Haven't seen much discussion on them, but, are M4 feed ramps highly desirable, or just something nice to have? Or maybe they are pretty much standard on newer ARs.

I don't see many of the manufacturers proclaiming their ARs have them.

They're nearly standard. Usually, you have to look for a receiver without  them if you use a standard rifle extension instead of an M4 barrel extension.

Recently got an upper on sale for backup that does not have the feed ramps.
From searching it seems that not having the M4 feed ramps is fine for semi-auto operation.
Now I'm thinking of building another pistol with it when I finish the 80% lower.
Would this combo work out fine, or should I just keep the rampless upper as a backup and get another upper?

Welp fwiw, my SP1 I bought new in 78, now has approx 12k thru it to date. Some of that was full auto(had a bud yrs ago with a registered DIAS as well as using his trigger group in it). Didn't have M4 ramps. Still doesn't. It has had exactly ZERO malfunctions to date. So I figure it can't be all that bad.

M4 ramps were/are a fix for certain feeding problems with high cyclic rate AR’s in FA.
I don’t recall seeing any posts of the problem in semi-auto.

Les Baer rifles don't have them.  They claim that it makes the projectile chamber at inconsistent angles into the chamber degrading accuracy slightly.  The reason military went with M4 feed ramps was a combination of shorter gas length and faster cyclic rate in full auto coupled with worn GI mag springs to increase function.  If these criteria aren't met, M4 feed ramps aren't a necessity.  On the other hand good luck finding a barrel without M4 feed ramps.

Here are a couple of things to remeber about feed ramps:

1. You CAN use a barrel that has M4 feed ramps with an upper that doesn't without an issue.

2. You CANNOT use an upper with M4 feed ramps with a barrel that doesn't, you WILL have feeding issues if you do.

Yes M4 feed ramps are generally on all barrels now and uppers.

Do you need it? I mean it doesn't hurt to have it and if anything it benefits you.

How descriptive a company might be depends on their market or how well they might describe their products. Generally speaking, companies that market towards the "milspec" crowd will indicate what their rifle may have to make sure folks know. Take BCM, they always make sure they list every possible milspec feature that a person might want from the steel type to various accessories or additions. However, Bushmaster might appeal to people who want a ar15, but not necessarily understand or know what those features might be. So although they have it, they might not care to tell their consumers that.

Las Baer is also a builder of 1911's with few years of experience in the M4 platform. Baer used to install castle nuts backwards and attempt to justify why it was correct to do so. M4//feed ramps add more reliability in any case.

Yep. Not sure sure Les Baer is exactly an AR guru.  1911s, I wouldn't question him.

Didn't it have something to do with tracer rounds being a tad longer so they added feed ramps?

When I started this silly hobby in 2007, M4 feedramps were available usually as an option.
Not as standard in most cases.
Times have changed.  As others have stated they are pretty much standard.

Just for s**ts and giggles I checked my HD carbine for ramps because I couldn't remember if the carbine had them.
The upper was bought at a 2009 Black Friday Sale.  Turns out the barrel is cut for feedramps.
I switched from the flattop upper receiver(that was cut for ramps) to a A1 slickside upper receiver with no ramp cuts.
Gun's been running fine for years with no hiccups. The ramps are similar to the ramps in lower right corner in the pic.
My other AR has the ramps in the barrel and receiver.

I came to the conclusion that I could live with or without them.  For the average AR Joe they probably aren't needed.
Nowadays most AR manufacturers will put them on their guns whether they're needed or not.
That makes the daily argument regarding whether or not M4 feedramps are needed a moot point.
YMMV.

Thanks!

Good info here.  Thanks everyone.

^this. I was just about to post this, it was purely the switch to M855/SS109 that necessitated M4 feed ramps.

William

Great info, thanks!  Posts like this one is why I love this place