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Gas operated wonder 9?

5K views 40 replies 10 participants last post by  vz58 
#1 · (Edited)
I've been taling about building a 9mm direct impingement upper since this past april and have been gathering parts since early summer. Over the past two weeks I've managed to make some progress on the build.
I started with: a 9mm blank; a $10 barrel extension from CDNN; a spare carrier and upper I've had for years; A set of used USGI bolts off of gunbroker; and a Caterpilliar diesel engine nut 13/16"-16tpi.
The barrel blank I started with was one of the .800" blanks that have been floating around for the past few years. This barrel is too thin to turn a shoulder into for the barrel extension to rest against. I decided to used a savage style locknut to solve this problem. An internet search turned up Catapilliar diesel engine grade 8 nuts for about $1.50 each so I got several.
After short chambering the barrel I threaded the last 1.150" of the barrel to 13/16"-16tpi so that the barrel extension was a little difficult to thread on. Then I screwed on the barrel extension so that the back end of it was 1/2" from the back end of the barrel and drilled the index pin hole to a depth of just past the bottem of the barrel threads. I also drilled a gas port about 1.5" in front of the barrel extension since it was already squared up. Then I screwed on the locknut as far as it'd go, and then screwed on the barrel extension with red locktite and secured it with the index pin made from 1/8" drill rod. After letting it set for a day I torqued down the lock nut and secured it with red locktite. Before the loctite set up I turned down the locknut to just under 1" with the barrel extension towards the tail stock so that any cutting force would be tightening the lock nut and not loosening it. Then I finished chambering the barrel after opening up the 556 bolt to .398" so the 9mm case would fit.
I then made a gas block for it out of steel and cut down a used rifle length gas tube to fit. The first few rounds of test fire showed that with a .090" gas port the bolt comes back enough to extract the case from the chamber and stop with the hammer half cocked so that the hammer catches the firing pin as it tries to go forward.
Thats where I'm at right now. The wonder 9 part of it is I wonder what it'll take to get it to run. More work and current photos to come!
 

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#2 ·
Very cool, and ingenious. I will be following this thread as I like to see creativity like this. Will you use ar15 9mm mags or some other?
Also off the top of your head do you think this would be possible with .45 ACP?
 
#3 · (Edited)
I had been talking it over with several members and would like to thank Viper Dude and Mad Machinest for their input. I would like to see if I can get pps43 mags to work, and use that same mag for 9mm, 762x25, and 45acp. Mad Machinest currently sells 762x25 and 45acp uppers so I would defer all questions about those to him. Once I get the bolt carrier to cock the hammer I'm going to use a modified sten mag for the mean time. I tried several carriers, Colt, FN, and unknown in both ar15 and M16 styles. The one that shows the most promise is a very well worn converted colt m16 carrier. It is pushing the hammer down enough to get caught on the firing pin collar (its a semi converted carrier) so it just needs a little more to lock the hammer on the disconnector. If it turns out it doesn't work, I can always cut the barrel off at the gas port and make a 16" blow back out of it.
 
#4 ·
I know that you are working with gas operation but if you did go to blowback how would you balance the recoil impulse, with heavier recoil spring or heavier bolt weight, or both?
Is there a formula for that or is it trial and error?
 
#8 ·
What I would do is calculate the energy the bullet has, and then that is the same energy that the bolt has to absorb. The .45 is a straight walled case, so the pressure acting on the base of the bullet is the same as the pressure acting on the bolt face. Calculate the energy that is imparted on the bullet and you can figure out a good bolt weight.

230 grains times 830 FPS = 190,900 divided by 20FPS [a high end of reasonable bolt speed] = 9545 grains

Your bolt/buffer combo should be at least 9545 grains, which happens to be 1.36 lbs. I'd try and go heavier than that by a little, as it seems a bit light to me, but my calculations up there didn't take into account the force of cocking the hammer and what little force the recoil spring puts into it holding the bolt in battery.

