This might be a very stupid question (read: is a very stupid question), but is there any possible negative effects that could come from clamping a forged lower receiver in a vise with nylon soft jaws but without a vise block of any sort? I'm assuming 80% builders have done it before.

I received my Badger Ordnance C1 modular selectors today, and after a frustrating time trying to get the roll pins inserted into the selector switches (who the hell uses roll pins to attach selector switches?) I got impatient, chucked the lower into my table vise, tightened it snug enough not to move, and tapped the pins in. Afterward, I remembered hearing some horror story somewhere about lower receivers getting crushed in vises, but I didn't recall the details. While I'm fairly certain I didn't cock up my lower since I'm pretty sure I clamped down on the most sturdy part of the whole thing and certainly didn't tighten it down as much as I could with my little 3" vise, I figured I should at least ask for a second opinion since I don't know how well lowers stand up to crush forces.

Here's the area where I clamped it, shown in red. About the only indication it was ever in a vise are a couple of tiny marks on the thumb shelf area of my pistol grip; haven't seen any cracks or anything on the lower proper. And yes, I know, I'll be getting a proper magwell vise block soon since it's about the only AR-specific tool I don't have at this point.

If you didn't reef down on it, I suspect you did no structural damage to it, but I would put it together with the LPK and function check it to make sure you didn't crush something that can't be seen.  That area is actually a stronger area of the lower, so odds are you are fine.

Thanks, that's what I figured. I gave the handle maybe a half turn after it made contact, not a ton of pressure since the job being done was super light duty.

The LPK is already installed; these selectors were a last-minute add-on. Everything in the FCG appears to be working properly, but the upper it's intended for is out of my hands at the moment. I can test fit it with my existing upper, though I'm assuming as long as the FCG is working right everything should be golden.

You would really have to crank down on that to do any harm.  There is plenty of meat there.

You should be fine.

I clamp lowers in my vise between rubber padded vise jaws in the same area to install and remove receiver extensions as per the manual with no issues.

Can you even bend that alloy?  Won't it just crack?

I have one in the shop with a bent magwell on it, guy dropped in on some rocks about 30 below him with out a mag in it and it bent the magwell enough that you can't get a magazine in it.

So it depends on a lot of things, but in certain instances they will bend before they break.

I've wrapped them in a shop rag and secured in a smooth-jawed vise with nothing bad happening. Pretty much at the area you highlighted. Just be smart about it.

I've chucked up old mags before and used them as lower blocks for installing some roll pins and tightening the receiver extension down, but other than that, I can do everything on a lower on my table or in my lap.

Best way to check is get a pair of calipers and check the size of the cavities.  The fcg will either fit, or it won't.

I have never had a problem clamping in a padded vise to install RE’s.

It was in nylon jaws?  You're worrying way too much.

I use a magazine well block to hold lowers while I work on them.  Although I can't picture how roll pins work with any kind of selector, I have plenty of (obscenity laced) experience with gas tube pins - getting them in or out.  I fixed that by getting a couple of bench blocks and some decent bar clamps because the issue there is keeping the $^*@ing parts from moving around while trying to assemble them.

With all of that, unless you used a vise with rough teeth and tried hard to crush the lower, you're probably fine.