Rifle bolt lug lapping - does it really increase accuracy?

[rickyw]

Any experiences with this? Does it increase accuracy significantly? For example, would it take 2.5 MOA 10 shot groups down to 1.5 MOA? I have a few rifles whose bolt lugs do not lock up evenly at all. But a couple possible issues….
A) there will be leftover compound you can’t remove on the locking recesses that will continue to lap your lugs as time goes by
B) your headspace will increase…maybe too much?

Any thoughts appreciated. Thanks

 

[RockAndSage]

Any experiences with this? Does it increase accuracy significantly? For example, would it take 2.5 MOA 10 shot groups down to 1.5 MOA? I have a few rifles whose bolt lugs do not lock up evenly at all. But a couple possible issues….
A) there will be leftover compound you can’t remove on the locking recesses that will continue to lap your lugs as time goes by
B) your headspace will increase…maybe too much?

Any thoughts appreciated. Thanks

It's pretty common practice in accurizing bolt actions, at least by custom smiths. I can't say that it would take a 2.5 MOA gun down to 1.5, but I've seen evidence it does help. Less than other things, but probably still a good idea in most situations if you're trying to squeeze out every ounce of accuracy you can.

All you're trying to do is to get the lugs to mate up evenly during firing. That keeps the brass alignment and pressures inside the chamber a little more even and concentric, which helps with consistency, which helps with accuracy. But with a decently built gun we're talking very small amounts of surface high-spots that are getting smoothed down. If the lugs are so badly machined to begin with that lapping them down to "even" increases headspace to dangerous levels, that gun was a lemon to begin with.

Regarding cleaning out, the compounds come out pretty easily, you just have to get to it. Brake cleaner, rubbing alchol in a spray bottle, q-tips, pipe cleaners, etc are pretty good at getting that stuff out.

 

[khuber84]

I have been around pos rem 700s with unsaquare lugs, crooked tenon threads, shit manufacturing shoot well under sub moa. If you have a 2.5moa rifle, have the rifle bedded, and try different ammo. If still persists, new barrel. During new barrel install have the action blue printed.

 

[NorthIdahoDude]

Not that I'm all that smart or nothing, but - my general experience is that a barrel or bedding problems can either one make a gun shoot like garbage. Pretty much everything else (including load development; assuming all loads are done with quality components in a reasonable recipe for the cartridge in question), provides fairly minor improvements.

For a classic example; I once owned a Kimber 84M in 308 - rifle was smoking hot garbage from the factory accuracy-wise (well, and otherwise too), 6 MOA at best. Had a top shelf smith go through it, and I forget what all he did (it was a 2 page list and expensive), but it included lug lapping, free floating, correcting so bolt didn't touch barrel, and probably a few more mechanical things like that I forget. After all that, and some load development, it was an honest 3 MOA rifle, which hey, was twice as good as I started, but still s**t. The guy I sold it to (with full disclosure) had the barrel re-bored as 338 Federal, and it was a 3/4 MOA gun after that. IOW - there was no amount of work on the rest of the gun that could make up for the fact that the barrel sucked.

Other example, is my Howa Super-Lite - horrifically bad bedding from the factory, and it was a 3 MOA gun. Didn't realize at first what it was, so tried some load development - minor improvements, but nothing major. Figured out it had terrible bedding, so I fixed the bedding (home-brew DIY job, nuttin fancy), and it's a 1.25 MOA gun now.

 

[KClark]

The greatest benefit may be to reloaders. If the bolt lugs are not mated to a locking shoulder that is perpendicular to the axis of the bore it may produce fired cases with the head out of square. Resizing dies will not re-square them. Unless they are orientated exactly as they were when fired the first time the bullet will not meet the rifling square. Tight neck and throat may help.

 

[rickyw]

This is all very helpful to me. Thank you

 

[khuber84]

The greatest benefit may be to reloaders. If the bolt lugs are not mated to a locking shoulder that is perpendicular to the axis of the bore it may produce fired cases with the head out of square. Resizing dies will not re-square them. Unless they are orientated exactly as they were when fired the first time the bullet will not meet the rifling square. Tight neck and throat may help.

An out of square case head on a cartridge that has 002-003 shoulder bump isn't going to matter. The case has no choice but to center itself inside the chamber, it's not squaring against the bolt face(the ejector is pushing it off the bolt face in most actions anyway, even if 100% square). The saami freebore on 95% these modern(in last 20 years) is 0.0005 over bullet diameter, so once the bearing surface enters the freebore it's going to have 0.00025" on all sides of the bullet shank, centering things up, it has no choice. This is why loaded bullet runout doesn't matter unless it's very exteme, it's going to straighten itself. The brass cartridge is going to pressure form fit whatever way it its oriented inside the chamber to the bolt face this firing. If the chamber is true to the bore, but untrue to bolt face, it will not affect accuracy nearly as much as you think. What will happen is the load forces to the bolt lugs will not be uniform, one will bear the brunt VS the other(S) depending on lug configuration. This will show pressure signs prematurely.

 

[RockAndSage]

This will show pressure signs prematurely

That's super interesting. What other problems could unlapped bolt lugs result in? Any suggestions one when it would be worth lapping, vs not?

 

[khuber84]

That's super interesting. What other problems could unlapped bolt lugs result in? Any suggestions one when it would be worth lapping, vs not?

