Sigma 9mm Hi-caps in a Steyr M9 - The final Chapter!


April 15, 2003, 04:12 PM
Well, I got some Sigma 9mm Hi-cap (17 round) mags to try to see if they would work in my Steyr M9 with a little modification. I thought I would be the "guinea pig" for the forum.

First, some measurements:

Steyr M9:
side to side: .857 inches
front to back: 1.265 inches

Sigma 9mm:
side to side: .800 inches
front to back: 1.265 inches

I put the mag in the gun and function tested it with rounds by holding the mag in from the bottom. I then pulled the slide back and let it go. (Don't try this at home, kids!!!) It stripped the first round off just fine and chambered it. I then racked the slide and it extracted the loaded round and chambered a new round just fine. Did it again. Still worked fine. After the last round was extracted and ejected, the slide stayed locked back.

Ok, so far so good. But what to do about the mag catch? As you can plainly see, the Sigma mag is slightly narrower. At first, I didn't think much of this. How could 5 one-hundreds of an inch make a difference? So I began playing with empty mags in the gun.

First I put in the Steyr mag. Upon insertion and once the mag is about 75% in the gun, you can visibly see the mag release button move as the mag pushes it out of the way to get by. You can also feel the tension of inserting the mag get a little tougher.

Next was the Sigma mag. Since I have not cut a mag hole yet, I don't expect that it will stay in, but it should tension the mag release button the same. I put the mag in and got to the 75 % point. There was no increase in tension in putting the mag in the gun and the mag release button did not visibly move. So I held the mag in place and played with the release button. At no point is there any tension on the catch, and the magazine has a slightly loose fit feel in the gun.

Soooooooo, what I have determined is that 5 one-hundreds of an inch might as well be a MILE when it comes to guns and mags fitting. :cuss: With the tests, I felt it not worth the try to put a new mag release hole in the mag because even if it did work, it seemed that it could possibly be wiggled and drop free when you least expect it. :what:

I have heard others mention putting something very thin and tough on the opposite side of the mag to make it fit properly, but IMO, this is modifying the mag and it may not fit in the original gun.

So this brings this part of my being the "guinea pig" to an end. I have not given up on coming up with hi-caps for the Steyr, but unless someone has ACTUALLY drilled the hole in the Sigma 9mm mag and can say that it functions 100%, I am not willing to wreck an expensive mag.

(On a happy note, I have heard that the Sigma 40 cal hi-cap mags ARE the actual dimension of the Steyr M9 mags and will work, but will require a mag catch hole AND slight crimping of the feed lips, but only enough to hold the 9mm and still allow the 40 to feed. This will be my next experiment.)

Anybody want to buy (or trade for) a couple of un-touched Sigma 9mm Hi-cap mags?!?!?!?!? :rolleyes:

Anybody got any Sigma 40 cal hi-cap mags they want to trade stuff for???? PM me if so.

Thanks to all who have answered previous questions. Let me know if I left anything out or you have any further information.

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April 15, 2003, 07:30 PM
I don't know about the 9, but I traded a couple of 40 hi cap sigma mags to my brother in law, we cut the new hole, stuck them in his Steyr, functioned fine.

April 15, 2003, 08:09 PM
Sorry to hear about that, Stresspuppy. :( I was hoping that the conversion would work so that other people could follow suit. :(

April 15, 2003, 08:27 PM

I have heard the same about the 40s, so I want to do the same. I hadn't heard anything specific on the 9s, just what people were speculating. Call me hard-headed!!! :banghead:


Not a problem. It was a gamble and I was willing to take it. I will try to get a line on some 40s and document in great detail what I do and what the measurements are for the mag release hole and post here. Experimenting if half the fun. Besides, while here in Florida, I have gotten to shoot TONS and have several more days. Nothing like having a father-in-law that is an FFL =AND= has his own shooting range on his property!! :what: :evil: Let's just say that much lead is flying and the paper doesn't stand much of a chance!!!!!

If anyone knows of some Sigma 40 cal hi-cap mags that are up for trade or sale, please let me know.

April 16, 2003, 05:11 PM
Hey Stress puppy, I guess you got your portable computer thingy working. Found a write up on your Steyr in Gun Test mag if you haven't read it yet. They were OK with it, lots better than they were on my S&W 342PD revolver (sorry to cuss guys, I won't use the "R" word again). They metioned it was the closer to a single action than a Glock and liked the sights at 7 yards, but not at 15 yards.
I'm not old enough to be, but I'm Stress Puppies 01FFL father-in-law. Hope to do more shooting when he gets back.
Allan Schisel

April 17, 2003, 12:32 AM
As far as the .40s go, I believe modifing the Sigma to fit the Steyr would work. I borrowed a mag from a friend and checked it out, couldn't find any reason why it wouldn't work. I didn't drill any holes b/c it wasn't my mag didn't think he would appreciate it. My question is what do you do with the "toe" on the Sigma mag leave it or cut it off?

