Ladepistolen er tung.

In Norwegian, definiteness is often attached to the noun itself.
So ladepistolen is ladepistol + -en.

That means:

• en ladepistol = indefinite
• ladepistolen = definite

So the ending -en is doing the job that the does in English.

Because Norwegian usually writes compound nouns as a single word.
Ladepistol is one compound noun, not two separate words.

This is very common in Norwegian:

• bildør
• sommerferie
• mobiltelefon

So ladepistolen follows a normal Norwegian spelling pattern.

The dictionary form is ladepistol.

With an indefinite article, you would typically see:

• en ladepistol

Depending on usage and gender choice in Bokmål, some speakers may also use:

• ei ladepistol

The form in your sentence, ladepistolen, is the definite singular form.

Sometimes, yes. If the noun is treated as feminine, a speaker may use the definite ending -a instead of -en:

• ladepistola

In Bokmål, many feminine nouns can also take the -en ending, and that is very common in modern usage. So ladepistolen is completely normal.

Because tung is the form used with a singular noun that is not neuter.

Compare:

• Ladepistolen er tung.
• Batteriet er tungt.

So:

• tung = singular non-neuter
• tungt = singular neuter

Because tung here is a predicate adjective: it comes after er.

Predicate adjectives in Norwegian usually agree with:

• number
• neuter vs. non-neuter

But they do not take the definite form just because the noun is definite.

So this is correct:

• Ladepistolen er tung.

You get tunge when the adjective stands before a definite noun:

• den tunge ladepistolen

Er is the present tense of å være = to be.

It is used with all persons:

• jeg er
• du er
• han er
• vi er

So unlike English, Norwegian does not switch between am / is / are in the present tense.

In this sentence, yes. The pattern is:

• Ladepistolen = subject
• er = verb
• tung = adjective

So the basic order is subject + verb + complement.

But Norwegian main clauses follow the verb-second rule, so if something else comes first, the verb still stays in second position:

• I dag er ladepistolen tung.

It would be:

• Ladepistolene er tunge.

Here both the noun and the adjective change:

• ladepistolene = plural definite
• tunge = plural adjective form

Useful pattern:

• singular non-neuter: tung
• singular neuter: tungt
• plural: tunge

You would use the indefinite noun form:

• En ladepistol er tung.

And if you use a feminine article, you may also see:

• Ei ladepistol er tung.

The adjective stays tung because the noun is still singular and non-neuter.