FWIW, the hammer-and-trigger pins breaking with 9mm uppers was evident from the beginning. That's why Colt quickly switched to stainless steel pins for all factory 9mm full-autos.

So at the very least, go to SS pins.

When I got my first 9mm M16 upper in 1996, I broke the factory Colt mild-steel hammer pin in the first few range sessions. If was scary to discover, because had I not found it, the holes in my receiver wall would have opened up quickly.

I then switched to KNS pins, and I'm running the same set of KNS pins more than two decades later.

Your Mileage May Vary.

Well, yes and no. Many other uppers and calibers are rough on the lower -- 7.62x39 have also done damage, as have extremely short uppers in any caliber.

Stoner's AR15 has an inherently shorter service life due to an all-steel FCG riding in, and supported by, alloy walls. This is not a problem for the U.S. military, but it is for transferables due to 922(o).

I have been dealing with M16s for decades, and cannot count the number of egged-out hammer-and-trigger pin holes I've seen. And not just due to 9mm upper use.

First, I strongly recommend that when you get a new-to-you RR, even if unfired/NIB, you immediately change out the hammer, trigger, disconnector and recoil springs, to have the maximum cushion for the moving parts.

Then replace them regularly, like you change the oil in your car or truck. In a 20" rifle config, you might make 8-10k rounds before needing the change them; in short-barreled 5.56, all 9mm and 7.62x39, that might be as little as 6k rounds, maybe even less.

One thing to watch for is a flattening on the top of the disconnector: If the hammer is hitting it hard enough to distort it even in the least, you have waited too long.

If you run .22RF and are using a lightened hammer spring, do not fire it in 5.56! Swap the spring every time you switch from .22 to another caliber.

Finally, because a transferable lower is worth $15k to $25k, I always run KNS pins (the Gen. 1.5, with hex screws at all four ends, if you can find a set).

Here's why: If you break a hammer or trigger pin, the gun will continue to fire. About all you may notice is that the ROF speeds up and slows down slightly (this does not always happen). However, the component on the broken pin will wobble up and down, side to side.

Think of how you loosen a nail in a board: You wobble it up and down and side to side until the hole opens enough to pull it out.

The locking KNS pins keep the pin stubs, even if they break, perpendicular to the receiver walls. In addition, the locking pins redistribute any impact to all four receiver wall holes, greatly lessening it. It's like four people lifting a tree instead of two people -- much easier.

Getting that peace of mind is worth $28 to me, because I can't replace my Colt M16A1.

Your Mileage May Vary.