4.375in DI 9mm AR

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propeine
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4.375in DI 9mm AR

Post by propeine »

So I wanted an MPX but I'm a cheapass. I had an SBR AR-15 lower already and a spare upper. The following story takes place over the course of 3-4 months with more tribulations than are detailed here but...

I purchased
A prechambered 9mm barrel from Ebay for $17
An AR-15 Barrel extension from brownells for $20
A spare AR-15 bolt for maybe $65
A KAK magwell adapter for $100
Some ASC mags for $14 each.

First, I ran a bunch of quickload simulations. My standard 223 load (25gr of H335 behind a 55gr bullet) through my standard SBR barrel has the gas port at 7" and the exit at 10.5". I took the quickload pressure data for this and imported it into excel. I then did a polynomial line fit, took the integral from 7-10.5 to figure out the "work" done by the gas on the BCG. Using this, along with known gas port sizes, I could reverse engineer the work (engineering version) done to send the BCG to the rear.

Then I started building 9mm load data around 147gr bullets and doing the same process to try and match the work done by the gas. I settled on a 9mm load with 4.8gr of Herco and the gas port .7" from the barrel extension. I went with a full .125 for the gas port.

Using this info, I could profile the barrel, fit the extension, thread the barrel, and drill the gas port. So I did. That left me with this.
Image

I needed a gas block that would have the gas tube be straight due to how close it was to the barrel nut. So I made one. Technically an AR10 gas block would have worked but I'm really cheap, I made the gas block diameter .936 instead of .750 which was dumb and I've been too lazy to recut it.
Image

Then the bolt needed opened up to fit a 9mm. That was straightforward so no pictures. Suffice to say I chucked it up in the lathe, indicated it in and skim cut it.

Now the KAK magwell adapter needed modified. Magwell adapters AND magazines are set up for blowback operation which leaves them sitting too high to clear a standard AR-15 BCG. I measured the height of a standard 223 mag to the top of the magwell catch, cut (with a dremel) the ASC Colt style 9mm mags to the same height, and cut the magwell adapter until it all fit the way I expected. Now the feed lips of the 9mm mag sit at the same height as the feed lips of a 223 mag. Not much to show here but I can give details if anyone is curious. Since 9mm and 223 are close enough to the same diameter, it strips rounds as expected and as long as the BCG is released from a full rearward position, the rounds jump into the chamber. If you do it slow, or the BCG doesn't go far enough back, the 9mm round turns and jams up the gun.

This is what it looked like without the gas block installed but I can't find a picture right now with it on.
Image

So with all that out of the way, I could feed ammo from the mag but hadn't test fired. Off to the range I went. Wouldn't cycle even with a suppressor on it with my AA7 loads I use for pistols and there was powder everywhere in the action (makes sense since there is only 74% burn according to quickload vs 98% for Herco). So I took all the weights out of the buffer and 4 coils got chopped off the recoil spring. Today it cycles 100% with a suppressor on and about 70% without. Seemingly providing a stiffer surface to recoil against makes it run better as the girl firing below had more jams than I did and I lean into it a hell of a lot more. It gets dirty faster than a 223 but I've put a few hundred through in a range session with it still cycling. Videos below (not me shooting)

Unsupressed - jams 2nd round due to BCG not going far enough back
http://vid1077.photobucket.com/albums/w ... hqjq7h.mp4

Suppressed - empty mag at the end
http://s1077.photobucket.com/user/prope ... sort=3&o=3

There are 2 more things to do for full functionality and one more thing to try for kicks
1.) Make a krink style "brake" to increase backpressure when unsuppressed
2.) "Pin" the false lug so that the 9mm brass doesn't drag on the barrel extension during extraction. According to another fella with a similar setup this is the primary cause of the inconsistent force sending the BCG to the rear. You can see vampire marks in some of the brass where it hits the lugs.

