Firearm assembly:

Necessary parts:
Receiver - Operational part of the weapon. Contains bolt assembly, breach and ejector (may require barrel with gas tube). Diameter >= barrel diameter, length <= barrel length.
Barrel - hollow tube, with or without rifling. May have gas tube. Diameter <= receiver diameter, length >= receiver length.

Recommended parts:
Shroud - Covers receiver and (optional) part/all of barrel. Contains rails, buttstock attachment, fixed carry handles etc. Rear length = receiver length, extra length <= barrel length. Rails vary in length and may fit multiple attachments.

Muzzle - Attaches to end of barrel to increase initial accuracy. May have additional muzzle attachments added later. diameter = barrel diameter.

Buttstock - Recommended for most firearms. May be collapsable. Decreases recoil but only decreases maneuverability when deployed.

Extra attachments:
Sights / Scopes - Aids aiming from the shoulder. Some types may be collapsable or otherwise adjustable. Various types may be fixed to the shroud or bolted to a suitable rail. Multiple scopes/sights may be used a once, provided all have the same height.

Silencers, flash suppressors etc. - Alters some affects of firing the weapon like muzzle flash or recoil, attaches to muzzle. Diameter = muzzle diameter.

Bipods - Greatly reduces recoil when mounted on a surface, but reduced maneuverability when deployed. May be collapsable. May be fixed to the shroud or attached to a suitable lower rail. Length > receiver / magazine height (whichever is greater) to be deployed while prone.

Foregrips - Reduces recoil. May be fixed to the shroud or attached to a suitable rail.

Grenade Launchers etc. - Provide an extra weapon. May be fixed to shroud or attached to a suitable rail, but must be infront of any other attachments on the rail. Some launchers etc. may not need to be fitted to a weapon to function, but switching between a firearm and an attached launcher is quicker than switching weapons entirely.

Lights / laser pointers etc. - May be toggled on / off to project light etc. May be fixed to a shroud or attached to a rail, but must be infront of other attachments on the rail.

Bullets / Damage Types:

Jacket Types:
Hollow Point - Causes reasonable damage to flesh but is ineffective at penetrating armour or shields.
Soft Point - Causes slightly more flesh damage than hollow point and is less likely to exit, but degrades weapon durability slightly more.
Armour Piercing - Causes less flesh damage but has much greater penetrating power, very likely to exit unarmoured targets.
Jacketless - A fully exposed bullet, likely to cause high flesh damage, but will disintegrate rapidly. Will cause rapid durability degradation.

Core Types:
Lead - Regular bullet core, average flesh damage and armour penetration.
Incendiary - Ignites a variable fuel on impact, likely to ignite flammable targets and deal more flesh damage. Less likely to ignite when used with armour piercing jackets, and will only ignite when stopped by a surface / target.
Explosive - Similar to incendiary, but explodes on ignition and is more likely to explode from initial impacts.
Electro-Magnetic - A specialised  explosive charge which disrupts electrical systems. Can be used to temporarily drop shields or disrupt engines.
Plasma - Ignites immediately after firing, creating a small fireball. Causes very high flesh damage and can ignite targets, but ineffective against armour. High-velocity plasma may be able to penetrate shields. Requires jacketless bullets.

Tail Types:
Tracer - leaves a visible trail behind the bullet. Cannot be used with jacketless ammo.
Fletchette - Stablising fins improve accuracy. May be combined with a tracer. Cannot be used with jacketless ammo.

Armour / Shields:

Body Armour:
Woven Fibre - Various fibres woven into clothing to give limited protection from penetrating attacks. Reasonable durability and very light.
Ceramic Plate - Effective protection against penetrating attacks for reasonable weight, but low durability.
Metal Plate - Effective protection against all attacks with high durability, but heavy.
Polycarbonate - Effective protection against all attacks with reasonable durability, and light weight. Can be made see-through if necessary.

Shields:
Radial Particle Shield - Creates a spherical wall of energised particles to deflect high velocity projectiles and pressure waves.
Directed-Arc Particle Shield - Creates an arcing wall between two points to deflect more effectively over a limited area.