Who said we'd never use this stuff ever again in math class? :lol:
 
#6 ·
If your using a standard AR-15-M-16 carrier?? you may want to lighten up the carrier a bit, the 9mm cartridge will only produce so much gas it may need more than it can produce with the 9mm powder burn.. You can ope up your gas port hole, and or move gas port location closer to the chamber.. location of the gas hole also will effect the cycling.. I think you can get it to work, but it's gonna be ammo sensitive... B2B
 
#7 · (Edited)
Wrench, if I end up going the blowback route (which I don't think I'll have to) I'll make my bolt the same weight as a colt or rra 9mm bolt and use a heavy 9mm buffer. I'll get some more pic's up later today of the gas block ect. I haven't profiled the gas block yet to fit under the handguard so it's pretty much like the block of steel above the barrel in the photos.
B2B, It's still a work in progress. The gas block I made is a set screw type for easy removal to open up the gas port and so that I can move it back as needed. The 1.5" location and .090" gas port were just best guesses and are subject to change as I debug the thing. I'm trying to get it to work with a standard carrier and buffer first, and If I can't I'll lighten those up as needed. Ammo sensitivity isn't an issue for me since I reload just about all the pistol ammo I shoot.

Edit: Here's a pic of the gas block so far. It really is a "block" at this point, but I'm going to mill away where the black sharpie marker is so that it'll fit under a set of standard handguards and to lighten it some.
 

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#9 ·
Hey Moleman,
Could you put an adjustment screw on the gasblock to adjust the amount of gas allowed through the block? Kind of like a mixture screw on a carburetor. You would open up the gas port hole in the barrel and rely on the screw to throttle it back a bit. Then adjust it to your particular load of the day.
 
#10 · (Edited)
Wrench, Yep I have thought about tapping the block for a modifed ss set screw to do just that. That is part of the reason the gas block hasn't been profiled yet. It's gonna be a lot easier to lay out the hole locations with everything flat and square. I did manage to tweek it a bit an now it is ejecting and cocking the hammer!!! Now I need to finish modifing the sten mag to work.
 
#11 ·
Awesome, I will wait to see what you do for the adaptation of the sten mag. I have a lower that I was thinking about making a .45 ACP upper for but my kid says I may have trouble with finding a reliable mag to use that will fit the ar lower. I do reload for the 9mm as well as the .45 so maybe I'll follow your lead with 9mm first. BTW thanks for posting your adventure!
 
#12 ·
Wrench, Hahn makes an adaptor to use sten mags, and they also sell one for grease gun mags. Don't know if the grease gun adaptor is a drop in or not. I'm leaning towards making an adaptor like the hahn. There are two styles of these. One thats basically a block like the Olympic mag adaptors, and another one that is ar15 mag profile and the mag sets about in the middle of the mag well. I'm going to make the block type.

I got some more work done on the 9mm build. I have been able to get it to fire semi-auto using the sten mag with a block of plastic jammed behind the mag to hold it in place temporarily! Since it did run, I went ahead and profiled the gas block so that it would fit under a handguard. The front sight is pressed on and held with a set screw at this point. Once I'm ready to parkerize it I'll drill the front sight pin holes. When I get time after my mag block is done I need to make a flash hider for it, but in the mean time the one thats on there will protect the barrel threads. Here's some pics.
 

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#13 ·
Really nice! Are you still at .090 for the gas port? The profile job on the block looks great.
The sten mag looks great also! Show pics of your mag block when its done. This is a really cool build and I really like it, good job so far!
 
#14 ·
Wrench, I actually opened up the gas port some, but it had no effect. So I removed the buffer weights and that got it working. I think since the gas tube is a little under 1/8" on the inside, only so much gas can get down the tube no matter how big the port is. I tried to contour the gas block much like it was layed out on the first blocky picture of it. I used a regular end mill to remove the corners, and then a 3/8" ball end mill to contour the top. I'm sure I have a couple 1/2" double ended carbide ball end mills somewhere, but haven't seen them since the move. I'm sure the shop gnomes will make them appear in my sock drawer or china cabinet in a month or two.

