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Discussion Starter · #1 ·
Warning, modify your gun at your own risk.

I was at the store a few months back checking out a S&W M&P 40. One thing of note, the magazine held 15 rounds with the same exact length magazine.

Today, I did some investigating and discovered that not only were the followers roughly the same dimensions, but both also had the same lockplate thickness.

I kept examining the differences to figure out why the M&P could hold 15 rounds while the FNX could only hold 14.

I compared the length of the springs next. I found the M&P's springs to be significantly shorter. One simple way to do the conversion is to simply purchase the M&P spring and make it work in the FNX.

I decided instead to alter my current spring.

I counted the coils and compared the angles. The angles of the springs were pretty much the same. However, the FNX had 13 coils, while the M&P has only 11. I clipped the bottom of the magazine spring, so as to enable the smaller portion of the top to still be able to collapse inside the larger coils. This is how the M&P spring is constructed.

Upon doing this, I then bent the leading edge of the coil back to become parallel with the adjacent wire, and bent the hook inward in the middle. (pictures will be posted next)

I then reassembled and loaded up the magazine, easily fitting the 15th round inside the magazine.

I then proceeded to test.

I cycled the weapon with live rounds in the magazine (pointed in a safe direction with safety on) as fast as I could to try and emulate actually firing it. All rounds fed. All rounds went into chamber and were extracted. There was still plenty of pressure pushing out the last round. The slide still locked back.

I will be conducting a live fire test with the pistol this weekend to determine if there are any reliability issues. Once I am finished, I will be converting the rest of my magazines also if all goes well. Why should FNX 45 users get 15 rounds while FNX 40 users only get 14? Phooey I say.

If there are any issues with my current alteration, I will be testing the M&P magazine next.
 

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My focus really wasn't on how easy it would or wouldn't be.....I am just questioning whether getting one more round in is worth jeapordizing the reliability of your firearm....for me it wouldn't be worth it.
 

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Discussion Starter · #9 ·
I think that the risk is small enough that its worth the try. But that's why you test it anyway.

Sent from my SPH-L710 using Tapatalk 2
 

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My focus really wasn't on how easy it would or wouldn't be.....I am just questioning whether getting one more round in is worth jeapordizing the reliability of your firearm....for me it wouldn't be worth it.
Exactly what I am thinking. Not worth it in my opinion. I can live with 14+1 capacity in the FNX.
 

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Discussion Starter · #12 ·
Exactly what I am thinking. Not worth it in my opinion. I can live with 14+1 capacity in the FNX.
I don't fault you for that at all.

However, if the S&W M&P is reliable with it's shorter spring, then it should be possible to accomplish the same capacity in an FNX magazine as well and have it be 100% reliable.
 

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Discussion Starter · #13 ·
Perfect! Just ordered them. Great idea and good job on the research.
After you receive your new springs, try and compare how much weight a similar length of both springs are. Compressing them each from that length down to the same smaller length onto a bathroom scale should tell you. If the M&P spring is heavier, I would venture to say that Cutting the FNX spring down is a bad idea. But if they are the same, maybe I'll just go for it.

I know I didn't explain what I meant very well, but I'm hoping you get it anyway.
 

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Discussion Starter · #14 ·
I tested it. It worked flawlessly. This is an already broken-in mag as well. It did fail to lock back the slide once, but so did my other two mags. After the first time, it locked it back just fine. tested it on the last 5 rounds about 10 times. No malfunctions.

I'll be doing my other 2 magazines now as well. I hope everything goes as well with them...

I don't understand why FN didn't just make the mag capacity 15? Maybe they were pushing people to buy the more expensive FNX 45? idk
 

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Discussion Starter · #15 ·
All three of my mags are now 15 round mags. I'll keep everyone updated with how reliable it is over the next few competitions.

Sent from my SPH-L710 using Tapatalk 2
 

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Discussion Starter · #16 ·
I ran a competition tonight with the modified magazines. No malfunctions or failure to slide lock. These springs are way over sprung. Shortening them to this length, it's like it was meant to be...
 

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Discussion Starter · #17 ·
it's almost been 2 months, I have shot 4 or 5 competitions, and have had no failures to slide lock, and no malfunctions. It's working great.
 

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Nice post, I think this one should be a sticky as it is a great idea. As for modifying magazines, cutting springs or removing capacity limiting blocks are a relatively normal modification for many who own firearms for various reasons as some do it to increase capacity, functionality or on a project where the spring is out of production. One more round matters to a person in a situation where they need that last round to save their life; just my 2 cents. If the mod fails then a person only needs to buy a new spring ($5.00). Note I am not limiting my comments just to FNH products.
 

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Now you've got me wondering if I could do the same thing with my 45...I like having the 15 rounds but I do have a Tim Taylor approach that "bigger is better!"

Thanks for all the info jahtech!
 
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