Lower Receiver Parts Quality Guide | Firearm Configurator

Upgrade priorities by part

Move through these categories in order. A reliable trigger and spring system beats fancy furniture every time.

Component	Baseline	Upgraded	Premium	Notes
Trigger	Mil-spec single-stage (ALG ACT, BCM PNT) – reliable, fairly crisp	Quality two-stage (Geissele SSA, LaRue MBT) – better control, still durable	Match two-stage (Geissele SSA-E X, Hiperfire Eclipse) – precision oriented	Prioritize upgraded triggers once shooters consistently manage fundamentals; premium triggers require diligent cleaning.
Action spring	Standard carbine spring from reputable kit (Colt, Sprinco White)	Sprinco Blue or Green tuned to buffer weight	A5 rifle-length springs for VLTOR / LMT buffer systems	Sprinco color coding simplifies fleet swaps. Replace springs on schedule before they compress permanently.
Buffer	Carbine H buffer (3.8 oz) with properly staked castle nut	H2 or A5H1 buffer for suppressed or mid-length setups	A5H2 / LMT enhanced buffer with tungsten weights for hard use	Track buffer weights in build logs. Don’t mix tungsten and steel weights unless you’re intentionally tuning recoil.
Lower parts kit	CMT, SOLGW, or Aero kits with machined pins and 8620 hammers	Forward Controls, Sons of Liberty, or Sionics enhanced kits with QPQ pins and improved selectors	KAC, LMT, or Geissele kits matched to ambi lowers with captured detents	Avoid bargain-bin kits—soft detents and roll pins lead to drift and lost parts during maintenance.
Receiver extension	7075-T6 mil-spec tube with staked castle nut (BCM, Colt)	VLTOR A5 extension or LMT carbine tube with dry film lube	LMT MWS FDE or Radian adjustable systems for precision stocks	Proper staking and thread locker prevent rotation. Record torque values if using end plates with QD sockets.

Replacement schedule

These intervals strike a balance between cost and risk. Adjust if you run suppressed, full-auto, or dirty environments.

Part	Training cycle	Duty cycle	Why it matters
Action spring (carbine)	10,000 rounds or annually	5,000 rounds or semi-annually	Springs take a set under constant compression; fresh springs prevent bolt speed spikes or short-strokes.
Buffer retainer & spring	Inspect every detail strip	Replace annually or at first sign of deformation	Cheap insurance—broken retainers cause bolt-over issues when rifles are dropped or cleared aggressively.
Trigger & hammer pins	Inspect annually	Replace every 15,000 rounds or if walking observed	Pins egg out soft lowers; quality pins with proper tension reduce rotational wear.
Selector detent & spring	Replace on rebuilds	Replace every 10,000 rounds or if selector feels mushy	Crisp selector movement improves muscle memory and ensures the rifle stays on safe when needed.
Buffer	Inspect for flat spots every cleaning	Replace if weights rattle or face shows peening	Flattened buffer faces are a sign of timing issues; address before bolt lugs shear.

Armorer workflow tips

Standardized processes keep rifles consistent across builders and shifts. Adopt these habits before your next maintenance day.

Source control

• Buy small parts from known OEMs (Schmid, Sionics, Colt) to avoid inconsistent heat treating.
• Track lot numbers—throw them in your log book or tape them inside parts bins.
• Test new kits on one rifle before rolling out across the fleet.

Assembly discipline

• Use proper punches and roll pin starters; mushroomed pins crush springs and detents.
• Torque castle nuts to 38–40 ft-lb, apply blue Loctite if policy allows, then stake in two places.
• Grease trigger sear surfaces lightly to reduce initial wear without attracting carbon.

Documentation