I forgot to mention earlier that there are two styles of original sten mags. One will accept a 45acp with a little tweaking, and the other will not. The sten mags I have will not accept a 45 round so I can't speak of what the second style looks like. I've seen a thread on the subject recently either here or on weaponsguild. The black plastic block will work as is if screwed to the back of one of the sten mags. I think I'm going to mill a slot in one for a mag catch though.
 
#15 ·
darn it moleman--and i said i wasn't going to build a PCAR in 9mm, now i might have to eat those words! LOL!

this looks to be a very interesting concept. looks to be near perfect for a NFA SBR with a can! or even a pistol build!


let us know how it runs out!
 
#16 ·
j427x, I haven't made a magwell block yet, but just by wedging a piece of plastic behind the mag to temp hold it in place I've been able to load up several rounds in a sten mag and shoot them. I haven't loaded up a full mag yet as I figured the mag would move before I got to the end of it anyway. I cast lead rn bullets in 9mm and I'm uncertain if I'll be able to use them as when I pull the bolt back slowly the bolt lugs dig into the lead bullets of my dummy rounds. It doesn't seem to effect the hp bullets. Yes it feeds the winchester ranger hollow points I've been testing with! I'll have to load some lead bullets up to see if they feed ok and how they work out with the gas port and if they shave off lead down into the gas system or not. I might be able to limit the effect of the lead bullet deforming by lower the mag as well. Heck it might not even do it when it shoots. Only one way to find out.
 
#18 · (Edited)
I ordered some 1"x1.25" aluminum bar to make a magwell block out of, but it'll be at least a week till it gets here. So in the mean time I rummaged around and found some 1/2" aluminum plate and cut out a couple 1.3" strips with my trusty hacksaw. After cleaning up my hacksaw job on the mill and screwing two pieces together. Squaring the darn thing up and screwing it together took up alot of time. When the aluminum bar gets here I'll make another one since it won't take quite as long.
I was able to fit the block to a 80% tactical lower and was happy until I tried to fit it in another lower without success, then another, then another, ect... Back to the mill to remove a bit more so the block would fit in all of my lowers. I still need to slot it for both mag catches, and trim what sticks out of the bottem. Then of course make a mag catch to hold the sten mags.
 

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#19 ·
I managed to finish a magwell block. It was a comedy of errors none of which were critical. The first was measuring the sten mag catch pocket location and then milling on the wrong side of the line:grumble: I also lined up the mag catch with a chip and not a layout line:censored2 But no deal breaker mistakes and I ended up with a functional magwell block.
I test fired a bunch of rounds and was getting on average about 2-3 rounds before the gun jammed. I have been using an old box of winchester ranger 147gr JHP, like the black talons only not black. I switched over to fmj and managed to get about 5 rounds off before the gun jammed again. A piece of copper jacket managed to find its way into the carrier and made the bolt turn hard. Thats not unusual for metal shavings to get in the carrier for the first few rounds so I'm not worried. I've had it happen before on 556 uppers and it usually damages the gas rings. These rings still look good though. I'll give it another shot tomorrow if I can get away again.
 

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#20 ·
I got to shoot the upper today. I took two 30rnd mags to the woods with me. The first mag I had a failure to feed at round #29. I loaded it back up and shot the last two rounds without any problems. Then I tried the second sten mag I have. It's a well used mag with what looks like at least 3 layers of paint on it. After about 6-7 failures to feed where the bullet tip either tipped down and jammed on the front of the mag or tipped down, but not far enough to jam, but the back of the casewould be lower than normal since the rounds under it weren't pushing it up, causing it to tip upwards and jam on the top of the barrel inside the barrel extension. I took about 15 rounds out and tried again, but I could not get that mag to feed at all. So I put all of the loose rounds into the first "good" mag and they all shot off without incident except for two which had been damaged from the first load attempt with the bad mag.
A little bit more work to do to get it feeding 100% but it's almost there.
 
