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Discussion Starter · #1 ·
I have a STI Spartan in 9mm that I'm looking into converting to 40S&W.
I am trying to figure out if I must build an entire top end or if I can just open the breech face on my 9mm slide and use a 40 barrel.

No matter what I do, I want to be able to use this for either 9mm or 40S&W.

So I am questioning if its safe to shoot the 9mm if I do end up opening the breech face to accept a 40.
 

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Kimber builds their guns like that. It's not optimal. You will most likely require a separate extractor tuned for each caliber to get it to function reliably.
 

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I would really recommend getting a whole top end. Opening a 9mm breechface to .40 will leave your 9mm case heads unsupported. You might get away with it for light loaded rounds but if you are shooting 9 major it could cause case head failure.
 

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I would really recommend getting a whole top end. Opening a 9mm breechface to .40 will leave your 9mm case heads unsupported. You might get away with it for light loaded rounds but if you are shooting 9 major it could cause case head failure.
A normal breechface offers very little radial support to the case head. This is not a problem as the case head is very thick and the brass is work hardened during case forming.

Consider that the extraction groove is not supported by anything - ever. It would take an extreme overload for you to be able to measure any swelling in the case rim diameter or at the extractor groove.





Consider that when a case lets go due to an overpressure event where the barrel doesn't burst, invariably it is the section of the case ahead of the extractor groove that fails, at the bottom of the chamber. The case head closer to the breechface may get squashed lengthwise and enlarge in diameter until it meets the sides of the breechface, but this is only in an extreme event.

During normal firing the sides of the breechface do not offer the case any material support - even if the case did make contact with the side of the breechface on the side opposite the extractor before firing and during the early stages of gas pressure build-up, the case self-centers in the chamber during firing and that moves the head away from the edges of the breechface.

An oversize breechface opening may affect reliable extraction or ejection, and possibly feeding; but it would not be unsafe to fire, all else being equal. The bottom of all 1911 breechfaces is always "unsupported"; right there near the barrel ramp. In other words, fully half of the casehead never sees any possibility for radial support. While barrel ramp support or the lack thereof matters, breechface radial support does not - only axial support matters with respect to the breechface.

The breechface sides and barrel hood extension may reduce the amount and size of shrapnel blowing up and sideways in the event of case head failure, but this function should not be affected if the breechface is 1 mm wider; as in firing a 9 mm using a .40 caliber appropriate breechface slide.
 

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9mm slide converted to .40

I have "opened up" the breechface to accommodate the .40 S&W on two 9mm/.38super slides using hand tools, with excellent results. I then fitted a .40 barrel, and the results were very good. I used a different ejector, and a pre-tuned extractor for each caliber.....the 9mm uses a longer ejector than the .40.

The amount of metal removal on each side of the breech is roughly .015" per side of the breech....this really isn't much metal too remove. Since I didn't have a mill, I made an extension tool that allowed me to use my drill press.

I took a 1/2" alumimum rod, and cut it to about 6" in length. I then carefully marked the center of the rod, using a small Dremel engraving cutter, and market the center of the rod. I then carefully drilled a hole in the aluminum rod the size of my Dremel carbide cutter bit. To be able to secure the bit, I ground a flat on one side of the shank of the Dremel bit, then filed a flat on the side near the end of the aluminum rod with the drilled core, and drilled two holes and used a 6-40 tap to allow two 6-40 allen head set screws. I then put the Dremel carbide cutter bit in the aluminum rod, and tighted the flat of the shank with the set screws to firmly hold the cutter in place.

Next, I inserted the rod with cutter in my table top Rockwell drill press, slipped the muzzle end of the slide up into the alum. rod, then adjusted the support platform to allow the back end of the slide to rest on the support (against a cloth) I turned the drill press on, and manipulated the slide against the stationary revolving cutter to remove metal from each side of the breech. The end of the carbide cutter is a flat safe edge, so there was no damage to the breech face, and the cutter only removed metal from the side of the breech.
I removed the slide often, and checked my progress by sliding a .40 brass case up the breech face. When enough metal was removed, the case slid up the breech face quite easily. The most time consuming portion of the project was making the aluminum extension tool. Opening the breech face of the slide took about 20 minutes.

The slide was fitted to an STI Eagle frame set up. So I was able to convert the gun from 9mm to .40 S&W by replacing the ejector, extractor, barrel and mags for the .40 set up. I eventually purchased another STI frame w/modular grip, and opened the breech on another 9mm slide and built a dedicated STI .40 for USPSA Limited matches.
 

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Discussion Starter · #6 ·
Thanks for the feedback guys. I think I will pick up a 40 barrel and modify my slide.
 

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Discussion Starter · #7 ·
I finally got around to getting the parts for this.

Parts bought:
Swenson Semi Drop-In Barrel 1911 40 S&W 1 in 16" Twist 5" Government Steel Matte Black
Colt Barrel Link 1911 .278" Steel Blue
Colt Barrel Link Pin 1911 Steel Blue
Swenson Drop-In National Match Barrel Bushing 1911 Government Steel Blue
STI Extractor .38 Super/40 S&W

I decided to try out the original ejector to see if it would suffice.
I pressed in the barrel link and gave everything a good cleaning. I stripped the slide, put a 40 round in the chamber of the new barrel and placed it in lock up in the slide. Clearances looked good and I couldn't see any interference. I put the slide on the frame and put in a recoil spring to test for barrel fitment. To my surprise it worked without a hitch. No binding once so ever. So the next step was to try and feed a 40 into the chamber. This is where work was to be done. The 9mm slide had some material on the shelf of the breach face that needed to be removed.
The amount of material that was interfering was very small. So I thought that the dremel would be the right tool for the job. So I had at it. Taking just small grinds at a time and checking for clearance in between. Eventually enough material was removed that the 40 round could be stripped from a mag with no interference.
To check for compatibility I put all the stock 9mm components back and made sure the gun could cycle 9mm rounds. No problem there.
I did notice the 40 rounds where hitting the ejector a little bit when the mag was fully seated. So i trimmed the bottom side of the ejector just a little to add some room. Again I cycle tested both the 40 and 9mm.
So after all the work was done, I cleaned everything up and took both sets of components with the firearm out back for test firing.
Tested some factory 40 loads with a 13# spring and it cycled beautifully. The only problem I had was the slide lock was coming up on a non-empty mag. Which I think could be due to the 45ACP mags that I was using to fire 40 rounds.
Once I verified the 40 was working well I swapped all the components back to 9mm to check its functionality. No problem there. Worked just as well as it did before.

I will post some pictures of the mod and any updates on how the gun is running. I will be shooting mostly 9mm for now till I get set up to reload 40.
 

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I would like to thank all that posted here.
I am thinking of opening up a 9mm slide to make a 10mm pistol.
This info is very helpful.
Thanks,
Michael
 
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