AR-15 Small Parts & Hardware: Gas Blocks, Pins, Springs and Screws Explained

An AR-15 is a few big parts — barrel, receivers, bolt carrier group — held together and made to run by dozens of small ones. When a rifle stops working, the culprit is rarely a major component; it is a tired spring, a lost detent, a gas block that walked, or a screw that was never the right one. This guide covers the small parts and hardware that decide whether the rifle runs: gas blocks, pins, detents, springs, safeties, trigger guards and stock hardware — what each one does, how the sizes work, and what to keep in the spares bin. It is a general educational overview; always confirm dimensions against your exact barrel and receiver before ordering.

Gas blocks: the number is your barrel diameter

The gas block sits over the gas port and routes gas back through the tube to cycle the action. It is bought by one number: the diameter of the barrel journal it clamps onto. The common sizes are .625, .750, .875 and .936 inch — measure your barrel at the gas journal with calipers, because the number is not a caliber and guessing wrong by one step means a block that either will not slide on or will not seal. The fit and the gas it delivers depend on that match. TACBRO's range covers the spread, from the .625 steel low profile micro gas block for pencil barrels to the .936 stainless blocks for heavy and bull barrels such as the .750 low profile stainless steel gas block pattern scaled up — each shipped with its gas tube roll pin and set screws, because those are exactly the parts that roll off the bench.

Low profile vs high profile, steel vs aluminum

Low profile blocks are slim enough to sit under a free-float handguard — the default for most modern builds. High profile blocks stand taller, replace an A2 front sight base, and can carry their own rails: the .750 high profile gas block with top and bottom Picatinny rails gives you a sight plane on top and an accessory rail underneath. Material is a heat and weight call. Steel and stainless steel shrug off the full heat of the gas system and are the safe default under a handguard; aluminum blocks are lighter and work well, but they expand at a different rate than a steel barrel, which is why their fit and set-screw torque deserve a second look after the first range session. Most blocks attach with two set screws on the bottom — a dimpled barrel gives those screws something to bite, and threadlocker rated for heat is cheap insurance.

Pins, detents and springs: the parts that vanish

Every AR control is a pin, a spring and usually a detent working together. The takedown and pivot pins hold the receivers; the buffer retainer detent keeps the buffer and spring captive when you open the rifle; the safety, bolt catch, magazine catch and trigger group each ride on their own small springs. These parts fail quietly — a spring takes a set after years of compression, a detent launches across the room during a build and is never seen again. That is why complete spring kits exist: a lower receiver spring kit covers the bolt catch, disconnector, buffer retainer, trigger, hammer, magazine catch and selector springs plus the detents, so one envelope replaces the whole lower's worth of small springs at once. Swapping a tired spring set is also the cheapest reliability refresh a high-round-count lower can get. Trigger and hammer pins deserve one upgrade note: standard pins can walk under recoil in some receivers, and anti-rotation pin sets in the standard .154 size lock the pins to each other so they cannot rotate or walk out — a small fix for a problem that otherwise shows up mid-string.

Safeties and trigger guards

The safety selector is the control your thumb lives on, and it is a spring-and-detent part like everything else in the lower. Traditional selectors rotate 90 degrees; push-button designs take a different route — the push-button speed safety switches from safe to fire with a straight push, shows an orange indicator on fire, and runs on the detent and tension spring included with it. Neither is automatically better; it is a training and preference call, and the detent and spring are what make either one feel crisp or mushy. Trigger guards are simpler: the standard flat guard crowds a gloved finger, and an oversize enhanced trigger guard opens that space up for cold-weather or tactical gloves while keeping a snag-free profile — one of the few upgrades you notice every single time you pick the rifle up.

Stock hardware: locking pins and A1/A2 screws

Fixed-stock rifles hang on one screw, and it is not an ordinary screw. A1 and A2 buttstock screws are vented — they carry a gas port that lets the buffer tube breathe — and they are different lengths for the two stock patterns: the 1.18-inch A2 buttstock screw and its shorter 0.48-inch A1 counterpart are not interchangeable, so match the screw to the stock, not the other way around. At the other end of the compliance world, stock locking pins lock an adjustable M4-style stock in a fixed position for featureless configurations; designs like the M4 stock lock pin do it without modifying or marring the stock, and they are made to fit specific stock families — USGI M4, SOPMOD-pattern and similar — so check your stock against the fit list before ordering, and check your local rules before building a featureless rifle at all.

Fit-tuning parts: wedges and charging handles

Two small parts exist purely to fix feel. An Accu-Wedge style rubber wedge drops into the rear of the lower receiver behind the takedown pin and puts tension between the upper and lower, taking the rattle out of a loose receiver set — less play means less vibration and a rifle that feels like one piece. Side-charging uppers move the charging handle out to the bolt itself, and an extended bolt handle gives your fingers more to grab than the small factory knob; it attaches with a single screw (10-32 x 3/4 inch on the TACBRO pattern), which makes it one of the fastest swaps on this whole page. Neither part changes what the rifle is — both change how finished it feels.

The spares-bin rule

Small parts follow one rule: the one you lose is the one you do not have. A spring kit, a spare gas tube roll pin, a couple of set screws and one buffer retainer detent in the range bag will cover most of the ways an AR stops being fun at the range — for a few dollars and almost no space. Buy small parts by measured size, keep the packaging until the part is installed and tested, and when a spring-powered control starts feeling vague, replace the spring before you blame the expensive parts around it.