M262 being hard to get, and M4gery being what many have, does anyone know a good recipie for reloading to M262 specs?
No, largely because the powder being used in military loadings won’t be available to you. You would have to start with the ballistics reported and the bullet used (the 77gr Sierra) and work your way up. M262 shot out of a 20” barrel (or an 18” barrel) shows 2,800 fps or more at the muzzle; SAAMI spec ammo tends to top out in the 2650 to 2700 fps range (depending on powder) for a 77gr pill. If you don’t hold a NFA license, you won’t be able to own a 14.5” barrel for a AR, so let’s not worry about that.
When loading to 2800+ fps is done with commercial powders you, the civilian, can obtain, you’ll see your primers flatten - indicating a hot load. Modern firearms should withstand the load, but they’d better be in spec on the headspace or you’ll start to see case failures. I’m not going to give a load that would replicate M262 velocities because I’m not going to assume any liability for failures of case, firearm or reloader. There are benchrest types out there who claim they can push a 77 or 80gr pill at nearly 3,000 fps, but I don’t think that sort of load would be at all wise, in either a bolt or semi-auto.
You can obtain a commercial loading of the 77gr Sierra bullet from Black Hills Ammunition, at a bit over 2,700 fps. Nosler is also shipping a 77gr round, at 2,600 fps.
Both of their products are of very high quality, so expect a high price accordingly.