones.

For most words, add s.
   elephant becomes elephants
   stereo becomes stereos

For words ending with a hissing sound, add es.
   box becomes boxes
   church becomes churches

For words ending in a vowel plus y, add s.
   tray becomes trays
   keys becomes keys

For words ending in a consonant plus y, change y to ie and add s.
   enemy becomes enemies
   baby becomes babies

For words ending in is, change the is to es and add s.
   synopsis becomes synopses
   thesis becomes theses

For words ending in us, change the us to i.
   cactus becomes cacti
   fungus becomes fungi