Is it more relevant to free-float on a rifle length than a carbine length?  Only asking because the handguard cap is further down on the rifle length and theoretically it seems like inconsistent pressure there would affect harmonics more.

Yes.

For a weapon largely used at distances less than 125 meters, isn't 2MOA enough? It's when you are qualifying at 25m and you need to get all the company thru Basic Rifle Marksmanship that you need the edge.

Yes, theoretically the longer barrel would suffer less inaccuracy if it was free floated. Keep in mind a free float can't make it more accurate than the barrel is capable. If someone is attempting to accurize a AR then a better grade of barrel would be the right money spent first, an $88 cheapy isn't going to be guaranteed .25 MOA. Even then a free float isn't as important as the sights/optic.

The ORIGINAL concept was to put the sights out as far as possible on the barrel, free float it so it won't move around, keep the harmonics neutral at the muzzle which may require tuning, and use the smallest practical aiming point for improved accuracy. What has been in practice is to use a 2MOA red dot on a 14.5" barrel with the longest ranges typically 125m because that's a far as the average combat shot might reach in dense woodland or urban build up. The fantasy of long distance engagements is now being supplied by the HK M110A1 SDMR which is now being issued.

We dont need no 5.56 sniper rifles. If you want to hit out to 500m, you still have to carry enough mass to penetrate, the same thinking that elk hunters use in the Rockies. Small coal don't bowl.

For the most part the Army accepted the quad rail because it was an institutional compromise trying to fit 38 pieces of different gear for dozens of MOS jobs both offensive and defensive. It being a free float wasn't a primary benefit, note it went right on down to the 11.5", too, which is a short range ship boarding and urban townhouse Osama shooter.

Keep in mind that all the BUIS mounted on the free float means the entire purpose is destroyed, a sling or grip on it will result in POI shifts. Recon builders don't often understand why their red dot works fine but the BUIS is less accurate.

Depends on what you are doing.

If you are shooting golf balls at 100 yards, that is just barely over 2moa.  Beyond that you are looking at misses.

If you are splitting playing cards at 25 or 50 yards, 2 moa is a miss iwht a .22 caliber bullet.

And yes, I shoot like this wiht my 10/22 builds - working on being able to do the same with an AR :)