For kicks, I may try some modifications to a magazine as shown below and avoid the magwell adapter all together. I would give up last round hold open but the BCG wouldn't have to travel as far either which should increase reliability. Plus less pissing around when switching back to 223 is a good thing and the mags are already custom so it isn't like I can just grab a new colt pattern mag and slam it in.
Image

Also, "normal" rails tend to have barrel nuts that are too long to fit in front of the gas block so that has been problematic. Matrix Arms barrel nut clears with easy but I mangled the 7" rail I bought when trying to cut it down to 4". I'll get around to it at some point.
BlogSarge
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Re: 4.375in DI 9mm AR

Post by BlogSarge »

Good God, man!

I admire your stick-to-it-ness and ingenuity! But just one word...CMMG!

Seriously, that's a hell of a project and I applaud you.
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Re: 4.375in DI 9mm AR

Post by poikilotrm »

I envy you for your skill set.
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propeine
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Re: 4.375in DI 9mm AR

Post by propeine »

BlogSarge wrote:Good God, man!

I admire your stick-to-it-ness and ingenuity! But just one word...CMMG!

Seriously, that's a hell of a project and I applaud you.
I didn't want a blowback! Otherwise yes CMMG, Spartan, CZ scorpion, many others. But, 9mm should not recoil more than a 223 and I think there is likely less port noise with direct impingement but won't know until this summer.

Thank you though!
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Re: 4.375in DI 9mm AR

Post by propeine »

poikilotrm wrote:I envy you for your skill set.
I've had the advantage of quality education in engineering but I'm all self taught from a machining standpoint. You can do it just takes perseverance.
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Re: 4.375in DI 9mm AR

Post by BlogSarge »

propeine wrote:
BlogSarge wrote:Good God, man!

I admire your stick-to-it-ness and ingenuity! But just one word...CMMG!

Seriously, that's a hell of a project and I applaud you.
I didn't want a blowback! Otherwise yes CMMG, Spartan, CZ scorpion, many others. But, 9mm should not recoil more than a 223 and I think there is likely less port noise with direct impingement but won't know until this summer.

Thank you though!

I have to admit that my suppressed 9mm Colt SBR is loud. I bet your design will help quiet it down. My Colt's heavy BCG slamming back and forth is not in any way stealthy!
propeine
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Re: 4.375in DI 9mm AR

Post by propeine »

BlogSarge wrote:
propeine wrote:
BlogSarge wrote:Good God, man!

I admire your stick-to-it-ness and ingenuity! But just one word...CMMG!

Seriously, that's a hell of a project and I applaud you.
I didn't want a blowback! Otherwise yes CMMG, Spartan, CZ scorpion, many others. But, 9mm should not recoil more than a 223 and I think there is likely less port noise with direct impingement but won't know until this summer.

Thank you though!

I have to admit that my suppressed 9mm Colt SBR is loud. I bet your design will help quiet it down. My Colt's heavy BCG slamming back and forth is not in any way stealthy!
I unfortunately am using my form 1 can, on a form 1 SBR with nothing else commercial to compare it to. This summer I'm hoping to make it to a metering session though and we shall see!
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Re: 4.375in DI 9mm AR

Post by propeine »

35NCO wrote:This is really impressive. Seriously, Great job man!

Can you post some of your data on your thought process for determining the energy necessary for gas flow and volume to move a specific mass over a given distance on this type of application? I have some idea how that works, but I was wondering if you could break it down with your example? I am not a mathematician, but would like to have a better understanding of this for my own experiments. I can see it...just not sure how to calculate it properly.

Are the variables of: (case of gas volume), (bore volume and distance), (gas speed to port and choked flow of port restriction) (time duration/Dwell of pressure curve), (gas speed down given length and volume of gas tube) (peak of wave), to remaining force for (work greater than reciprocating a given mass of a bolt group) and then (compression force of the recoil spring) as one unit?

Thanks again for sharing!
The engineering term for moving the BCG would be "Work" as in force over a distance.

I know that my 10.5" 223 SBR barrel has a carbine gas port at 7" and the barrel ends 3.5" later. I know that it has a gas port with a diameter of .075 and that it is overgassed without an adjustable block on it. This is with my standard load of 25gr H335 and a 55gr FMJ Hornady.

