It's the loaded chamber indicator, in this specific case the small slot or hole drilled at the edge of the chamber to show a round is loaded.

There are a number of posts on the internet trying to find barrels without the LCI - and its alleged that it causes more residue to be expelled, and that it's directly up and back onto the RMR.

I can't find anything in the way of testing, even a side by side comparison with otherwise identically equipped pistols and a "seems like it does" conclusion. Its not about their safety or tactical use, but residue.

Anyway, the RMR fans are distressed about residue buildup on lenses. If you have changed from a LCI barrel to "normal" one with an RMR installed, some reports would be interesting. Best we can get might be pics of lens wipes before and after, and an idea of how many rounds it takes to obscure things.

I am curious as well. I posted this in another recent thread and the discussion died shortly thereafter.
===========================

Being curious about the issue of soot being deposited on the top of the slide through the loaded chamber viewing slot in the barrels of P320 pistols, I took note of mine after a range session today. The pictured unfinished stainless steel slide is from a P320 Carry model chambered for 9mm. I cleaned the slide before the session, and then fired 84 rounds of training ammunition, mostly 115 grain Blazer aluminum which some shooters claim is unusually dirty.

The below photo was taken before the top of the slide was cleaned after the session, but my hand may have contacted that area when performing reloads during today’s drills. I would have expected, though, for any effect that that contact had to be uneven and to leave at least partial visible residues rather than being perfectly clean. The darker band in the center of the slide is an artifact of the lighting that consisted of flood lamps to the left and right of the setup. It is not visible residues.





After photographing the slide I scrubbed it with a white cotton patch saturated with 99% isopropyl alcohol which is an effective way of removing soot-type firing residues. The patch was only faintly discolored. By comparison I used a clean patch on other areas of the slide, including forward of the ejection port and on the sides. Significantly more dirt/residues were transferred to the patch from those areas.

My experiment doesn’t demonstrate anything about the possible effect of firing residues through the loaded chamber viewing slot onto optical sights because different mechanisms may somehow be involved (even though it’s not clear to me what they would be). It was, however, consistent with my previous experiences with the lack of firing residues’ being deposited on the slide itself behind the slot in the barrel hood extension.

I have heard it on the internet, and I also had a sig rep in an armorer's class admit that it does. Another member here (iirc it was hrcjohn) in the other thread said he had experienced it, but I believe the gun he was using had been factory equipped with the full LCI in the slide, and it had been removed. What I don't think we have an answer for yet is if having a slide without the LCI, but just a barrel with the cut in the hood, will foul the glass on an RDO. Sigfreund's test would imply that it's a non-issue, but like he said he doesn't have an optic mounted, so it's not really definitive.

Personally, I'm glad to not own any barrels with that cut, however even if one did, and it did foul, it would be more of an annoyance than a significant problem. Using the Binden aiming principle, one should be able to make hits even with a fully occluded optic.

ETA: I'd also theorize that if it is an issue, ammunition choice would likely be a factor. Light-loaded cheap target ammo that doesn't produce enough pressure expand the case mouth to a tight seal of the chamber would be likely to allow more gas and soot to escape to the rear than a high quality defensive load.

Don't know about the LCI issue, but I do know I get some residue on my Romeo Zero after shooting a box or two of ammo in a 365 with no LCI.

Not a lot, just enough to be noticeable, so it sounds possible to me.

It doesn’t matter.
The lens gets dirty regardless of the notch being there, or not. Qualifying on quantifying the filth is an exercise to pass time.
Assuming both eyes remain open, the front lens could get black as coal and not take away the ability to place the dot on the target, and hit it with extreme accuracy.

A fair point. As I understand it, the first real red dot sights that were fielded by our military were not see-through, and it was akin to having the front lens cap down on a modern one. Not ideal, but with both eyes open, useful enough.

it does leave powder residue on the optic.

quote:

Originally posted by Pete K:
It doesn’t matter.
The lens gets dirty regardless of the notch being there, or not. Qualifying on quantifying the filth is an exercise to pass time.
Assuming both eyes remain open, the front lens could get black as coal and not take away the ability to place the dot on the target, and hit it with extreme accuracy.

