Early February, I purchased a Sig P320 X-FIVE. After shooting several hundreds of rounds through this highly accurate handgun, I found that the slide would not remain open on the last round. I had 30/30 slide not remaining open in a row. I posted this to the Group earlier this year. I called Sig Customer Service and they said to send it in to them and I did. Sig replaced the gun sending me a new 320 X-FIVE and it works great. Zero problems with both 115 and 124 grain bullets. I liked the 320 X-FIVE so much I purchased a 320X-Carry which also shoots flawlessly, out of the box. I also just purchased another 320 X-FIVE this past week.

Saturday, I returned to the indoor shooting range to sight in the new sig P320 X-FIVE. After adjusting the rear sight for windage, the 320 X-FIVE was right on the bulls-eye. I shot several hundred rounds of 124 grain bullets through it and I began to notice that the slide was not remaining open after the last round. I was shooting just one round magazines and it was not remaining open probably 80% of the time. The SST Range Officer noticed and commented on the problem. We field stripped the 320 X-Five and oiled it, everywhere, The barrel looked worn for about 2 inches and we liberally lubricated the outside of the barrel and reattempted the one-round magazines. It continued to not remain open after the last round.

We then replaced OEM guide rod and spring with a Gray Gun Rod with a 15# spring to see if this would make a difference and it did not, the slide failed to remain open on the last round fired, respective of the magazines used. I replaced the Gray Gun Rod with a 15# spring with the OEM Sig rod and spring.

Becoming frustrated with the same thing happening with the second Sig P-320 X-FIVE, neither the Range Officer nor I could come up with the reason for such failure to remain open on the last round shot. I immediately departed the range and went to the retail side of the indoor Range and purchased a box of 50 147-grain 9mm bullets. It sort of solved the problem but maybe one out of four or one out of five shots with the 147 grain bullets, the slide did not remain open on the last round. I used most of the box of 147 grain bullets shooting single shot magazines. I shot with both hands, strong arm and weak arm being careful not to have any hand parts by the slide release. The range officer did the same. We did have more slide remaining open on last round but still many of the times, once in 4 or 5, the slide failed to remain open on the last round. We then shot 20 rounds of single shot magazines using 124 grain bullets and in all 20 cases the slide failed to remain open on the last round. This is identical to what happened with the P320-X-Five that I returned to Sig Earlier this year. I am frustrated with this problem. Can you please offer a solution for me other than returning the P320 X-FIVE to sig?

NOTE: Sig customer service has always been good to me When they replaced the faulty P320 X-FIVE earlier this year, they sent me a totally new handgun and returned the 4 magazines from the defective gun, so that left me with 8, 21-round magazines.

I've shot a couple mags through a SIG P320, but have zero experience with the X-FIVE and so my comments aren't based on platform experience but general T/S checks.

As you have the benefit of 2 examples, one with consistent lock back and one not, I would lay them side by side, checking/ comparing all the various related parts. Specifically I'd be comparing the guide rod spring assemblies, slide catch levers and return springs, and all the various magazine components, paying particular attention to the slide catch lever to follower interface point.

I'd also be tempted to select one magazine and look for a way to boost the mag spring force slightly..perhaps using a make-shift shim and test that mag to see if increased mag spring force had any effect on the lock back issue.

You might also consider taking some hi-res pics of the various parts and the interface of the mag follower to the slide catch lever and posting them here. It is sometimes difficult to T/S remotely and perhaps others with X-FIVEs might be able to compare theirs to your pics.

Wish I could help. I have never experienced this with my X-Five or X-Carry.

I have 2 x5's. One of the originals and one that came with the black barrel and trigger "upgrade". I am left handed and shoot them in uspsa with a thumbs forward grip and the slide never locks back, although I rarely shoot to slide lock. However, when I intentionally have my thumb off the slide release it will stay locked open. I haven't tested it enough to know if there is 100% consistency, but I am 100% sure that 99% of the time my thumb is on the slide release. Even if it didn't lock back I wouldn't really care as it's a competition gun for me, but it's very strange you got two with the issue. If you put an empty mag in and pull the slide back does it lock back 100% of them time? If so the only possible issue would be either you are hitting the slide release or the spring is too strong and the gun doesn't fully cycle? Very strange.

Does it do it with all of your magazines or just one? Is the follower in the mag weak and not pushing the stop up after the last round fired?

Good suggestions. I was 100% consistent with not having any hand part anywhere near the slide release. The range officer was watching my fingers also. When he shot it, it did the same. I put a 15# spring in it and it did the same....not lock open. It comes with I think a 22# spring. Maybe I should change slides with my 320 X-Five that does not have that problem. If I manually pull back the slide with a mag in the gun, it stays locked back 100% of the time. Even with a lighter 15# spring and 147 grain ammo, it failed to lock open at least 20% of the time?????

When I get a new gun, I first run a variety of factory loads through it, to see what it seems to like best, then go to work, and develop a reduced power handload, of about 132pf. It seems that every gun has its likes and dislikes. Some of my fellow shooters load for the softest recoil, but, I'm usually loading for best accuracy.
My xfive wouldn't always lock back, with several flavors of factory loads. I replaced the guide rod, and went with a 13# spring. It now locks back 100%. I would THINK that a 15# spring would be fine with factory ammo, but, all factory ammo isn't the same.

I have a VTAC and XCarry. I’ve had issues with the X magazines needing to have a break in period. If This is happening with the new magazines that came with the new gun, it might be the mags. I fully load my X mags and leave them overnight. This has resolved the issue so far. I’ ve bought regular full size magazines and put x floor plates on them and had no issues.

I also use the X5 in competition. What I noticed is if you are reloading, it is possible that the rounds are not kinetic enough to fully rack the slide, as mine was. The ejected cases landed about 2' away.

Purchased GrayGuns Fat guide rod, replaced the #15 spring with a #12 and all is great.

YMMV