1911 noob slide lockback question

[Almostelmo]

Finally got out to the range last night with my new (to me) SIG ultra compact 1911. I shot 170 rounds and had two instances where the slide locked back with rounds left in the mag. Both times the round at the top of the mag had slid forward and up and I'm assuming contacted the slide stop. I've read about filing the protrusion on the slide stop a bit to clear the nose of the bullet, but I'm concerned with why the rounds slid so far forward?

I plan to take a look at the clearance with the slide off, slide stop in place and a loaded mag in the mag well later but I'm just wondering if anyone has any input on other possible causes. They are the factory mags and I don't know if it happened on the same mag or not. Could there not be enough spring pressure to keep the top round in place during recoil?

 

[Independence Arms]

Are you sure you didn't activate the slide lock with your hand position?

 

[Almostelmo]

I thought about that after the first one and changed my weak hand thumb position but it happened again after that. The round at the top of the mag was slid forward maybe a 1/4" from the rear of the mag. I wish I'd taken a picture...I didn't immediately realize why the slide was locking back and was more concerned with whether the round would chamber (which it did by simply pushing the slide release)

 

[M1911]

Try a different magazine before you start filing on the slide stop. It may be magazine related, rather than slide stop related.

 

[Almostelmo]

I am definitely going to get some other mags in the near future. In the meantime, I'm going to number the mags I have and pay closer attention if it happens again.

 

[nheric]

i have seen a couple slide stops that had too much material where the mag follower hits and some bullet types would activate the slide stop on recoil. remove the slide install the stop and a loaded mag and look to see how close the bullets are to the stop

 

[LittleCalm]

This is not uncommon with 1911s. You get bullet creep from the force of the last round that is fired and then the next round contacts the slide stop. Filing down the top part of the inside of the stop solves the issue.

 

[Almostelmo]

Well, I took a closer look. The slide stop is already beveled. The first pic shows the clearance with the rounds seated all the way back, the second pic shows how far forward the round has to be to contact the slide stop. The third pic shows the profile of the bevel.

Im heading down to AFS in the morning to shoot so I'll keep an eye on it.

 

[Almostelmo]

I shot about 140 rounds through it on Saturday and had the slide lock back only once with one round left in the mag. The slide lock was just barely holding the slide. I also hit the inside face of the slide lock with a sharpie so I could see if there was any contact from the tip of the bullet. There was a very distinct line of copper on the middle edge so I think I'll file it down a bit and see what happens. I'm still not sure if this issue is me hitting the slide lock (unlikely based on my grip), if it's the magazines (I'm going to pick up some different ones soon) or if it's the slide lock contacting the bullet.

I'm curious as to how the round traveled as far forward as it did with the slide locked back? Would the small amount of forward travel before the slide lock catches be enough to push it as far as shown in the pic below?