Designing A Rifle From Scratch(ish)

[Sturgeon]

Despite being "done" I made a minor improvement to the FCG:

 

[Sturgeon]

Final-final weights (no magazine, no ammunition):

 

SCAR-alike config w/ ACR stock: 2.74 kg / 6.03 lb

14.5" / 13" w/ ACR stock: 3.05 kg / 6.72 lb

10.5" / 8.5" w/ ACR stock: 2.62 kg / 5.78 lb

[Sturgeon]

Detail of the dust cover:

 

[Sturgeon]

The four horsemen of the charging handlepocalypse:

 

 

 

 

[Sturgeon]

Weight of rifle with 14.5" medium weight barrel / 13" rail / ACR stock configuration: 2.98 kg / 6.57 lbs.

Weight Seahorse (same barrel profile):

 

(3.03 kg / 6.68 lbs)

 

So my design came out a tenth of a pound lighter than a comparable AR-15. Noice.

 

 

[Bronezhilet]

And now: Drawings, so you can actually make the thing!

[Oedipus Wreckx-n-Effect]

4 hours ago, Bronezhilet said:

And now: Drawings, so you can actually make the thing!

Pay up.

[CDNcommissar]

On 11/8/2018 at 4:59 PM, Sturgeon said:

Working on the gas system today:

 

Inspired by the HAC-7?

[Sturgeon]

28 minutes ago, CDNcommissar said:

Inspired by the HAC-7?

 

Nope. It's based on a Magpul patent.

[CDNcommissar]

1 hour ago, Sturgeon said:

 

Nope. It's based on a Magpul patent.

Neat!
Do you have a link to the patent handy?

[Sturgeon]

1 hour ago, CDNcommissar said:

Neat!
Do you have a link to the patent handy?

 

Here.

[Sturgeon]

There went uncorrected a minor error in my comparative weight calculations where I didn't account for the 12 rail covers. I weighed one, and each is 4.46 grams, all 12 are 0.118 pounds.

Which means the actual adjusted weight of Seahorse there is 6.56 pounds - slightly lighter than the F-4. Having said that, the AR has a lighter rail design, so I'm calling it even.

[Miroslav]

Hey mr Sturgeon-sir,

I'm slowly drawing up a rifle design as well, although I'm progressing much slower. It's meant to be in a traditional stock layout, so minimizing the overall receiver length is very important, as it directly increases the length of the entire rifle (unlike pistol grip layouts that let you extend the receiver and occupy the stock area behind the users wrist).

When I reread this thread I found the posts on firing pin in carrier vs firing pin in bolt. I've been going over the same thing over the last days, as I want the carrier to be as short as possible when the bolt is in the unlocked position. I tried to sketch a hammer that would curve over the bolt catch housing, but I couldn't make it look trustworthy. It rotated 90 degrees from the forward to the down position...

Long story short: I don't care about bolt/carrier mass ratio, because I have an op-rod where I can hide the carrier mass. How could you mount the firing pin in a rotating bolt in a more user friendly way than than the M14/AK punch pin? I've thought of having the bolt stem threaded and having the end of the bolt screw off when you want the firing pin out, but if the firing pin holds the cam pin in place, you'd need to screw apart the bolt each time you want to separate it from the carrier. As there's probably a lot of grime and gunk near the bolt, and as it's pounded by the hammer all the time, those threads are probably going to get quite stiff.

 

[Miroslav]

I found an answer to my own question, that I think could be satisfactory. The ARX rifles from beretta has a cut in the cam pin. The firing pin spring keeps the firing pin in a position that holds the cam pin in place, but if the firing pin is pushed in, the cam pin can be removed. I could do some variant of this.

 

I also need a way to keep the firing pin from reaching the primer if the hammer is dropped prematurely. With a carrier mounted firing pin, this is not a problem. With a bolt mounted firing pin, this has to be addressed. The M14 has a tab sticking out radially from the firing pin. That tab has to go through a slot in the receiver for the tip to hit the primer, but the firing pin rotates with the bolt, and the tab won't line up with the slot unless the bolt is in the locked position. The whole point of mounting the firing pin in the bolt was to shorten the bolt carrier, so I don't want a part of it sticking out. rearwards to block the hammer...

 

Any other ways of doing this than adding a bunch of small parts to the trigger group?

 

 

[Sturgeon]

13 hours ago, Miroslav said:

Hey mr Sturgeon-sir,

I'm slowly drawing up a rifle design as well, although I'm progressing much slower. It's meant to be in a traditional stock layout, so minimizing the overall receiver length is very important, as it directly increases the length of the entire rifle (unlike pistol grip layouts that let you extend the receiver and occupy the stock area behind the users wrist).

When I reread this thread I found the posts on firing pin in carrier vs firing pin in bolt. I've been going over the same thing over the last days, as I want the carrier to be as short as possible when the bolt is in the unlocked position. I tried to sketch a hammer that would curve over the bolt catch housing, but I couldn't make it look trustworthy. It rotated 90 degrees from the forward to the down position...

Long story short: I don't care about bolt/carrier mass ratio, because I have an op-rod where I can hide the carrier mass. How could you mount the firing pin in a rotating bolt in a more user friendly way than than the M14/AK punch pin? I've thought of having the bolt stem threaded and having the end of the bolt screw off when you want the firing pin out, but if the firing pin holds the cam pin in place, you'd need to screw apart the bolt each time you want to separate it from the carrier. As there's probably a lot of grime and gunk near the bolt, and as it's pounded by the hammer all the time, those threads are probably going to get quite stiff.