#23 ·
Viper Dude,
I ended up bending the feed lips on the bad mag to match the mag that would feed. That seems to of fixed that mag. I tried it out and it feeds well now. I tried some of my 124gr lrn bullets with a 4.0gr charge of 321. That light load wouldn't cycle the action, but a 115gr fmj with 5.2gr of 231 (which is a starting load) cycles it just fine. The +p ranger ammo that I initially was using is noticably louder with a more energetic case ejection. I think I have a good cartridge load range that'll work the action. After the last gas port adjustment I was getting some bullet jacket in the gas system which was beginning to disappear. I had no copper in the gas system today, but I did have some lead dust stuck to the bolt tail where the key dumps the gas. If I'm going to work up a lead plinking load I think I'll have to debur the gas port first.
 
#24 ·
Some .50BMG guys load turned steel bullets and report no abnormal barrel wear, I'm thinking something like that would deburr a gas port pretty well. Turn a few bullets right at .355, anneal them, load them up with a minimum load and see if it knocks the burrs off.

Oh and remember that the gas key might restrict a bit of gas flow too, what with the bend in the gas channel. Might want to look at the insides of it and maybe port it out.
 
#26 ·
Viper Dude, Yes I was planning on using a small dental burr to do it! I got lazy the last time I drilled the gas port and just used a cordless drill instead of squaring it up in the mill. I think I have the gas port canted slightly. Before that there were no bullet shavings in the gas system and I enlarged it several times. Not a big deal and it seems fine now with fmj. I'm cheap though and like to shoot cast bullets. The gas port is around .108"-.110" so a slightly modified 1/8" shank dremmel bit would also work.

[486], You're thinking this through! When I was working on the bolt I measured a gas key and the inside after the turn is around .122". The gas tube is a little under 1/8" (.118"ish) and is really the choke point for gas getting from the barrel to the bolt at the 90 degree turn in the gas block. The gas port is about as it should be and I don't think a bigger one would improve things. The upper seems to be working fine at starting loads all the way up to +p loads. I really don't want to overgas it more so it'll work with very light plinking loads. The upper worked fine with +P loads with a gas port of .100". I opened it up some so I could use slightly lighter loads than standard. The light plinking loads don't even break 900fps but work in some of my other 9mm builds so I'll just use them up and increase the powder till they work in this upper. It shouldn't take much.
 
#27 ·
Maybe try drilling the gas hole after the bend in the key larger, then when making the turn the gas doesn't meet a restriction, because they probably just run a drill bit in from each end to make it, where the 2 holes meet [where the bend is] might not be full bore of either drill size.

With a little googling [as I've never had an AR gas key off of the bolt carrier] I found a pic that shows the hole sizes. You might also want to take a small round file to the bolt carrier's gas hole and elongate it to the front to make for a more gradual radius there with the larger hole in the gas key.
Ar 15 gas key image by Phil_in_CS on Photobucket

Might get the nice light loads cycle. That'll save you a bit in powder if you shoot as much as I know making a 9mm AR would let me shoot! 8D
 
#28 · (Edited)
[486] The photo of the guys messed up carrier you linked to shows how the gas passage is angled in the photo and since it is an angled cut across the 1/8" hole the hole is actually .123"x.178" (still an 1/8" hole as far as a passage goes). I can stick an 1/8" bit through both ends of the gas key and see it in the hole of the other side which means the same funky .123"x.178" hole is inside the key as well (still a 1/8" passage as far as the gas goes). I measured a gas tube as I was pulling the .118" measurement from memory. It turns out its closer to .113"-.114" I just remembered it was smaller than 1/8" as I had to turn down some 1/8" drill rod to cap the gas tube so I could press it in. That means the passage in the key is still around .010" larger than the gas tube and probably .015" larger than the gas port.
The 4gr of win 231 just isn't powerful enough to cycle the action. Not the guns fault it is a very light load. I typically load lead plinking rounds as slow as possible as it uses less powder and you don't get much leading to speak of compared to driving them faster. The 4gr loading is maybe going somewhere around 870fps and I was using as it cycled my sgn-9 and suomi, plus you get 1750 rounds out of a pound of powder occording to the math. Even if I go up to 4.6gr I'll still be right at a starting load for some of the reloading books I have and will get 1500+ rounds out of a pound of powder. I can live with that.
 
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