April 17, 2003, 09:43 PM
For me personally, I hate the way the "toe" looks on the Sigma mag when it is inserted in a Steyr, so it will have to go. My choice would be to find a way to create a new base plate and not modify the actual Sigma baseplate, but since I am not quite that adventurous, I will probably just use a little "dremel-magic" on it to make if fit flush.


Thanks for the tip on the Gun Tests article. I will read it (and the one about your S&W). I personally like the Steyr a lot, but since I am coming from shooting a Kimber Pro Carry, it has been a little bit of a switch. But the gun grows on me every time I shoot it, so I will stick with it for awhile.

April 19, 2003, 07:24 PM
Replace the Steyr frame with a Sigma and it should work.;)

April 21, 2003, 10:59 AM
I bought two Sigma 17 rounds mags about a month ago. They cost me about $100 and I am a little nervous about cutting them up. For me the best alternative was using the Steyr .40 Cal Mags. I could fit 12 rounds in them with very little modification. CDNN has tons of mags.

I am really excited for this AWB to sunset. They have all sorts of mags available right that are marked LEO only. September of 04' if this thing sunsets I am going to put in a preorder about a month early.

April 21, 2003, 11:07 AM

Don't cut them! Not worth it. The Sigma 40 mags should work, but I haven't tried yet.

As for the M40 mags, wasn't worth it for 2 rounds to me. But that is just me.

As for the sunset on the AWB, I will believe it when I see it. This is such a hotly contested issue that I am not getting my hopes up. I want it to sunset, but wanting and it happening are two completely different things!!!

April 21, 2003, 04:36 PM
Maybe you could somehow increase the size of the mag catch. That should solve your problems right? Is it plastic of metal? Either way though you may be able to make it work.


Steyr Hahn
April 27, 2003, 06:01 PM
Well my M9 came home for the first time last evening, and we are just now back from the range! I can't seem to stop smiling!
Now all I need to do is convince the wife that we can't go on living the good life if we don't have a complete set (a M40 and M357)!

As to the high capacity magazine problem, the Beretta 92 mags work just fine. They look a little odd, the way they hang out the bottom of the grip because of the weird angle of the magazine floor plate. However, a capacity of 15 rounds does make the look grow on you!

Ok, it took me about 30 to 45 minutes with the Dremmel tool and some cold blue to have something ready to go to the range this morning. The first attempt was/is good, function is satisfactory although not perfect, yet. I did experience out of 5 full mags (75 rounds total) feed problems 6 times. The first loaded mag had 2 misfeed’s, the second was perfect, as was the third, the forth had 1 misfeed and the remaining three failures were on the 5th mag. I suspect that I'll need to lower (remove more metal from the topside of the "catch cutout") the position of the mag about a 1/64th of an inch.

Anyway, the very good news is that the modification to the Beretta mag was quite easy and I can still use it in my Beretta!

For any that may want to “try this at home” I'd be pleased to pass on a few Jpeg's of how/what I did, and what to be careful of in your endeavors.

:D :D

April 28, 2003, 02:46 AM
Alex, I happen to have a few beretta high caps hanging around, could you go into more depth about how you did it?

Steyr Hahn
April 28, 2003, 08:43 PM

I be pleased to... However, it would be so much easier if I could email a series of pictures to you (or anyone else that may want to know) along with explanation.

I'm working on wading through the miasma of posting the jpeg's here, but that may take a bit time.

You may email me at...

mons meg
April 29, 2003, 01:27 PM
You have an email, Mr. Poopsky. :)

Can't you just use the attach files feature when posting?

Steyr Hahn
April 29, 2003, 09:38 PM

Unfortunately my day job has kept me more than busy the last 2 days. So I've not had the time to write up the procedure as I wanted to, I will have it completed within a day or two and will post it here. I'll also email a version to any that have/may request it.

The attachment below is the finished product

mons meg,

Many thanks for the suggestion, I had no idea it was that simple to post a picture on this forum!


Steyr Hahn
May 2, 2003, 09:16 AM
Ok, I've finally written this up in a semi-coherent manner. If there are any that may have lingering questions after going through this, please feel free to email me or post here.

I’ll start this saga by saying that even if you make a total mess of things as regards your attempts to making a Beretta 92 magazine work in your Steyr M9, all will not have been lost. Because as long as you do not touch the feed lips of the magazine you will still have a fully functional 92 mag when your finished/flustered, albeit it’ll have an extra “vent hole” in it.

