375 H&H AR

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45flaco

New member
Hey guys. Built my 375 H&H AR, now just have some minor work to do before hopefully test firing this weekend.
Just need to make a handguard, install detents, safety, and FA, and grind feed ramp a bit.
Its a left side, forward charging piston.
Handle does reciprocate, as it is directly attached to the piston.
Gas port is 18 inches from the back of the barrel extension, gas port is 0.082", because I figure the 375 has a lot longer pressure curve, and piston rifles don't tend to need as much gas to operate anyways. If it doesn't work, I'll drill it out a bit. But I don't think it will be necessary.
 

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stagpanther

New member
An AR in 375 H&H?? You have a wilder imagination than I do!! I'm assuming you must have started with something like a noreen lower that gets you into non-standard cartridge length? I can't even imagine how to engineer a magazine to work with that. :eek::D:eek: Did you take a look at the 375 Ruger? Easy to work with cartridge (though pretty wide so there may be an issue with bolt face).

That said, I love your idea--I would have never have thought of that.:D
 
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45flaco

New member
So, it chambers a roudn from the magazine, fires, but doesnt eject under its own power. Basically a magazine fed straight pull currently.
@stagpanther, I actuallybdidnt start with a Noreen. I would have liked to, but their rifles cost more than my truck. So I started with 14 dollars of 1/8" steel flat and some castle nuts for a lower. The magaxine is from Wyatt's Outdoors, although any 3.850 CIP/AICS magazine will work, even the savage ones with a little modification to the locking tab.
I did think of 375 ruger, but Ive done a bolt rifle in both calibers, and the 375 H&H just feeds much smoother. The H&H is longer, more tapered, and has a minimal shoulder, which means it feeds beautifully in prettyvmuch any rifle. The main benefit of the Ruger is that it fits a 3.34" magaxine. But since Im doing everything custom, that wasnt really a concern.
 
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45flaco

New member
stagpanther said:
An AR in 375 H&H?? You have a wilder imagination than I do!! I'm assuming you must have started with something like a noreen lower that gets you into non-standard cartridge length? I can't even imagine how to engineer a magazine to work with that. :eek::D:eek: Did you take a look at the 375 Ruger? Easy to work with cartridge (though pretty wide so there may be an issue with bolt face).

That said, I love your idea--I would have never have thought of that.:D
Click to expand...
If I had started with a Noreen lower, the 375 Ruger would have likely been my go-to, as I think they make a 3.34" length action.
 
4

45flaco

New member
4V50 Gary said:
Position of the gas port and gas port size is important. You're a pioneer! Keep us informed.
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Yoy are correct, gas port placement was a lot more important than I thought. Im used to building AKs, where I just pick a piston and gas tube length that looks cool, and drill port accordingly.
I have sinced moved the gas port to 15.5", standard rifle length. And am trying tp rework the piston.
 
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stagpanther

New member
So you haven't actually achieved auto-feed/cycling with it?

375 Ruger would have likely been my go-to, as I think they make a 3.34" length action.
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The ruger does work in a conventional long action--and is also a fairly straight-forward case design--except it is "fat" and my guess would require some artful work on a magazine to work. It would also need a magnum type bolt, I'm not familiar with such a thing in AR's.
 
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45flaco

New member
stagpanther said:
So you haven't actually achieved auto-feed/cycling with it?

The ruger does work in a conventional long action--and is also a fairly straight-forward case design--except it is "fat" and my guess would require some artful work on a magazine to work. It would also need a magnum type bolt, I'm not familiar with such a thing in AR's.
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Have not got it cycling. Right now it's a straight pull. I have a huge dislike of the 375 Ruger after trying to build a bolt action for it, and it took a LOT of work to make it feed. Whereas 375 H&H took nearly zero effort to work in a mosin, and the AR was maybe 10 minutes of polishing a feed ramp and bending a 338 LM magazine's feed lips to fit. Feeds smooth as glass.
The 375 Ruger could use the same bolt head as the H&H, as it was meant to be a mini action knockoff. They're both 0.532" rim diameters. But since it's only advantage is fitting in a standard action, I'd rather opt for smooth feeding and extraction, and the worldwide availability of the old H&H.
The difference in cost between making a 3.34" AR and a 3.85" is about 3/8 of a cent of sheet steel.
Both would be full custom, as I'm building anyways, not buying.
 
4

45flaco

New member
Feeds wonderfully now. But can't get enough pressure through my homemade gas block to eject the old case. So it's sitting at a CNC shop while some guy designs a better gas block and also a charging handle, since he's already got it I the shop
 
4

45flaco

New member
Got tired of waiting for the CNC guy to make a gas block for it.
Took the rifle home, modified some parts from autozone and put an adjustable gas block on it. Gas tube is just 3/16" brake line lol.
But it works beautifully now.
Test fired with 20 rounds of factory Hornady DGS ammo, 10 rounds of 200 grain sierra handloads, and some random cast bullets.
They all worked decently.
Then I loaded up some DGS above max, as kind of a proof test.
Fired 2 of them with gas block turned almost off.
Still cycled, but kicked like hell. Have included a link to that video, will have more organized testing once I'm finished with current job.

https://www.youtube.com/watch?v=bLe0Px2VWv4

The sparks are because my previous gas port at 16" was plug welded, but the proof rounds blew that plug out. Eventually I'll just start over with a new barrel now that I know what distance to put the gas port at.
 
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stagpanther

New member
Your's is about the most ambitious and coolest build I've ever seen on this forum--my hat is off to you! Among other things, just getting those bullets to stack and feed from a magazine must have been a major engineering feat in itself. I built a 284 win AR and it works great--but I can't get more than 3 cartridges at a time to successfully feed from a magazine due to magazine bulge.

So what did you end up using for a lower?
 
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45flaco

New member
Made the lower myself, welded some steel together.
I do have plans for a CNC cut version at some point, as my lower has the buffer tube tower slightly misaligned.

As far as feeding, it wasn't actually a terrible chore, although the lack of factory box magazines was a bit irritating. Using a modified 338 magazine from Wyatt's Outdoors, and it's a single stack so I had to modify the barrel extension much the same as you would see on a .50 beowulf.

Also had to make a new paddle-style mag catch for the new mag. Being intended for a bolt action, there was no way to put a side bolt catch on it
 
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45flaco

New member
I did incrementally open it up, and unfortunately it still didn't like some ammo even at 3/16" port diameter, which is where I stopped. Ended up moving it further up the barrel to get higher pressures, which means I'm now changing buffer weights and spring.
I think the issue is that some H&H ammo is loaded to drastically lower pressure than others.
 
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45flaco

New member
I've found that it likes factory DGS ammo, loves whatever the hell PPU puts in their 300 grain, and doesn't like Federal at all. I think federal loads to like a 375 winchester/38-55 level, as there's noticeably less recoil and they don't want to cycle. Hopefully the new, shorter gas system fixes this, it seems promising so far
 
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