It matters to me. My concern would be that repeated cleaning of the Lens on an RDS would degrade/damage the lens coating, especially considering you'd need to employ a combination of mechanical cleaning, potentially combined with the use of a cleaning solution, up to and including a gun cleaning solvent at some point. Regardless of the cleaning method, whether it be a utilizing a lens cleaning solution (which I doubt would be very effective at removing powder residue and/or carbon), or Hoppe's No. 9, that can't be good for the lens coating long term.

quote:

Originally posted by nhracecraft:

quote:

Originally posted by Pete K:
It doesn’t matter.
The lens gets dirty regardless of the notch being there, or not. Qualifying on quantifying the filth is an exercise to pass time.
Assuming both eyes remain open, the front lens could get black as coal and not take away the ability to place the dot on the target, and hit it with extreme accuracy.

It matters to me. My concern would be that repeated cleaning of the Lens on an RDS would degrade/damage the lens coating, especially considering you'd need to employ a combination of mechanical cleaning, potentially combined with the use of a cleaning solution, up to and including a gun cleaning solvent at some point. Regardless of the cleaning method, whether it be a utilizing a lens cleaning solution (which I doubt would be very effective at removing powder residue and/or carbon), or Hoppe's No. 9, that can't be good for the lens coating long term.

Run a few thousand rounds through a pistol with a red dot. You will see that everything you are concerned about is a non-issue.

^^^Please explain how this is a non-Issue and all of my concerns are without merit...

quote:

Originally posted by nhracecraft:
^^^Please explain how this is a non-Issue and all of my concerns are without merit...

1) Residue is deposited on the lens every time a round ejects. My experience is that the LCI notch does not add any noticeable amount of debris.
2) I’m not sure what the manufacturers recommend to clean the lens, but I use windex and a microfiber towel. Years later, and dozens of cleanings have not put a mark on it. Brass has scuffed the paint, but lens remains intact.
3) Hoppes or other solvent is never needed.

I like my stuff clean too. But, even if I spray painted the lens black, it doesn’t affect the ability to put rounds on target, at speed, as long as I have both eyes open.
In practical application, lint and dirt on the emitter side of the lens annoy me more.

This message has been edited. Last edited by: Pete K,

Other than to experiment with for the purpose of advising others, I don’t have any intention of using an optic on a handgun, but for whatever value it may be to others, alcohol is a prime ingredient in many lens cleaners, and I can attest that 99% isopropyl alcohol (and probably 91%) works pretty well to remove soot-type firing residues from guns. I don’t use it on the interior of bores or on pistol breech faces, but it’s about all I use on other parts of the gun. I believe, therefore, that it would work well to remove residues from an optical sight. I have used it to successfully remove residues from handgun mounted lights.

All lens cleaners should be used carefully with optical devices. The usual guidance is to apply them sparingly so they don’t run off the applicator and into the interface between glass (or plastic) and its mount. Some seals/adhesives used to mount lenses can be attacked by strong solvents or cleaners. If used carefully, though, it can’t hurt to try 99 or 91% isopropyl.

I believe Windex and similar cleaners contain a bit of detergent and may work better to remove residues, but I don’t use them on guns because of their common water content. They should be fine on an optical device, though.

Its less about if a lens picks up residue and more about does the LCI contribute more? Its generally accepted residue escapes the chamber during extraction.

Once the brass is released, some pressure escapes past it, out, and possibly up. As the cycle continues the case mouth passes the short opening, once past the hood its wide open. That seems to be a small window of pressure vs the entire extraction cycle.

The observation is correct that lenses are getting residue, my interest is how much extra the LCI contributes. It's like other subjects brought up on the internet, is it really worth the bother?

Doesn't mean someone won't make tear off screens like moto bikers use.

quote:

Originally posted by Tirod:
how much extra the LCI contributes.

Good point worth emphasizing.

quote:

Originally posted by Pete K:

quote:

Originally posted by nhracecraft:
^^^Please explain how this is a non-Issue and all of my concerns are without merit...

1) Residue is deposited on the lens every time a round ejects. My experience is that the LCI notch does not add any noticeable amount of debris.
2) I’m not sure what the manufacturers recommend to clean the lens, but I use windex and a microfiber towel. Years later, and dozens of cleanings have not put a mark on it. Brass has scuffed the paint, but lens remains intact.
3) Hoppes or other solvent is never needed.

I like my stuff clean too. But, even if I spray painted the lens black, it doesn’t affect the ability to put rounds on target, at speed, as long as I have both eyes open.
In practical application, lint and dirt on the emitter side of the lens annoy me more.

Yep.

Of all the things to worry about, this isn’t one of them.

To the OP, I shot about 500 rounds through a M&P CORE pistol over the weekend. Ammo was 147 grain Freedom Munitions. The optic was no worse than if I’d shot 500 rounds through a Glock or P226.