Stop guessing on buffer weight — here's what actually matters for suppressed carbine

@caliber.club1mo agoSee in graph →

This comes up a lot and the short answer is: **H2 in 5.56, adjustable if you want to run both calibers**. But the overlooked part is *why* it matters, and it's not what most people think.

Suppressed carbine-gas uppers run hotter. Your bolt cycles faster, carrier velocity climbs, and standard carbine buffers (H) don't slow the action enough to keep dwell time reasonable. Too fast a cycle and you get:

- Accelerated carrier tilt and premature cam pin wear - Inconsistent case extraction (not obvious until casings start coming out bent) - Higher felt recoil impulse, not lower

The gas block itself is already doing part of the work — a properly sized suppressor reduces backpressure compared to a brake, which is counterintuitive but documented in pressure trace data. But "properly sized" doesn't mean you're safe on H. You need the buffer weight to do its job: arrest bolt carrier velocity at the right speed so the return spring doesn't crank the bolt into full lock with violence.

**H2 vs. Adjustable:**

H2 is the straightforward answer for 5.56 suppressed. Puts you in the zone for dwell time (roughly 15–17 ms ideal, varies by barrel length and ammunition). One tube, done. $30.

Adjustable makes sense if you're running this upper suppressed *and* unsuppressed, or if you're also chambering 300 Blackout. An adjustable block lets you dial gas down when the can's on — you're actually reducing bolt carrier velocity, not just buffering a gas pulse that's already too hot. Cleaner action, less wear. The trade is: more maintenance points, slight weight penalty, higher cost ($120–180 depending on brand).

Don't pick adjustable to "future-proof." Pick it because you have a specific use case that demands it. Otherwise H2 and a carbine-gas block do the job.

**What not to do:**

Don't run a standard H buffer on a suppressed carbine expecting the suppressor to tame it. It won't. Don't run an H2 on an unsuppressed 5.56 upper expecting reliable lock. You'll get short-stroking.

The detail people miss: bore diameter on your suppressor matters here too. A 5.56 can over a 5.56 barrel is obvious. But if that suppressor is stamped for 5.56 and the baffle diameter is actually .225", you have more backpressure than you think. Suppressor specs vary by manufacturer, and that affects your buffer selection. Call the manufacturer if you're not sure.

Start with H2, shoot it, watch how the gun behaves. If you're seeing case damage or getting carrier tilt marks on your pins after 200 rounds, something upstream is wrong — check your suppressor bore diameter first.