The finished product, showing the location of the new catch cutout is shown in picture #1.

The method I used to locate the new cutout on the 92 mag was to remove the slide/barrel assembly from the receiver. I then inserted the Steyr factory mag and “eyeballed” the location. I then replaced the slide/barrel and checked the fit/feel. I repeated this process with the Beretta mag, removed the mag catch lever, marked the mag for the location of the new cutout and got to work with the Dremel tool and some “cold bluing”. This, was not the best method! In retrospect I’d recommend against it, even thought it did work for me.

For attempt #2, with the benefit of hindsight, I intend to do things a little differently.

If you compare the 92 mag to the Steyr mag you’ll see that the feed lips on the 92 mag are inclined slightly downward. This has, so far, not proven to be a problem. With what I’ve learned from the 1st attempt, I believe that the method below will yield the most consistent results. The location of the “Ejector Tab” just to the rear of the “Slide Release” lever should be used to “index” the location of the feed lips(pic #2 & #2a).

On my M9, the space from the top of the feed lips of the Steyr mag to the bottom of the ejector tab is .045”. With my first attempt, I set up the 92 magazines feed lips resting against this ejector tab. This setup was/is the reason I had some feed troubles. In addition, it also was necessary to remove some metal from the backside of the mag so that the slide/bolt face would clear the magazine. With a lowered position of the 92 mag cutting at the rear of the mag should not be necessary.

Step one?
Measure the distance of your M9 magazine(s) feed lips to the bottom of the ejector tab. I used automotive type Feeler Gauges for this, laid them across the top of the feed lips and slid them under the ejector tab and came up with .045”.
On to the tricky part, you will now need to remove the magazine catch lever from the receiver, preferably without shooting it’s return spring and stop button across the room (pic #3). Insert the Beretta mag and index to almost the same location (I suspect that a gap of .20” to .35” will prove correct) as my Steyr mag measured .045”. I recommend that the Beretta mag set about .010” to .015” closer to the ejector tab than did the Steyr mag. You can always remove a little more metal later to open the gap. However it’s a bit tougher to add metal to close the gap if you remove too much at this point!
Now that you have the 92 mag in position you’ll need to mark the new catch hole on the mag, I used a sharp scribe to scratch surface of the mag.
It’s now time to warm up the Dremel tool, I started with a cutoff wheel to remove the largest portion of material. I finished up with the smallest diameter cutting bur I had and a hand held “pin file”.
The only really critical cut you are making is the top of the cutout (the cut parallel with the feed lips and closest to them). The other three sides of the box should be as close as you can get them but there is some “fudge” room to work with. The method I used was to under cut the top cut, try it for fit, shave a little and try again repeating until it was “right”. Start with a tight gap of say .010” and if possible take the Dremel to the range and make a small adjustment then test fire until its right.

As I said in the first post (at THR), this project was one of those “quickie” lets see if it could possibly work type projects that worked better than expected. If you attempt this modification and come upon questions, you may contact me and I’ll render whatever knowledge I may possess to assist you.

a.k.a. Steyr Hahn


Steyr Hahn
May 2, 2003, 09:18 AM
Picture #2

Steyr Hahn
May 2, 2003, 09:20 AM
Picture #2a

Steyr Hahn
May 2, 2003, 09:22 AM
Picture #3

July 28, 2003, 04:12 PM
The saga continues.
I successfully (as far as I know) fit a S&W Sigma 15rnd mag to function in my Steyr M40. Blacked the mag with marker and inserted it into the well, then forcefully depressed the right side mag catch to mark the proper position on for the slot. Compared that to the factory mag using the top of the feed lips as a reference, and found it to be reasonably close. After disassembling the mag, I used a dremel cutoff wheel to make a series of small cuts and then finished with a burr bit to open up and smooth out the edges. I didn't make any attempt to square up the notch, just made sure the sides were fairly even and burr free, so total dremel time was less than 10 mins. Through tremendous luck, the mag fit on the first try and has fed 30 rnds without a bobble, holding open on the last round. I polished the mag bright and I'm currently grinding on the bottom pad to slim it down but, no way aside from adding an epoxy spacer will it fit flush.

I'm going to try running more rounds through it to test for reliability and then try to duplicate it before I get too excited.
For the paranoid, the mag still fits and functions in a Sigma, thus no known laws were violated in this modification.

Jason Demond
July 28, 2003, 07:28 PM
Good job!

If you enjoyed reading about "Sigma 9mm Hi-caps in a Steyr M9 - The final Chapter!" here in archive, you'll LOVE our community. Come join today for the full version!