CraigC
Sixgun Nut
Cool, keeping an eye on this one!
Another scratch build; Ultralight folding .22 semi-auto.
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MachIVshooter
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Bolt is done
To my knowledge theres never been a published "ruling" from the ATF saying categorically no open bolt semi auto guns. They dont approve them from import as it is their prerogative. There were those several specific rulings in the early 80's mostly aimed at Macs/cobrays etc.
https://www.atf.gov/rules-and-regulations/rulings
It should be mentioned that BATF "Rulings" do not actually carry the force of law but when you playing on their playground they don't take it kindly if you don't play by their rules. As with every law or "rule" its a risk to benefit analysis.
https://www.atf.gov/rules-and-regulations/rulings
It should be mentioned that BATF "Rulings" do not actually carry the force of law but when you playing on their playground they don't take it kindly if you don't play by their rules. As with every law or "rule" its a risk to benefit analysis.
Legal issues aside, I expect you'll find it a challenge to get a decent trigger pull from an open bolt semi.
In a closed bolt the job of controlling the bolt can be divided between two springs, the main recoil spring and the hammer / striker spring. The hammer / striker spring can be just heavy enough to provide reliable ignition, which promotes a good trigger pull.
In an open bolt, the entire job is done by ONE spring and the trigger has to contend with it, resulting in having to deal with a MUCH stronger spring than it would with a closed bolt. You need lots of engagement area to cope with it and combined with the heavy spring it results in a long, creepy and heavy trigger pull.
In a closed bolt the job of controlling the bolt can be divided between two springs, the main recoil spring and the hammer / striker spring. The hammer / striker spring can be just heavy enough to provide reliable ignition, which promotes a good trigger pull.
In an open bolt, the entire job is done by ONE spring and the trigger has to contend with it, resulting in having to deal with a MUCH stronger spring than it would with a closed bolt. You need lots of engagement area to cope with it and combined with the heavy spring it results in a long, creepy and heavy trigger pull.
chicharrones
needs more ammo
Hmmm. Thinking of spring piston airguns, they are somewhat related to the issue of an open bolt being held by the trigger mechanism. What I mean is that a spring piston airgun has the strongly sprung heavy piston being held by the trigger mechanism.
Some of those guns have heavy, creepy triggers and some don't. And for those that do, a Charlie Da Tuna trigger fixes it.
My point as an observer is, that with good trigger leverage ratios and adjustable sear engagement, it would seem that an open bolt firearm with a decent trigger is possible.
The tough part may be all that mass in the bolt moving when the gun is fired and having precise enough follow through, on the part of the shooter, to be as accurate in actual shooting as one would like.
Just thinking out loud.
Some of those guns have heavy, creepy triggers and some don't. And for those that do, a Charlie Da Tuna trigger fixes it.
My point as an observer is, that with good trigger leverage ratios and adjustable sear engagement, it would seem that an open bolt firearm with a decent trigger is possible.
The tough part may be all that mass in the bolt moving when the gun is fired and having precise enough follow through, on the part of the shooter, to be as accurate in actual shooting as one would like.
Just thinking out loud.
Because of the heavy spring, you can't get away with the tiny amount of sear engagement you can with a closed bolt. It may be possible to get a decent trigger, but it will be a challenge.
FWIW, I have 4 various open bolt 22s. I've polished and adjusted all of them and triggers still range from poor to marginally acceptable.
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MachIVshooter
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Well, I tried to rotary table the receiver bore with a 7" long 4" LOC 1/2" carbide 4 flute end mill. Kinda expected significant deflection, and I was right; I have a hole that is .751" at the bottom and .768" at the top. Oh well. Reamer ordered, will try again next week.
Would a hammer-head type of grip be better for your design?
Obviously, you could design your own; maybe something with lines that flowed better with the rest of the gun (you also wouldn't machine attachment points at the rear).
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MachIVshooter
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Nah, those things are fugly!
I find many of the AR grips out there very comfortable and ergonomic, and building the rifle to use them saves me having to design and shape one. This thing is going to be very mechanical in appearance, so the utilitarian style of the AR grip works.
I do, however, have an opinion question: Should I stick with my original plan of a dovetailed rear sight mount, or machine a picatinny rail atop the receiver? I'm kind leaning toward rail right now
badkarmamib
Member
The rail would give you more optics mounting options, and, judging from your past photos, you are up to the task... Depending on what you plan to do, you could add a rail further down the barrel, so you could use AR-style flip-up sights to create a lower folded profile.
Is this going to be an irons-only rifle? If so, I'd drill and tap the receiver for Tech Sights or similar. It will take extra material to machine a rail on top, and extra material is extra weight. If you intend to mount an optic on the gun, machining a rail into the receiver will probably be beneficial.
