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Hornady 45 cal 230gr FMJ-RN
Bullseye 5.2, 5.4 & 5.6gr

Everything seemed OK at 5.2gr
All rounds OK at 5.4gr, except the slide locked-back with a single round left in the magazine.
At 5.6gr I had the slide lock-back several times during the 2 magazines of rounds. Also, on the last round I had a stove-pipe (not sticking up, but jammed between the breach and the chamber).

Haven't heard of premature lock-back being a sign of overpressure, but thought I'd ask the experts!
In total I fired 76 rounds today, and this was at the end, so the gun was not particularly clean at this point. But I would think my Kimber Ultra CDP II would handle more than that without needing to be cleaned.

Maybe the extra recoil causing the slide-stop to bounce?

Any ideas?

Check your slide stop for signs of brass. The nose of the round can bump on the stop and engage it.

Premature slide lock is a known issue with Kimbers. Give them a call & they will send you a new slide stop with a slightly modified profile.

DocV is right. It's caused by the the nose of the top round hitting the slide stop.

Check your grip also.

It's possible to accidentally put pressure on the slide stop during recoil.

This is usually more of a problem with small gun / big hand combinations, but it's something that should be considered if you're not seeing brass marks on the slide stop. Use of an aftermarket extended slide stop can also contribute to this issue.

Are you talking about copper from the bullet? The brass shell is about 1/4" behind the stop.

sjfcontrol wrote:Are you talking about copper from the bullet? The brass shell is about 1/4" behind the stop.

Yup. The rounds can move forward in the magazine when firing but I would be concerned if the nose of the shell was 1/4" behind the stop. Here is a photo of a 1911's slide stop in a magazine that is being inserted:
In the picture, the magazine is not yet seated. Here is a picture with the magazine fully seated.
As you can tell the rounds in this picture are jacketed hollow points.

Here is a FMJ brass jacketed Montana Gold in a magazine that is not yet seated:

If round moves slightly forward in the magazine it can impact the slide stop. If that is happening you might find marks on the slide stop lug. If so, you can try lightly filing your slide stop to profile it away from the nose of the bullet or just get a replacement slide stop from Kimber. If you reload, you can see what happens if you shoot SWCs.