There are several ways an aftermarket recoil lug can be a vast improvement over a factory lug. First you need the right type of recoil lug to be of benefit.
1. Must be heat treated
2. Must be surface ground on both sides to insure its consistant in thickness to within 0.0002" or better
3. Bore through lug should also be true.
A factory lug, such as on a Rem 700, is not heat treated, it is not surface ground. Because it is not heat treated, it can bend and flex much easier then it should. This not only is a problem with recoil lug but also in how the stock handles recoil.
It is also not surface ground. Most of the time they are decent but rearly will they match a good qualiy after market lug. This is critical because when you torque a barrel down on a lug that is sandwiched between the receiver and barrel, if the lug is not consistant in thickness, the barrel will be stressed when it is torqued. Never a good thing for accuracy.
With an aftermarket lug, there is none of this bending or flexing under recoil. There is also no stress imposed by the lug when the barrel is torqued onto the receiver, given that the receiver face, threads and barrel threads and shoulder are true.
other benefits of an aftermarket quality lug, they generally extend farther down then the factory lugs so they offer more area for the recoil energy to be transposed to the stock from the barreled receiver. The more area, the easier it is on the stock to support this energy.
Also, they generally have tapered sides so they are much better for glass bedding.
You can also get larger diameter lugs then what is standard so if you use a larger then standard diameter barrel, it will look much better on the larger diameter aftermarket lugs.
For the money that these lugs cost, if your rebarreling a rifle, there is really no reason not to upgrade to a good lug, again, this is only one part in the entire system and your rifle system will only be as true as the least true feature of the rifle. as such, its best to correctly accurize the receiver and fit the barrel threads properly to get the most out of an aftermarket lug.
Hope this helps some.
Allen Precision Shooting
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