1/2 scale AR-15

[MachIVshooter]

It's 1:30 AM and I just blew the aluminum dust off myself, so this is only going to be a teaser thread for now

Decided this afternoon that I needed a new project, and this seemed as fun as anything. Started off with a hunk of .780" 7075-T6, just went to town with a schematic in hand.

First hour:



and this is where we're at after about 7 hours on the mill:





It is going to become a functional firearm, chambered in .17 HM2. Waiting on a reply from ATF FTB about whether or not a buttstock would make this an SBR, being as this firearm will be designed and intended to be operated as a pistol, a 5.2" long 1/2 scale stock being aesthetic only, not a functional stock at all. We'll see, hopefully I don't have to tax stamp this critter, but I'll do what I have to.

I have a long, looooooong way to go, but it's off to a good start. I will do a full detail build thread as things progress!

 

[ArchAngelCD]

Boy I'm betting a good CNC machine would cut a lot of time off that project.

Looks good, please keep us updated and keep the pictures coming...

 

[The_Next_Generation]

Awesome! I'm excited to see where this goes!

 

[silicosys4]

Neat! What are your plans for the barrel? Are you going to find a donor barrel and turn it down, then cut and rechamber it? I'm interested to see how you approach the magazine too, I hope its still a 30rd double stack that looks like a mini AR mag
I'm thinking it would be interesting to try and keep the internals as true to the AR15 as possible, somehow re-engineer a rimfire firing pin and rimmed extractor into a shrunken AR bolt, but it might be easier to shrink one of the existing rimfire lookalike's internals, or shrink a .22lr conversion bolt...would that then be strong enough? Blowback or GI...it would be cool as heck to see it stay GI, lock up and everything.
Very cool, can't wait to see where this goes.

 

[chicharrones]

Looking forward to seeing all your hard work, MachIV.

 

[MachIVshooter]

Boy I'm betting a good CNC machine would cut a lot of time off that project.

Only if you already had the CAM file. CAD/CAM is very painstaking.

Truth is, so long as you're dealing with straight lines and symmetric radii, manual machines are faster for one-off stuff. Crazy, swoopy lines are very difficult on manual equipment, but there really aren't any on an AR receiver.

What are your plans for the barrel? Are you going to find a donor barrel and turn it down, then cut and rechamber it?

Yeah, ideally I'll find one that's already .17 HM2 chambered and just turn it into an M4 profile at 7.25" long.

I'm interested to see how you approach the magazine too, I hope its still a 30rd double stack that looks like a mini AR mag

It'll be slightly staggered, but no where near 30 rounds. The rim makes that impossible in a mag that even resembles a 1/2 AR mag.

I'm thinking it would be interesting to try and keep the internals as true to the AR15 as possible, somehow re-engineer a rimfire firing pin and rimmed extractor into a shrunken AR bolt, but it might be easier to shrink one of the existing rimfire lookalike's internals, or shrink a .22lr conversion bolt...would that then be strong enough? Blowback or GI...it would be cool as heck to see it stay GI, lock up and everything.

I do plan to scale down lower internals, and the bolt will be a homogenous piece that closely resembles a 1:1 AR BCG. It will be blowback, though. Trying to incorporate a gas system on this scale and with rimfire is a little more than I feel I can handle.

 

[Willie Sutton]

Did you scale drawings for dimensions? Any particular ratio if so? Use the same ratio for longitudinal dimensions as for the radius dimensions?

Beautiful work, and fully agree with using a manual mill for one-off work like this.

Willie

.

 

[MachIVshooter]

Did you scale drawings for dimensions? Any particular ratio if so? Use the same ratio for longitudinal dimensions as for the radius dimensions?

Everything is 1:2. I didn't actually have a 1/2 scale drawing; just split numbers from a 1:1 scale schematic as I went. Most of the radii are true to the 50% dimensions, except those for which I did not have an exact cutter. The large radius at the extension housing is .750", so should have been .375" on this, but I damaged my 3/4" collet and had to use a 13/16 cutter, so that radius is .406" instead.

Fortunately, the AR is a pre-CNC design, and that is reflected in the dimensions. Not a simple shape, mind you, but pretty easy to duplicate on manual machinery with a rotary table, tilt/swivel vise and a decent selection of cutters.

I had a lot of down time waiting on parts today, got quite a bit more done. About all that's left is tapping the RE housing (which will be 5/8-24 NEF), getting a long enough 0.040" drill to do the BHO pin hole, run a 1/4" cutter through the front take down pin boss, and then lots of tedious sanding to get tool marks out.







Gonna have to use a trigger shoe, I think:



And boy is it light! Just 40.5 grams (1.4 ounces)

The extension housing was a bugger. I should have done that part earlier, when there was more material to bite. As it were, I had to do the best I could with a micro vise on the rotary table:





Wasn't easy. Dang thing kept tilting with even a little pressure, so I ended up taking cuts of .005"-.006" at a time. Well, that table only turns 4 degrees per handle crank. That's right-90 handle turns for one table revolution. So I decided to power it with my cordless drill. Worked great until my finger got in the way of that handle flying around at 1,200 or so RPM. A bit tender now.

So, I'll probably get started on the upper receiver in the next couple days here. Will continue to update as progress is made!

 

[rcmodel]

Keep going!

When the swelling goes down in your finger, it will probably fit in the tiny trigger guard again!

Outstanding work sir!

BTW: Make it a .25 ACP!

The HM2 might be a little energetic for a blow-back.
The .25 is purfect for that, and rimless too, so a higher cap mag would be possible!

rc

 

[herkyguy]

one of the coolest threads in a long time......