Using quickload, I ran that same load and got the pressure over distance curve. Quickload also does tabulated data that can be imported into excel so I imported the data from 7" to 10.5". It looked like the following.
Image
At the bottom you can see the equation for a polynomial line fit of that data. You can also see that at 7" port pressure is around 25ksi. Also you can see that this particular load I use isn't even really near SAAMI maximum but it works well for me.

For starters I then new that I could run a maximum of .110 gas port size which has a cross sectional area of almost 2 times a .076" gas port. Without getting out all of my engineering texts on orifices and pressure drops I played it safe that I needed ~75% of the port pressure of a 223 AND sufficient gas volume to work the BCG.

Then I began to play around with 9mm loads and powders that I had in quickload. AA7 seemed promising at first but didn't have a good burn ratio in a short barrel. Some VV powders looked good but those aren't available here. Herco seemed to be a decent option especially at 110% case fill. I did the same thing with Herco that I did with the H335 load and used excel to get a line fit. I can't find the excel data right now but here is a snip from QuickLoad
Image

So now I know where the pressure is at least 75% for the gas port location but do I have enough time at pressure to move the BCG. Using the line fits from excel, I calculated the integral from 7-10.5 for the 223 line fit and the same for the 9mm from .75 to 4.375. This was my best guess of how to do all of this in theory. I'm not a weapons designer thats for sure.

Well it turned out I was "close enough" to get in trouble but not all the way there. I wound up having to clip a half dozen coils off of my recoil spring and remove the weights from my buffer. Now that I've made further tweaks I'm going to try a new buffer spring. Most of the tweaks involve removing friction which when you're working with so little energy makes a large difference.

All of these excercises would be much easier with 10mm or even 45ACP since there is more gas volume to work with. In addition if I weren't worried about keeping things subsonic I'm sure some 9mm Major (USPSA/IPSC loads) should run like a top and I plan on trying some in the coming months. Something like a 124gr bullet with 6.5ish grains of power pistol should sing as the pressure stays higher for longer.
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Re: 4.375in DI 9mm AR

Post by fishman »

do you think this would work with a glock mag lower?

i love the idea of a 9mm or .40 AR but wouldnt want a blowback gun if im going to suppress it.
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Re: 4.375in DI 9mm AR

Post by quiettime »

fishman wrote:do you think this would work with a glock mag lower?
Probably not, my Glock mag lower has almost straight shot from mag to chamber.
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Re: 4.375in DI 9mm AR

Post by fishman »

quiettime wrote:
fishman wrote:do you think this would work with a glock mag lower?
Probably not, my Glock mag lower has almost straight shot from mag to chamber.
What if the mag was modified like OP did.
If the catch on the mag was moved up would it have similar geometry?
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Re: 4.375in DI 9mm AR

Post by fishman »

After some thought, I don't thunk the glock mags will work. The glock mags feed from the center, not lining up with the feed ramps.

Do the asc mags feed from the center of the mag (like a glock mag) or from alternating sides (like an AR15 mag)?
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Re: 4.375in DI 9mm AR

Post by propeine »

fishman wrote:After some thought, I don't thunk the glock mags will work. The glock mags feed from the center, not lining up with the feed ramps.

Do the asc mags feed from the center of the mag (like a glock mag) or from alternating sides (like an AR15 mag)?
Alternating sides. You would have to open up the back of a glock mag an awful lot to clear a standard AR15 bolt. Remember the lugs aren't oriented at 6 o'clock when the bolt is stripping rounds from the mag. The opening is very narrow since they're center feed.

The best way to do this IMO would be to make a custom lower with the mag catch dropped. I'm not spending the money for a custom billet lower though.
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Re: 4.375in DI 9mm AR

Post by garredondojr »

this is great news. I bought a reamer and blank to try the exact same thing. one thing I was reading years back when Ronald Williams (a big pistol caliber DI builder) was building di 9mm uppers was that dwell time is the biggest hurdle in the 9mm system. I personally plan on trying the 165gr xtreme plated bullets to test the theory or I also have been using 158gr rnfp ranier plated bullets pushed through a .356 sizer.

the mag issue is what worries me the most. most mags are center feed which isn't in the ar's favor. I did consider removing the bottom lug but that would be a last resort option. if there were stagger feed magazines out there that would fit it could help since it would feed on either feed ramp.