 

 

You might consider using a linear hammer arrangement like the vz. 58. In this arrangement, the hammer rides in the BCG rather than the receiver, and catches on a spring-loaded element in the receiver. Where it differs from a Mauser-esque striker arrangement is that it is a separate piece from the firing pin, and impacts it like a traditional hammer.

With this arrangement, you could have the hammer cock and make up the distance between the back of the BCG to the FCG. Then the hammer collapses into the BCG minimizing its length and therefore the length the receiver has to be to contain the action's stroke.

Linear hammers do have some disadvantages - notably they load in the forward stroke using energy from the recoil spring, rather than on the rearward stroke using propellant energy. This means if you're not careful about your spring weights and mass ratios you could end up having trouble with failures of the bolt to close. The vz. 58 for example sometimes experiences this.

 

As for firing pin retention (your actual question!) maybe try what Glocks have on their slide and use a sliding plate?

[Sturgeon]

13 hours ago, Miroslav said:

I found an answer to my own question, that I think could be satisfactory. The ARX rifles from beretta has a cut in the cam pin. The firing pin spring keeps the firing pin in a position that holds the cam pin in place, but if the firing pin is pushed in, the cam pin can be removed. I could do some variant of this.

 

I also need a way to keep the firing pin from reaching the primer if the hammer is dropped prematurely. With a carrier mounted firing pin, this is not a problem. With a bolt mounted firing pin, this has to be addressed. The M14 has a tab sticking out radially from the firing pin. That tab has to go through a slot in the receiver for the tip to hit the primer, but the firing pin rotates with the bolt, and the tab won't line up with the slot unless the bolt is in the locked position. The whole point of mounting the firing pin in the bolt was to shorten the bolt carrier, so I don't want a part of it sticking out. rearwards to block the hammer...

 

Any other ways of doing this than adding a bunch of small parts to the trigger group?

 

What does your bolt arrangement look like?

[Miroslav]

On bolt arrangement: I've been thinking I should go with a six lug bolt, with the lugs arranged like the seven lug bolt on an AR-15. This is inspired by the recessed lug that some manufacturer (I've forgotten which one) uses on the lug opposed to the extractor. As far as I can calculate, this setup is plenty strong. Bolt carrier should look kind of like a .308 AR carrier cut off behind the firing pin and instead of a gas key it has a protrusion where the op rod/piston hits it.

 

I have some vain dreams of spending a ton of money to have a prototype of this rifle built. As I'm based in Europe, I don't have a cottage industry of AR-15 manufacturers that I could pay to make me a barrel extension with a broach. Because of this, I'd have to CNC mill/lathe both the bolt and barrel extension (and then maybe heat treat and remill, I don't know, not really an expert on metallurgy).

 

Right now I'm leaning towards making the housing for the bolt catch be less obtrusive, by moving it to the side (with a little arm that reaches in towards the spot where the follower actuates the bolt catch) so that the hammer doesn't have to reach too far to hit the firing pin. I think the bolt carrier/hammer interaction during the recoil stroke will work out. The hammer will be pushed way deep down by the bolt carrier, but I guess you could let it follow the BC by a small bit before it's caught by the disconnector and later held by trigger. This will reduce the distance the hammer will have to travel upon firing to make it to the firing pin. Of course, I haven't calculated the hammer travel speed, so I don't know how big of an effect it would have on lock time, which in itself is not that important. All in all, this would mean keeping the AR style carrier/firing pin. If I can't make a trigger group/hammer that seems plausible enough, without compromising receiver overall length, I'll have to go with a different arrangement for the carrier.

 

The linear hammer thing is interesting.

 

Civvie semi autos usually have the bolt catch by one of the front corners of the magazine. I suspect they do that to keep the receiver short. They also often have a bolt carrier that is shorter than the long action ejection port, but with a telescoping dust cover on the carrier.

[Sturgeon]

Enlarged the ejection port and added the gasket (finally):
 

 

 

 

[Sturgeon]

[Sturgeon]

Dat feedway:

 

[Miroslav]

Yeah that looks like quite the distance. Is it meant to reduce the angle the cartridge has to tilt to get from the double stack to the center position?

[Sturgeon]

1 hour ago, Miroslav said:

Yeah that looks like quite the distance. Is it meant to reduce the angle the cartridge has to tilt to get from the double stack to the center position?

 

It's actually just there for disassembly and to give enough thickness to the lower. You could probably shorten things, if you wanted.

[Alex C.]

I find it interesting that the charging handle has given you the most trouble on this project and in the end you went with a conventional design. IMO your L1A1 style one that requires the cutout in the endplate is terrific (especially if you could incorporate the L1A1's folding mechanism that folds flat after charging).

 

 

[Sturgeon]

7 hours ago, Alex C. said:

I find it interesting that the charging handle has given you the most trouble on this project and in the end you went with a conventional design. IMO your L1A1 style one that requires the cutout in the endplate is terrific (especially if you could incorporate the L1A1's folding mechanism that folds flat after charging).

 

 

 

It would have been terrific but it prevented the use of ACR stocks, which is what I was going for. The ACR stocks were too fat.

[Sturgeon]

7 hours ago, Alex C. said:

I find it interesting that the charging handle has given you the most trouble on this project and in the end you went with a conventional design. IMO your L1A1 style one that requires the cutout in the endplate is terrific (especially if you could incorporate the L1A1's folding mechanism that folds flat after charging).

 

 

 

I also knew the charging handle would be a big issue (they've given me issues on previous less-developed designs), but I still managed to underestimate it.