Bearing surface contact varies from rifle to rifle, and the percentage of contact doesn't always correlate with accuracy. Here's a basic primer on lapping the lugs.
We have been talking about trying to get to that maximum accuracy potential of our rifles and invariably when techniques are discussed that are supposed to help us get there someone brings up lapping the bolt lugs.
Taking a rifle from the box to its full accuracy potential is a process of many individual steps that cannot be rushed. Often someone throws out a term or procedure like “Lap the lugs!” or “Bed the action!” as if this is the magic voodoo to cure all the accuracy ills.
Well, welcome to reality, boys and girls; many times what some people consider as required for good accuracy is completely unnecessary and may actually harm your rifle, to say nothing of ruining your accuracy. Every rifle is an individual unto itself and what works for one might not work for another.
Lug lapping, in my experience, is the one procedure that is best left as the very last resort, and then it may be best to leave it out completely. Improper lug-lapping can ruin a bolt-action rifle. The concept is simple and logical. To promote the best circumstances for accuracy, the rifle bolt when in battery under a loaded round should be solidly and evenly held in the action, true with the central axis of the bore and holding the cartridge case at minimum headspace.
Several factors are involved here; the face of the bolt must be square with the long axis of the bolt (some say the firing pin axis, but if the bolt was drilled true, they should be one in the same), the chamber must be cut aligned with the bore, the barrel and receiver must be mated properly, etc.
If any one of these things is botched then you can lap the lugs until they disappear, it won’t matter a bit. Lapping is the act of using some type of abrasive compound on the bearing surfaces of the bolt lugs to cut away metal to try to get the bearing surface of the lug in as close to full contact as possible with the mating surface in the receiver. Lapping can be done on precision machinery, but for the most part shooters use the compound/work-the-action method to cut the bearing surfaces.
Here is where we start getting into trouble; how much contact is “enough?” We have seen rifles that shot wonderfully with about 50% contact on the lugs, guns that shot “ho-hum” with 90% contact and several guns that were brought in ruined from too much lapping.
Modern rifle bolts such as those from Remington, Winchester and Savage with modern steels will lap differently than those from old Mausers, where the heat treating is different. What is important to remember is that changing one thing in the precision dimensions of a rifle may have unexpected effects on other dimensions. Proper chamber fit and headspace must also be considered if we are going to muck about with bolt fit in the receiver.
When we build a custom rifle “from the ground up” we try to get the action right first, then chamber, thread and fit a barrel. Lapping the bolt lugs for maximum contact is one of the operations we perform when we are getting the bolt and action ready, along with truing the action surfaces, squaring the bolt face, smoothing the bolt ways, squaring off the receiver face (all of which must be done before the lugs are lapped) and on Mausers, grinding the rear tang for a lower profile.
The lapping is done with a special jig and spud and once we get above 80% contact we stop. So far it seems to be working; one Ruger old model 77 action we barreled in 6.5-284 Norma that received our standard action treatment produces five-shot groups at 300 yards with handloads that measure .775” center to center.
I am convinced that lug lapping alone in this instance had almost nothing to do with this superb level of accuracy, rather the combination of materials we used and all of the treatments and procedures together we followed in the building culminated in a very accurate finished product.
I have been asked many times to give the proper technique for lapping bolt lugs and I’m sorry, but I will not publish such information, for the simple reason that this procedure can so quickly ruin a rifle. There are several write-ups on the Internet and the various forums found there and some of the lapping kits sold through gunsmithing firms such as Brownells provide this information, but I will not.
As I have said, I believe that lug lapping is the “last resort” in working a rifle for accuracy and may or may not deliver the results one is looking for. If your rifle is not shooting to suit you it would be my guess that lapping the lugs would be pretty far down the list of possible fixes for the problem.
After all, take a look at any model 788 Remington, with its nine rear-locking lugs. I’ve never heard of anyone trying to lap those lugs and the 788 is one of the most accurate rifle designs ever made.
All too often we look for the “magic bullet” to make our rifles shoot better, looking for the easy solution instead of the smart solution. Sometimes we need to accept reality and realize that it might not be the gun that needs help. Give it some thought.
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