I have 2 3-lug actions from a popular custom action maker, but have north of 500 rounds on them, one lug has all the dlc gone, the other a small amount, the other looks new. Both actions are this way. I've had the action inspected, measured, by a qualified machinest, as well as measuring them with my own depth gauges. The action is square, but the lugs on the bolt are not, idk if the holding fixture wasn't totally solid during machining or what. Both these actions show psi sooner than the cartridge/barrel length should. These are typically observed psi signs: bolt lift, case head swipe, ejector marks. With one lug taking the majority the load, that lugs flexing till lug #2 starts supporting, this allows the case to elongate over the static headspace spec, which is causing these signs, my theory and a few smith's as well. I have spoken with the action manufacturer, and even sent the actions back with very little support and no remedy offered. I've just axed them off a list Id ever use again, unfortunately I cannot sell these rifles or action knowing the issue is there, wouldn't be right. They could be fixed by a quality machinest when a re-barrel happens, as there will be a headspace change.

I don't feel that there are any safety issues here, as long as chamber pressures arent crazy. When using older style saami cartridges, it can cause some cartridge to chamber misalignment. These cartridges had cone style freebores, they basically start very large and taper down at quicker rates than modern chambers, lot more room for angles to start stacking up. These rifles with some tinkering still usually shoot well enough. As I previously stated, bedding and a free float barrel, plus quality ammo, 1-1.5 moa should be attainable in most rifle systems if using bullets supported by the barrel twist.



Lapping is going to smoothen the lugs and polish them, but lapping isn't likely going to remove enough material to actually true them up if there is much more than very minor inconsistent contact. This will require some, or a lot machining, and doing this is going to change headspace since it'll likely move the lug abutments back in the action, and shorten the bolt lugs up a bit, so we're talking a thou or two minimum, in extreme cases, much more. This is why most people accurize/blueprint(fancy name of truing) upon the event of a new barrel install. The tenon threads great slightly oversized, and aligned straight to the new trued lug abutments that interfaces with a bolt that has the rear side of lugs trued to the bolt face. This should have everything in as close of alignment possible, but you're stacking 3 machining processes atop one another, so there is going to be tolerances in there, as things rarely ever come out 0.00000 perfectly straight.

 

[RockAndSage]

I have 2 3-lug actions from a popular custom action maker, but have north of 500 rounds on them, one lug has all the dlc gone, the other a small amount, the other looks new. Both actions are this way. I've had the action inspected, measured, by a qualified machinest, as well as measuring them with my own depth gauges. The action is square, but the lugs on the bolt are not, idk if the holding fixture wasn't totally solid during machining or what. Both these actions show psi sooner than the cartridge/barrel length should. These are typically observed psi signs: bolt lift, case head swipe, ejector marks. With one lug taking the majority the load, that lugs flexing till lug #2 starts supporting, this allows the case to elongate over the static headspace spec, which is causing these signs, my theory and a few smith's as well. I have spoken with the action manufacturer, and even sent the actions back with very little support and no remedy offered. I've just axed them off a list Id ever use again, unfortunately I cannot sell these rifles or action knowing the issue is there, wouldn't be right. They could be fixed by a quality machinest when a re-barrel happens, as there will be a headspace change.

I don't feel that there are any safety issues here, as long as chamber pressures arent crazy. When using older style saami cartridges, it can cause some cartridge to chamber misalignment. These cartridges had cone style freebores, they basically start very large and taper down at quicker rates than modern chambers, lot more room for angles to start stacking up. These rifles with some tinkering still usually shoot well enough. As I previously stated, bedding and a free float barrel, plus quality ammo, 1-1.5 moa should be attainable in most rifle systems if using bullets supported by the barrel twist.



Lapping is going to smoothen the lugs and polish them, but lapping isn't likely going to remove enough material to actually true them up if there is much more than very minor inconsistent contact. This will require some, or a lot machining, and doing this is going to change headspace since it'll likely move the lug abutments back in the action, and shorten the bolt lugs up a bit, so we're talking a thou or two minimum, in extreme cases, much more. This is why most people accurize/blueprint(fancy name of truing) upon the event of a new barrel install. The tenon threads great slightly oversized, and aligned straight to the new trued lug abutments that interfaces with a bolt that has the rear side of lugs trued to the bolt face. This should have everything in as close of alignment possible, but you're stacking 3 machining processes atop one another, so there is going to be tolerances in there, as things rarely ever come out 0.00000 perfectly straight.

Man, that's extremely interesting, thank you for the detail. So, would a fair interpretation be that the bottom line here, is don't worry about lapping unless you just want to smooth the action out a bit, especially with a more modern chambering? Because by the time out-of-true lugs actually impact accuracy or pressure signs, the amount of machining necessary to fix the problem is way beyond just a lapping job?

 

[khuber84]

Man, that's extremely interesting, thank you for the detail. So, would a fair interpretation be that the bottom line here, is don't worry about lapping unless you just want to smooth the action out a bit, especially with a more modern chambering? Because by the time out-of-true lugs actually impact accuracy or pressure signs, the amount of machining necessary to fix the problem is way beyond just a lapping job?

Agreed this will really only make this feel a lil better. I mean throwing some lapping compound on there isn't gonna hurt. Pretty sure a proper lap job would require removal of barrel, tearing down the bolt, and directly applying lapping compound to front of lugs then using the bolt to work it in using a pressureized force applied by fixture to the bolt face. Once complete, I'd recommend full cleaning of action/bolt.