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MachIVshooter
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It will, but not much. I'll also run a 3/8" .050" chamfer end mill through the middle of the rail, like I did on this AR:
I took the time to draw my receiver in paint, since it's a pain to produce multiple copies for the interior & exterior profiles, plus internal parts overlays. Doing a pic rail will also smooth out the top profile, rather than having a hump to transition into the upper barrel block lug. This is the basic outside profile now:
And showing fire control parts, bolt, and internal cuts for bolt bore and magazine well. As before, showing everything in the fired position. Obviously, the sear cut in the striker will be about 1-1/2" forward of it's representation in the image:
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chicharrones
needs more ammo
I prefer a rail anytime to make optics mounting easier.
However, if your gun barrel folds upwards and lays on top of the receiver, then that optic will interfere with the fold going a full 180.
However, if your gun barrel folds upwards and lays on top of the receiver, then that optic will interfere with the fold going a full 180.
Bobson
Member
I like the idea of drilling for tech sights. Rails and optics are great, and if this was anything but a .22, I'd say go for it. But this build, to me, sounds like more of a minimalist workhorse. Idk why, but to me, optics on a rifle just make it look high maintenance. Not necessarily in a bad way, just in a way that detracts from "minimalist workhorse."
jmorris
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I am pretty sure you can pull off your goals with what you have. I pulled out one of my AR-7's and the receiver is not what I would call bare minimum and things like safety, trigger, mag release and pep sights are made of steel and don't have to be and it still comes in at 14.8 oz. The barrel, front sight and locking collar are another 8oz. That would give you 9.2 oz for stock tube and grip even if you made the receiver that heavy.
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MachIVshooter
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Being held up on the receiver waiting for my reamer, I decided to go ahead and make fire control parts. Also remembered to snap a couple of in-process photos so those who are curious may see how something like an AR-type selector is made on manual machines. Lots of careful rotary table work!
Add then the rotary indexer is used to locate and machine the detent holes & channel:
The trigger is 7075-T651, connector and sear are 4140 steel, which I opted to harden & temper using a mapp gas torch and color indication
And overlaid on my drawing
Still have to cut the sear notch in the striker, and haven't made the mag catch yet. Also have to make the spring for the connector, and do some hand finishing to make the trigger comfortable. But that about does it for internal parts. Far fewer than my O/U pistol build!
Add then the rotary indexer is used to locate and machine the detent holes & channel:
The trigger is 7075-T651, connector and sear are 4140 steel, which I opted to harden & temper using a mapp gas torch and color indication
And overlaid on my drawing
Still have to cut the sear notch in the striker, and haven't made the mag catch yet. Also have to make the spring for the connector, and do some hand finishing to make the trigger comfortable. But that about does it for internal parts. Far fewer than my O/U pistol build!
Yes, as a matter of fact I have. The point is not that firing pin springs are in some way weak, but that open bolts require the trigger to deal with even MORE spring pressure.
jmorris
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I have some bolts that require tools to help over come the spring tension to get them apart, a lot more force than my open bolt SMG's require to operate.
One is a lot of force on a tiny bit of mass the other is a lot less force on a relatively huge mass.
Doesn't really matter for the op at this point though, looks like he is just about done with that part of the project and nicely done at that.
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MachIVshooter
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Trigger profiled & finished, magazine catch and barrel latch made. Mag catch is 7075, barrel latch is 6/4 Ti
barnbwt
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There's a bit more to it than that;
-Open bolt sears tend to be subject to more impact force catching the rebounding bolt, requiring a lot of surface area to carry it
-Open bolt sears tend to have a lot of overtravel engagement to ensure the bolt can't easily override or jar off the sear
-Open bolt trigger systems tend to be very simple, with little mechanical advantage pulling the sear down
So you end up with a long, rough, and hard trigger, followed by a rather slow lock time & shifting of the weapon's CG with each shot. Not the best for accuracy, but don't tell that to Winter War Finns picking off/mowing down Russian advances with subguns (the previous points are a big reason early subguns were so heavy, to help offset the trigger & open bolt foibles)
TCB
chicharrones
needs more ammo
Amazing work, MachIV. 
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MachIVshooter
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Reamer showed up early, so while the boy was napping, I decided to get a start on the receiver. Had to square up a 1.55x3.25x7.65 block of 7075 first, and reduce width to 1.25". I started it the same way, using the 7" carbide end mill on the rotary table, as I've no twist drills that large with 6" of reach. Made a hole within 0.010" of final diameter at the top, then took the chunk off the rotary and put it in the vise to ream. This time we have a perfectly straight .750" hole
Good thing the block wasn't any taller, as this took pretty well all the room I have on Z axis. A hole much deeper would've required the 90° head, which would also mean removing the rotary table and vise. I prefer not to!
Anyway, he was still out, so I got the two radii on the front cut as well
By the time I finished that, though, he was stirring, so I'll have to get back after it once he's down for the night.
Good thing the block wasn't any taller, as this took pretty well all the room I have on Z axis. A hole much deeper would've required the 90° head, which would also mean removing the rotary table and vise. I prefer not to!
Anyway, he was still out, so I got the two radii on the front cut as well
By the time I finished that, though, he was stirring, so I'll have to get back after it once he's down for the night.
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