Has anyone ever attached enough numerous heavy (but still practical!) accessories to a slim keymod or modular rail (like Noveske, BCM, Midwest, etc.) to actually cause it to bend or deform?

What exactly would you think is going to bend? Having a light and a front grip of some kind should never cause a rail to bend, if it does you need to take it back. If this isn't what you are referring to maybe you can explain further in more detail.      

NCH

Many times the slim rails are broken because someone inappropriately installed them.  Too much torque is common, parts crack, etc.

But on their own, they are very durable.

Don't feed the troll.

No.

So rails won't sag under the weight of a 16oz bipod and mount, 9 oz IR light and mount, and 20oz scope and mount?

A rail will flex under enough pressure. That's the point. It moves, the barrel doesn't.

If you put a scope on the rail, you're doing it wrong.

BCM initially planned on using a truss design but didn't want to look too German.

I was thinking more for NODs.

Due to the lack of positive responses, I'm fairly sure that rails will remain intact for regular use unless you plan on mounting a Spike's Tactical 37mm and a Picatinny chainsaw off to the side.

I mounted a barbell with 4 plates to the end of my KMR incase I ever need to hit some clean and press during a firefight.

WTF is up with these threads lately?

Light rails have less material. It seems reasonable to ask if they are just as strong, and if anyone has tested them to verify it. It's very unfortunate that there is so much negativity toward the OP for asking a reasonable question.

Why do you have a 20oz. scope and mount on your handguard?  

Reminds me of the broken Daniel defense lower rail from the RIS II and a bipod.

OP, the bipod won't cause you any issue.
That said you should mount the scope to the upper receiver. If you need more eye relief get the correct cantaliever scope mount.

Topic Moved

I suspect that most quality modular free float rails, whether Keymod, M-Lok or proprietary, will hold up well.  My concern would be with the attachment mechanism, rather than the rail itself.  Large, longer proprietary barrel nuts with multiple attachment points make sense.  So do anti-rotation tabs.   I'd not be worrying about flex as much as rotation or separation from the receiver because of the attachment method.

Sure, a rail can be made too thin and would be subject to flex or bending if made too thin and light.  I would think that the market would separate those out pretty quickly, given the abuse that some of us put our equipment to.  The issue would be a rail mounted folded front sight no longer being sighted in if the rail shifts, for whatever reason.   Impact of the rail with an object under stress would bother me more that a few things hanging on it. Since I have such a front sight, and wanted a modular free float rail, I went for more sturdy construction, big fat, multi-attachment proprietary barrel nut and tight fitting anti rotation tabs.  That was more important than saving a couple ounces over some of the "lightweights", although it is still ligher and more silm by far than the typical quad rail of the same length.

Why are you mounting your scope to your rail?