There seems to be a lot of misinformation reguarding rear lugs.
Rear lug receivers have been around for quite some time, long before most folks shot heavier bullets in their M14's. They became popular in the mid 1980's because they allowed match grade rifles to be removed from the stock and reinstalled without the need to fire 15 or 20 shots to get the rifle settled back down. The lug also improves bedding life and coupled with a torque screw, holds the action more consistantly in the bedding to give a slight improvement in accuracy. The torque screw is an important feature to have in a lugged receiver. It's too bad SAI doesn't offer this option.
Lugs are not for everyone. In most cases they are simply not needed, but for a master class competition shooter they definately supply a slight edge. Heavy barreled rifles in McMillan stocks make the best use of the lug. Lighter wood stocks may develop cracks behind the lug and that along with increased cost is one of the big reasons I don't recommend lugged receivers for the average shooter.
As usual, Ted provided excellent information. i would like to add a little more.
The rear lug was "invented" or was first used by the Navy Armorer, Don "Mac" McCoy in the 60's when he was doing it on M1 Garands and that included some with torque screws and some without.
The first 5 rear lugs on Marine Corps rifles was after one of our Senior Gunsmiths STOLE the idea from Mac McCoy and we put 5 of them on rifles just before we left for the Nationals at Camp Perry in 1975. We had JUST gotten official authority to use heavy barrels on NM M14's from The National Board for the Promotion of Rifle Practice a week before the Nationals, even though the Army had been using heavy barrels that whole year and maybe part of the year before. We had been arguing that either they or the NRA HAD to offcially either make them legal or not, for the entire shooting season in 1975. They waffled on it until that time. Then the CO of the Marine Corps MTU told the NBPRP either they made an official decision or we would field "OUR Service Rifle" the Model 40A1, in competition. Well, THAT finally got them off their duffs and they officially allowed heavy barrel M14's. We got that word Wednesday morning when the following Monday we would leave to go to the Nationals.
We NEVER had the money the Army AMU had, so we could only afford 10 heavy barrels for our number one Team Match Team. We began with NOS rifles and worked 12 to 14 hour days through Sunday and actually got them all tested and approved over the test rack by late Sunday afternoon/early evening. It was decided to weld lugs on five of those rifles as an experiment. We weren't sure if the lugs would cause other problems, so they wanted fixe without lugs in case of problems. If there were problems, the five rifles without lugs would be issued to our five best shooters for Individual matches. Of course, the shooters had to actually zero those rifles during practice at the Nationals. However, I have always said I'm glad NO ONE but we few Team Armorers ever saw those first five lugs as they were really CRUDE and not nearly as nice as Mac McCoy made them. We got exceptionally picky about rear lugs in the future and the sides and back had to be almost dead flat and checked with a precision ground block of steel.
I join Ted in NOT recommending a rear lug for a receiver that is going into a G.I. wood stock, because they cracked the stocks so often from the rear left corner of the lug. We rounded the corners as much as we could and it was still pretty common they cracked G.I. wood stocks in maybe 20 - 30 percent of the stocks. Of course in the super fat Walnut Stocks we used, it wasn't a problem. I have used them in a lot of laminate stocks and have yet to see one crack those stocks as long as the lug was shaped correctly.
A rear lug does not increase accuracy. It does increase the number of rounds you can fire before the receiver has to be skim glassed and that is the main virtue of the rear lug.