i'm currently playing with a wildcat that will use ppsh43 mags so I may give them a shot first as it will already be setup.

keep it up and keep us updated!
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Re: 4.375in DI 9mm AR

Post by propeine »

garredondojr wrote:this is great news. I bought a reamer and blank to try the exact same thing. one thing I was reading years back when Ronald Williams (a big pistol caliber DI builder) was building di 9mm uppers was that dwell time is the biggest hurdle in the 9mm system. I personally plan on trying the 165gr xtreme plated bullets to test the theory or I also have been using 158gr rnfp ranier plated bullets pushed through a .356 sizer.

the mag issue is what worries me the most. most mags are center feed which isn't in the ar's favor. I did consider removing the bottom lug but that would be a last resort option. if there were stagger feed magazines out there that would fit it could help since it would feed on either feed ramp.

i'm currently playing with a wildcat that will use ppsh43 mags so I may give them a shot first as it will already be setup.

keep it up and keep us updated!
I'm familiar with Ron. My new feed ramps are inspired of a combo between his DI45 ramps and Uzi ramps.

I made some of these mags myself and am working out logistics to make many of them...double feed colt style mags (mine are ASC) with a cut up 223 mag for the mag catch. It moves the magazine forward quite a bit which then requires some pretty serious work to the extension to build a better feed ramp. However, reducing 3/4" of bolt travel is significant. I have shot 124gr 9mm supersonic loads (think IPSC 9mm Major) and 147gr/165gr subs. The supers work a good bit better but that isn't the goal for me. I found no difference between the 147s and the 165s from a reliability standpoint.

Image

The gas tube is not where it normally lives. I was just too lazy to take the muzzle device off. Also you can see the pin that keeps the brass from dragging on the lugs as it extracts.
Image

I'm working on a rev 2 that should be more reliable with a wider variety of 9mm ammo.
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Re: 4.375in DI 9mm AR

Post by garredondojr »

thank you for those photo's. I didn't even think about the brass having issue on ejection but that makes perfect sense as it's so short it won't clear the lugs before its far enough back to be ejected. I could see that causing alot of reliability issues.
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Re: 4.375in DI 9mm AR

Post by propeine »

garredondojr wrote:thank you for those photo's. I didn't even think about the brass having issue on ejection but that makes perfect sense as it's so short it won't clear the lugs before its far enough back to be ejected. I could see that causing alot of reliability issues.
This is what brass looks like when that pin is not in there. The pin is just an 1/8in or maybe 3/32 roll pin but cuts down on friction and you need all the energy you can get.

Image
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Re: 4.375in DI 9mm AR

Post by garredondojr »

so I got to thinking. have you considered ppsh mags? I think this could do 2 things for you. 1 having a stagger feeding rather than centerfeed. and 2 it allows a max of 1.4" coal so you could do like the 9mm major guys and seat bullets out further thus yielding more useable case capacity which could be compensated with more powder and slower powders which may give you the gas volume your after.? reliability should be good as it has similar case taper to the 7.62x25 and they should stack nicely. just a thought.

you might even be able to run some 180's from a .357 sized down to .355-.356. now that would be a thumper and penetrate like crazy!

hmm I have a 200gr mold now that I think about it. going to have to load a dummy at home and see if it will fit in my throat. now look what you have done!
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Re: 4.375in DI 9mm AR

Post by propeine »

garredondojr wrote:so I got to thinking. have you considered ppsh mags? I think this could do 2 things for you. 1 having a stagger feeding rather than centerfeed. and 2 it allows a max of 1.4" coal so you could do like the 9mm major guys and seat bullets out further thus yielding more useable case capacity which could be compensated with more powder and slower powders which may give you the gas volume your after.? reliability should be good as it has similar case taper to the 7.62x25 and they should stack nicely. just a thought.

you might even be able to run some 180's from a .357 sized down to .355-.356. now that would be a thumper and penetrate like crazy!

hmm I have a 200gr mold now that I think about it. going to have to load a dummy at home and see if it will fit in my throat. now look what you have done!
I'm not sure if you could get the bolt to clear the feed lips or not but it is worth a look for the longer length. Of course that would mean throating the hell out of my chamber too.