 

[Stacer]

Totally want to see where this one goes

 

[ifit]

WOW...now that's awesome, thanks for sharing

 

[Darkbob]

Do you have any idea how long it will take the ATF to respond regarding the stock?

 

[19-3Ben]

I love that without the context, this image could also look like a GIANT man holding a standard AR receiver.

 

[jerkface11]

Is it going to be gas operated?

 

[Walkalong]

Seriously cool.

 

[Potatohead]

Wow! Wish I possessed some sort of skill like that! I'll take anything-

 

[MachIVshooter]

Is it going to be gas operated?

No, just blow back. At least for now. While most of this build is able to be done by just scaling everything down 50%, there's a lot more to it with the gas system, much of it stemming from the .273" .17 HM2 rim being 73% the diameter of the 5.56, which would make it very difficult to incorporate a rotating bolt able to house & extract a cartridge of .273" diameter while still fitting in a carrier only .465" across. It would also mean machining some very tiny parts and then having to heat treat them; The tiny parts machining I can do, but hardening and tempering I'm not well set up for. I have little more equipment for that than a 19th century blacksmith, and a lot less experience. I have some good steels, but none that would be appropriate for a miniature AR bolt without heat treating & tempering. Blowback, however, can use even mild steel. Of course, I'll be using 4140PH stock.

Also, due to the .17 M2 chambering, the barrel profile will be a bit heavier than true 1:2 scale, and leaving enough meat around the chamber means using a 3/4-28 thread instead of the scale appropriate 5/8-36. But because the delta ring and barrel nut can be one piece, the exterior dimension should still be able to be made accurate.

Do you have any idea how long it will take the ATF to respond regarding the stock?

Well, they emailed me back and said it will have to be a snail mail submission, to which they will respond in 30-90 days. Yay.

I'll SBR if I have to, but I think it would be pretty asinine for them to qualify something like this as a rifle when it is not only meant to be fired as a pistol, but would be nearly physically impossible to use as a rifle with a 1/2 scale buttstock. About as much useable "stock" as the commercially available title I mares leg pistols.

 

[Sebastian the Ibis]

Wow! You have some talent....

I'll SBR if I have to, but I think it would be pretty asinine for them to qualify something like this as a rifle when it is not only meant to be fired as a pistol, but would be nearly physically impossible to use as a rifle with a 1/2 scale buttstock. About as much useable "stock" as the commercially available title I mares leg pistols.

I'd include a copy of the photo above of your finger in the lower. I didn't understand how small it was until I saw that.

 

[Darkbob]

Well, they emailed me back and said it will have to be a snail mail submission, to which they will respond in 30-90 days. Yay.

At least their e-mail response was fairly quick.

Hey, if they make you SBR it, you could mock up a 1/2 size Sig arm brace to use until the stamp comes back.

As was mentioned above, I wish I had a small amount of the skill it takes to do this. Impressive stuff to me.

 

[SleazyRider]

Subscribed! You are my new machinist-hero --- keep us updated!

 

[Shear_stress]

By far the coolest firearms project I've seen in a while. Sort of a "tactical" spin on the .17HMR (not HM2) double rifle made by Peter Hofer in Germany:

http://www.hoferwaffen.com/hofer_52s.php?id=14&lang=en

Please keep us posted as the project comes together.

 

[MachIVshooter]

Well, today was trigger day. I actually started on the upper receiver, then realized that my smallest boring bar is too large for a .465" hole, so had to leave that alone with a 7/16" hole poked through a block of 7075-T6:



Will resume when the new mini boring bar arrives.

Anyway, I realized how little attention I'd actually paid to the intricate design of the AR trigger. To make one from a piece of stock is a whole lot of different and time consuming operations.

Started out with a piece of .500" D-2 tool steel, and cut a small area down to the proper width. D-2 actually cuts really nice, but you have to be generous with lube/coolant, or it will eat even carbide cutters in short order.

Just as with the lower, when working with these tiny, multi-angled pieces, it's best to leave them attached to the rest of the squared-up work piece for as long as possible:



Profiling radii is easy on a rotary table.........after you've spent forever screwing with the set-up. I'm fortunate, though; with a cross slide table, it's relatively easy to center up, and even easier to move to another spot on the work piece.



One more cut, and it's free!



Finished piece:



The channel cut in the top side of the trigger was fun. Moving a 1/16" end mill through tool steel is a bit of a nerve racking experience. Amazingly, I did not break a cutter.



And installed, after cold blue. The pin is a .450" section of .081" drill bit:



There is a spring installed, too. That was actually not too bad to make by simply uncoiling a small spring I had in my misc. spring drawer and reprofiling it to match the AR trigger spring. I did manage to send one end of it a good way into my index finger, though. Doh!

If all goes well, tomorrow will be..........wait for it.........hammer time!

 

[shotgunjoel]

At the end of this I'd be very curious the total cost in time, material, and tools to finish this project. You should keep track.

 

[MachIVshooter]

At the end of this I'd be very curious the total cost in time, material, and tools to finish this project. You should keep track.

Time I definitely intend to document. So far:

Lower: 13 hours
Trigger: 5 hours

Tooling was mostly on hand already. All I've had to order so far was $40 worth of 3/4-28 tap & die, and $60 boring bar.

Materials, again, mostly on hand, difficult to calculate cost. The only real expense is going to be a barrel, which unless I find a good deal, I'm looking at about 3 bills for a .17 HM2 10/22 tube.