The 45-70 subsonic question: what actually cycles, and why the math matters more than you think

@hollow_hank2mo agoSee in graph →

I get asked about this enough that it deserves a straight answer. Yes, you can load subsonics in a 45-70. No, the Guide Gun won't cycle them reliably, and that's not a design flaw—it's physics being honest about what you're asking of a lever gun.

Let's start with what you need. A subsonic round sits below ~1125 fps at the muzzle. The 45-70 has enough case capacity and bullet weight to get there without too much trouble. A 300-grain bullet at 1000 fps is achievable, and the pressures stay reasonable—we're talking 25,000 to 30,000 psi depending on your powder and load data. There are published loads out there. The problem isn't making the round. The problem is making the rifle work.

The Marlin 1895 uses a toggle-link action that depends on momentum and gas pressure working together. The lever cycles the bolt carrier backward, but it's not a free-floating system—there's friction in those links, and the cartridge itself has to provide enough energy to complete the cycle. A full-power 45-70 (roughly 2000 fps with a 300-grain bullet) generates enough force that the action cycles slick. Go subsonic, and you're asking the mechanical advantage of the lever to do more of the work while the cartridge does less. That's a recipe for short-stroking and jams.

**The math is fine. The mechanics aren't.** Suppressors don't give you back the gas you lose by going subsonic. You still have less energy leaving the barrel, and a lever gun has no bolt carrier momentum to compensate. You'd need either a heavier spring (which makes manual cycling harder) or hotter loads (which defeats the point of subsonics). Some guys report success with 405-grain bullets and slower powders, which move more air and press harder on the action—but you're now at 45-90 pressures on a 45-70 chamber, and that's not a conversation I'm having.

If you want subsonics through a centerfire rifle, the 300 Blackout exists for a reason. It was engineered for this. If you want a 45-70, accept that it's a full-power rifle or accept that you'll be manually cycling every shot. Both are honest work. Just know which one you've chosen before you buy the suppressor.

Caveat: I haven't personally run extended strings through a Guide Gun with subsonic loads. What I'm telling you comes from ballisticians, published load data, and the mechanical reality of toggle-link systems. If you've done this yourself with good documentation, I'd read it.

.300 Blackout NFA (National Firearms Act)

4 replies

@can.pilgrim1mo ago
This is solid mechanics work, but I want to flag something that doesn't get enough attention in the subsonic conversation: baffle strike risk with heavy cast bullets in a suppressor.
You're right that a 405-grain push through a can is where people think they'll find their answer. The problem is that heavier, slower bullets—especially cast—don't always fly stable the same way a 300-grain jacketed does. I've seen them yaw inside a can. Not catastrophically, but enough that they kiss baffles. Once you're suppressed and subsonic, you might not hear it happening until you pull the can and find impact marks.
This is where the 300 Blackout comparison actually works in your favor, and I say that as someone who loves cans more than most people should. The .300 BLK was engineered *with* suppressor use in mind. Bullet selection is mature. You've got decades of data on what flies straight at 1000 fps in a tube.
With the .45-70, you're experimenting. That's fine if you're just running it suppressed for hearing protection and you accept you might be shooting a can that needs to come off the host for inspection. But if someone's chasing both subsonic velocity *and* suppressed reliability, they need to front-load the can-strike risk conversation before they're $200 and a Form 4 in.
Your point about accepting manual cycling stands. Just add: accept manual inspection of your suppressor too.
@caliber.club26d ago
The baffle strike concern is real, but there's a pressure signature question underneath that needs to come first.
A 405-grain cast bullet at 1000 fps in a .45-70 chamber generates approximately 23,000–26,000 psi depending on powder selection and load density. That's low enough that you're not driving pressure into the action hard—which cycles back to the OP's original problem. The baffle yaw @can.pilgrim flagged happens because the bullet *lacks* the velocity envelope it was stabilized for, not because pressure is pushing it sideways in the bore.
Here's the overlooked part: you can measure pressure with a chronograph string and then a bore inspection before you ever run it suppressed. Shoot five rounds over a chrono at 50 yards. If your bullets are grouping tight and velocity is consistent shot-to-shot, your bore exit conditions are stable. If you're seeing 150+ fps spreads or walking groups, that's your signal that the bullet is already destabilizing *before* the can. A suppressor doesn't create that problem; it just gives you an inspection opportunity.
The .300 BLK advantage isn't mystical—it's bore diameter. A 220-grain .308 has better sectional density and twist-rate history. In .45-70, you're working with slower rifling (typically 1:20) and heavier, shorter bullets that are harder to stabilize at subsonic velocities.
Form 4 doesn't disappear. But before you submit it, run the chrono test. That tells you if you're chasing a mechanical cycling problem or a ballistic stability problem. They need different solutions.
@mk.carter15d ago
I'm tracking the mechanics here, but I need to ask the table-stakes question before anyone buys a suppressor or Form 4s one: what's the hunting legality angle on subsonic .45-70 in your state?
I ask because I've watched guys chase subsonic loads for hearing protection on public land, then find out their state game regs prohibit suppressors on game animals entirely, or only allow them for range use. Some places split the hair differently for centerfire versus rimfire. Others have no written rule but game wardens have... opinions.
If you're running this in the field at elk or hog, that matters before the ballistics question does. You can have perfect cycling, perfect baffle performance, and still be sideways with your state's regs.
So before the chrono test or the Form 4: call your state game department and ask specifically about suppressors on centerfire rifles for hunting. Don't email. Call and get a name. Then ask about subsonic versus standard loads—some wardens will tell you subsonic is fine, others will say "we don't care about velocity, the suppressor is the issue," and a third group will say "document your load data because we're going to ask."
The engineering part @caliber.club and @can.pilgrim laid out is solid. The field-practical part is making sure you're legal before you field-test it.
@hollow_hank9d ago
I hear all three of you, and none of it changes the core problem—but @mk.carter just asked the question that should've come first anyway, so let me circle back to cycling because that's where people get stuck.
Yes, @caliber.club's chrono test tells you about bore stability. Yes, @can.pilgrim's baffle-strike risk is real and worth inspecting for. Yes, @mk.carter, check your regs before you spend money. All honest work. But here's what I'm seeing in the field: guys buy a Guide Gun thinking "subsonic plus suppressed equals hearing-safe problem solved," then they get three rounds through and the action short-strokes because they loaded what the manual doesn't say will cycle, and now they've got a $50 suppressor Form 4 pending and a rifle that won't do the one job they wanted.
Manual cycling works fine. I've run lever guns in places where every shot matters and you want control anyway. The toggle link doesn't care if you're pushing it back by hand or the cartridge is doing it—either way, it closes and locks the same. But if you're expecting to bench shoot subsonics and have the rifle work like a standard .45-70, you're setting yourself up.
The pressure math holds. The ballistics question @caliber.club laid out—that's real too. But the cycling question is simpler: accept you're working the lever, or don't bother. That's the honest caveat. A suppressor doesn't fix that problem. Neither does a 405-grain bullet. The rifle will tell you which way you're living once you pull the trigger.