What nails does the Milwaukee 2909-21 take?

It uses coil roofing nails from 3/4 to 1-3/4 inches, the standard range for asphalt shingles and some siding. Wire-collated coils are the norm.

Does the kit include a battery and charger?

Yes, the 2909-21 is a kit, so it ships with an M18 battery and charger. Check which battery size is included, since runtime depends on it.

Is it as fast as a pneumatic roofing nailer?

It drives nails at a rate close to a pneumatic gun for normal shingle work. The difference shows on long runs, where you stop to swap batteries instead of refilling a compressor.

Can it shoot through thick shingles or over old layers?

It handles standard asphalt shingles and typical re-roof layers. Very thick or cold, stiff shingles may need the depth setting backed off and a fresh battery.

How does it compare with the older M18 roofing nailer?

The 2909-21 is the Fuel brushless version, so it is more efficient and consistent than the brushed model. It also adds tool-free depth adjustment and dry-fire lockout.

What battery should I run for a full roof?

A high-capacity M18 pack gives the longest runtime, and carrying a spare is standard practice. Smaller compact packs are lighter but drain faster on long runs.