I replaced the lens o-ring of the S21A because it was kind of thin and was stuck to the lens for some reason. I used the spare o-rings of the FC11.

I added some lubricant to the o-ring because people said it might improve waterproofing.

The order of the parts goes bezel(?), then o-ring, and then the lens, then reflector. Please correct me if I'm wrong.

Here is a closer view of the gap between the body tube and the head. I also lubricated the o-rings there and at the tailcap side.

There's a gap because of the thicker spare o-ring of the FC11 that I put in the S21A, and thus, I couldn't tighten the brass pill any further.

My question is, will that gap will make waterproofing worse?

What it looks like from the front. The FC11 o-ring doesn't stick out much, and it looks like there's a pretty even seal.

Top view.

XHP50.3 HI CRI 9050 at 4000K when it's on at 10%.

O-ring doesn't seem to affect the beam that much.