I'm going to buy another barrel sometime soon and start at 9" with a gas port of .1 and trim back 1/2" at a time until it stops working then open up the gas port to .125. I've got a new gas port location to try as well.
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Re: 4.375in DI 9mm AR

Post by garredondojr »

ppsh mags have been used for years. especially when their was super cheap surplus tok ammo. there are a few companies that make mag adapters. I'm going to make my own though. You can notch the mags to work with the factory mag release even.

as far as throating you may be surprised. I've owned a few 9mm's that had very generous throats. i'll make a few mockup rounds and see how they do in my guns. might have to make a case gauge off of my reamer and see how it is also. just didn't want to get started on that project until I'm done with the others I'm working on.

I plan on making a reflexed gas block and port it at an angle so the port ends up just past the case mouth. want to get as much gas as possible to this thing.
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Re: 4.375in DI 9mm AR

Post by garredondojr »

so I crunched some #'s for you. looks like seating heavier bullets out further will yeild a decent increase in gas volume.

4.375" barrel
1.3" gas port

147 rainier 1.155" 4.8 herco

20,515 psi port pressure
.294 ms dwell time
4208 psi at exit
1049 ft/sec


165 xtreme 1.3" 8.2 no.9

22,121 psi port pressure
.295 ms dwell time
5644 psi at exit
1039 ft/sec

180 speer HC 1.395" 7 no.9

19,839 psi port pressure
.323 ms dwell time
4630 ft/sec
952 ft/sec
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Re: 4.375in DI 9mm AR

Post by propeine »

garredondojr wrote:ppsh mags have been used for years. especially when their was super cheap surplus tok ammo. there are a few companies that make mag adapters. I'm going to make my own though. You can notch the mags to work with the factory mag release even.

as far as throating you may be surprised. I've owned a few 9mm's that had very generous throats. i'll make a few mockup rounds and see how they do in my guns. might have to make a case gauge off of my reamer and see how it is also. just didn't want to get started on that project until I'm done with the others I'm working on.

I plan on making a reflexed gas block and port it at an angle so the port ends up just past the case mouth. want to get as much gas as possible to this thing.
Word to the wise also from experience, the closer to the case mouth you put the port, the faster the back side of your bolt will foul up with junk. When you pull the bolt out the equivalent of carbon sand falls out. Ill order a PPSH mag and see what I can make happen. Moving the mags forward has helped reliability as well. Its a fun gun to shoot regardless.
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Re: 4.375in DI 9mm AR

Post by propeine »

garredondojr wrote:so I crunched some #'s for you. looks like seating heavier bullets out further will yeild a decent increase in gas volume.

4.375" barrel
1.3" gas port

147 rainier 1.155" 4.8 herco

20,515 psi port pressure
.294 ms dwell time
4208 psi at exit
1049 ft/sec


165 xtreme 1.3" 8.2 no.9

22,121 psi port pressure
.295 ms dwell time
5644 psi at exit
1039 ft/sec

180 speer HC 1.395" 7 no.9

19,839 psi port pressure
.323 ms dwell time
4630 ft/sec
952 ft/sec

Maybe I had better get some 180s to try! Thanks for running numbers too!
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Re: 4.375in DI 9mm AR

Post by garredondojr »

the 180gr posted is actually a 35 Remington bullet so I'm not 100% sure if the ogive of the bullet will be in front of the case mouth. looking at the picture in relation to what quickload shows for bullet seat depth it looks like it will be ok though. how cool would a 9mm with a spritzer bullet be! best part is I'm very confident it will fit in a sammi throat due to it's shape.

That accurate no. 9 must be like pixy dust. it allows a lot of powder in a small space.

this is the bullet http://www.speer-bullets.com/ballistics ... spx?id=116

oh and you have to get the ppsh-43 mags. the 41 mag is a different animal and will not work

I bought mine from here. they were sold out of the singles so was kinda forced into the 3 pack. http://www.robertrtg.com/store/pc/PPSH- ... 4p3362.htm
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Re: 4.375in DI 9mm AR

Post by garredondojr »

any updates on this?
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