A proper squad auto uses the issue caliber out of a compatible ammunition feeding device as the issue rifle, in my opinion. These days it's a 5.56. Magpul makes a decent 40rd mag and 60rd drum. It should be a dedicated open bolt design, as well.
The MG42 was always a general purpose machine gun, not a squad auto.
The Ultimax was a great design that has some promise, though limited use (the USMC recently rejected the latest version). The Stoner/KAC design is decent...though I think overly complicated. Belt feed puts it in the LMG role rather than squad auto, regardless of how it's marketed. Like the M249 SAW/FM Minimi, the mag feed feature is a bit of an afterthought.
I like the concept of using one gun in three roles as the Marines are attempting with the IAR...assault rifle, squad auto, and DMR...but in actual practice they all end up somewhat compromised. Its most suitable use is likely as a DMR.
The best practice, as some smaller units have been doing for ages, is firearms specialized and dedicated to their intended purpose. A 10.3" barreled CQB rifle, 14.5" general issue rifle, and 16" DMR seems to be a good formula. As for a squad automatic...I don't think there's a really suitable option out there currently. Maybe the M27 is the best thing we've got.
The Army loves spending untold millions in their endeavors to procure the latest in game changing technologies just to moderately upgrade existing inventories. They are currently hunting for a new squad automatic...though I have little faith that it will be a fruitful process.
All this parts matching/coordinating talk makes building something good from an 80% lower sound a bit of a challenge. More research than just looking through the Brownell's catalog.
If you buy a complete upper there's likely no worries, but manufacturers have been on a trend to use large gas port sizes in order to accommodate a wide range of configurations and ammo types (I.E. cheap commercial spec ammo). Stick with known good configurations from reputable manufacturers and you'll be fine. Buy the operating parts from generic/unknown/no name/Chinese parts from Amazon and Ebay and you might be in for a hard time. The most critical components are the barrel, bolt assembly, and fire control group. The rest just holds it together. Some cheap kits come with a solid buffer (no internal weights) and crappy spring.
Check out inrangetv on YouTube for the WWSD project. It's an attempt to lighten up the ever increasing mass of the AR platform. Not a how to video series, it's a what is optimized parts for purpose experiment. Rick has a rifle that takes a similar approach & is close to perfect.
Buy a quality barrel, bolt group, and fire control parts. Everything else is personal preference & can be as cheap as you want.
I prefer a drop in trigger pack that can't get jammed up if a pebble or primer gets into the receiver & offers a better trigger pull. I have other opinions, but that's the only universal fix to a serious function problem I can think of that I can suggest to anyone.
As folk from parts of the world where they had registration, then confiscation, keep saying, it's the guns you don't officially have that you need. Because your registered collection is at risk.
It's the real reason the gun grabbers want private sales to be illegal.
If the bad guys in government access the records that theoretically, legally, they weren't supposed to have, like the paperwork from Dick's Sporting Goods, then I'm going to have a very unhappy interview with the Green New Gestapo.
It was my habit for years to buy guns on sale, like a shotgun package with deer & bird barrels, and give them as presents to long time friends and relatives with known good character.
Co-worker invited to go deer hunting & had no suitable gun? Here you go, New in box. ( 5 years old doesn't matter )
Before the current Governor took his oath and lied through his smile about upholding the Constitution, I saw the writing on the wall.
I was also scheduled for surgery, so being acutely aware of my mortal nature, gave away most of my collection. Hand guns by legal paperwork, as required, but long guns then needed none, and none was signed. I'd heard plenty of horror stories about rare guns being destroyed or lost if the deceased had a disputed will, or no will, or the executor wasn't the one chosen ( had himself passed away ) or each gun wasn't listed and specifically provided for. And where the survivors had no interest in crazy Uncle Karl's hobby, and didn't know the value. Real world cases where very valuable guns like WW1 Lugers with provenance, taken in battle and papered as war trophies, got stolen for pennies by unscrupulous people who prey upon the bereaved.
Thus the unhappy folk who may someday question me using illegally obtained records. "Where is the Mossberg you bought in 1993? The Assault Weapon you bought in 1989? The .22 Assassination Rifle you bought in 2003?"
My memory isn't what it used to be. I remember Belisarius & his campaign in Syria but not the date. The full name and address of that would be deer hunter? Don't recall.
Will they charge the other guy with rape for giving him the double bird. This crazy train is off the rails. I thought that rape or attempted rape made deadly force authorized. Should he have signed "IF" before the "gun hand" gesture?
I've watched one of the preeminent SWAT teams in the nation shooting fancy precision rifles. I was not particularly impressed.
But, yeah...police in general are pretty sloppy when it comes to entering/clearing rooms and such...using cover effectively, etc. Their man concern is keeping the peace, apprehending suspects, and documentation...so, in that respect I suppose they do what they can.
As I posted elsewhere, the thing to do to combat this, after politely asking your Representatives to vote against it, is to anonymously report your Representatives as dangers to themselves and others, and your fear that they shouldn't have a weapon.
Arguably, most of these people today are really mentally ill. Sociopathic lying, narcissistic personality disorder, an overwhelming need to micromanage other people's lives, how many symptoms are they showing?
I don't know what the shrink buzzwords are for someone who can never admit to themselves and anyone else, the possibility they might be wrong. The sincere? Belief they are better, smarter, more woke, and morally superior than everyone else?
That kind of sickness surely needs to be considered a Red Flag that these people can't be trusted with the instruments of deadly force.