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15 round fnx 40

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.
I just left bass pro a few minutes ago and saw 15+1 round fnx 40 and i thought it was a mistake. I am a little upset since i just bought a new fnx 40 2 weeks ago and it only came with 3 14 round mags and its a new gun i didnt buy it from bass pro but still. I know one more round is not a big deal but i could need that one extra some day .
 

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Discussion Starter · #22 ·
I didn't know they started supplying them with 15 round magazines. It only makes sense. The spring on the others is just too long.

Sent from my SPH-L720 using Tapatalk 2
 

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15 round fnx40

I didn't know they started supplying them with 15 round magazines. It only makes sense. The spring on the others is just too long.

Sent from my SPH-L720 using Tapatalk 2
I didnt know either untill last night. It makes me want to return mine that I bought less than a month ago, I didnt buy it from Bass Pro or maybe I would have ended up with the 15 round one.
 

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I literally just bought my FNX 40 yesterday. I am gonna be a bit unhappy if FN is really shipping them with 15 round magazines. I know the mod is no big deal to do, but its less than a day old.

Can anyone confirm from FN that they are shipping the mags with a 15 round capacity?
 

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Is this for real? I just checked FNH USA again and there's no mention of a 15 round mag for the FNX-40. Where are these mags available?
 
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but its less than a day old
Check the date that comes with the fired case.

I'm sure it was made way before yesterday. :biggrin:
 
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Is this for real? I just checked FNH USA again and there's no mention of a 15 round mag for the FNX-40. Where are these mags available?
yes it is real i saw it at bass pro and another
local gun shop. And u wouldn't waste your time and money on the m&p springs I did it and it doesn't like it is going to be very reliable, I would just get the fnx 40 spring and clip one coil off my fnx40 factory mags with no mods will hold 15 but the mag has to be forced in which I would not do if you dont want to damage your gun, so i think just cutting one coil would be good enough in my opinion. But I think it is bass pro's website that says they come with 3 15 round mags now and a few other sites also I think cabela's was another maybe. please let me know if anyone has used the M&P springs and had any luck.
 

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I have 4 mags for my FNX-40 and I put the 15 round S&W M&P springs in 2 of them. I had to mod the mag follower by drilling a 1/16" hole in it to match the M&P springs. So far I have had no malfunctions with the 15 round mags. I put personal defense loads in the un-modified 14 rounder's and I put practice ammo in the 15 round mags.
 
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