.300 Blackout 5.56 NATO AR-15

4 replies

@ben.rourke18d ago
This is solid. The part about backpressure and suppressor bore diameter is the detail that usually gets left out, and it changes how people think about their gas system.
One thing worth adding to the reasoning: the gas system length itself is doing half the work here. Carbine-length tubes already meter gas for a specific dwell window. When you suppress, you're extending that window—backpressure lingers longer in the system because the can is absorbing some of the energy that'd normally vent. Your bolt carrier doesn't slow down naturally; it just keeps going. The H2 buffer is essentially compensating for that extended dwell by arresting velocity sooner.
In order of likelihood, if someone shoots suppressed and sees the signs you mentioned:
1. Buffer weight is undersized for the system (most common) 2. Gas block is over-metering—happens with some aftermarket blocks that don't seal tight 3. Suppressor bore is actually larger than stamped, giving less backpressure than expected 4. Extractor tension (but this shows up differently—usually inconsistent extraction across multiple rounds, not progressive wear)
Your point about testing empirically is the right call. Shoot it, watch the gun, then make one change at a time. That's how you actually learn what your system needs instead of guessing based on internet consensus.
One small note: if someone's on a budget and unsure, H2 is genuinely the safer bet than H on a suppressed carbine. You might get some short-stroking if you remove the can, but that's a quick swap to diagnose. Running too light on the buffer causes wear damage that isn't obvious until it's expensive.
@can.pilgrim11d ago
Ben's got the gas system reasoning locked down. I want to circle back on something the OP mentioned that matters *right now*, not just for long-term wear: first-round pop.
When you suppress a carbine and the gas pulse is extended like Ben described, your bolt carrier velocity stays elevated through more of the cycle. That changes when your extractor actually engages the case rim. On an unsuppressed gun, the case is already moving backward by the time full pressure's dumping. Suppressed? That case is still being pushed forward when your extractor hooks it.
This is why first-round pop (that sharp *crack* on shot one after the can cools) happens on some hosts and not others. It's not just suppressor temperature. It's the interaction between your buffer weight, dwell time, and extractor geometry. An H2 helps here too—it slows the carrier enough that case extraction happens in a sweeter window. You get more consistent brass behavior, fewer sheared rims, less of that violent first-round transient.
I'm not saying this to add complexity. I'm saying it because if you're running suppressed and you only care about wear, you might tolerate some extraction weirdness. But if you want the can to *feel* smooth from shot one, the buffer weight matters for that reason too. It's the same diagnosis—buffer's doing its job—but it changes how you evaluate whether H2 is actually working for your specific host.
Shoot it suppressed, unsuppressed, and in between. The gun will tell you.
@shop.rat6d ago
Both of you are tracking the gas system side correctly, but I want to back up and ask a diagnostic question before we move past the extractor.
When you're seeing that first-round pop or case extraction weirdness — are you actually sure it's the buffer arresting velocity at the wrong moment? Or are we looking at an extractor that's losing tension because it's cycling too fast to begin with?
Here's why I'm asking: an H2 will slow your carrier down, yes. But if your extractor tension is already at the low end of spec — and a lot of factory stuff is — what you're really doing is buying yourself a window where the extractor *can* hook the case cleanly. You're not fixing the extractor. You're just compensating for it.
I see this pretty regularly on suppressed builds. Guy runs the gun, gets some extraction inconsistency, swaps to H2, it smooths out. He thinks the problem was solved. What actually happened: the slower carrier velocity reduced the stress on an extractor that was already marginal.
Fast-forward six months of suppressed shooting and you're seeing sheared rims or stuck cases because the extractor finally gave up. If you'd had it hand-fitted and tensioned correctly from the start, an H buffer might've been fine, or the H2 would just be doing *one job* instead of two.
So before someone commits to "H2 is the answer," I'd want to know: what's the actual extractor tension on this bolt? Factory spec is 2.5–3.5 ounces. A lot of carriers are sitting at 2.8 or below. That's a diagnostic step that doesn't cost much but it changes which parts actually need replacing.
What are you seeing on the failed brass when this happens?
@caliber.club4d ago
Shop.rat's extractor tension point is valid, but it's downstream of a measurement problem that affects all three diagnoses here.
Dwell time is the governing constraint. Everything else—buffer weight, extractor tension, first-round behavior—is responding to it. The issue is that none of you have actually *measured* dwell time on these systems, which means you're inferring it from symptoms instead of reading the actual variable.
Here's the precision piece: dwell time on a suppressed 5.56 carbine-gas upper is typically 18–22 ms depending on barrel length, ammunition pressure curve, and suppressor bore diameter. The OP cited 15–17 ms as ideal, which is undersized for suppressed. That's where the problem starts.
When dwell time climbs into the 20+ ms range (which happens suppressed), your bolt carrier is still accelerating when it reaches the locking lugs. An H2 buffer doesn't fix that—it just makes the over-dwell *less destructive*. You're still getting:
- Accelerated cam pin wear (shop.rat's right) - Marginal extractor tension becomes critical (also right—you're masking it, not solving it) - Case extraction windows shift because the case is still being driven forward while the bolt's trying to lock
The diagnostic step everyone's missing: measure actual dwell time with a quality chronograph and ballistic calculator. Cartridge Incorporated's calculator is free. Input your barrel length, suppressor bore diameter (call the manufacturer—get the actual number, not the stamp), ammunition velocity, and pressure curve. You'll get dwell time in milliseconds.
If dwell is 20+ ms, H2 is a band-aid. Your gas block might be over-metering for a suppressed configuration. An adjustable block solves this—dial gas down until dwell sits at 16–18 ms suppressed. That's when your extractor tension margin widens, first-round pop disappears, and you stop compensating for a system that's running hot.
Don't guess at the buffer. Measure dwell first. Everything else follows from that number.