Gunsmithing Bolt Lug Contact of High End Customs: Surprising Results

[Straight Shooter]

In a perfectly trued action the top lug will not touch with the test you are doing. The only way the top lug could touch in a perfectly trued action is if the bolt to receiver fit was so tight it would not allow any movement. A production receiver can have as much as .010" lateral movement in the bolt. If you lap the lugs with the trigger in forcing the back of the bolt to the top of the rear receiver ring you will put about .002" tilt across the actual bolt face. This is usually not desirable. I want it square while firing. I like to Cerakote the bolt and the inside of the action to help reduce this clearance. It knocks it down to about .004" and acts like a bushed bolt.

In a target clearance action like a Bat there is a lot less bolt slop and you will see a lot more contact on the top lug with the trigger in place. A Surgeon action or a Stiller Tac action has a bit more slop to allow for dust clearance and they will show much less top lug contact with the test you are using.

 

[300sniper]

1. No one has directly addressed the issue of one high-end custom having more contact on the much-discussed bottom lug (effect of sear displacing bolt body upward), while the other one has more contact on the top lug.

Since these swipe marks in the Sharpie ink are only indications of what is going on while the bolt is in motion (being locked and unlocked) and not necessarily what is going on when stationary (locked into battery), my hunch is it has more to do with forces on the bolt handle than the sear. But again, there are many variables.

 

[Grump]

Well, Grump, somebody did address that point. Like, 8 days ago:

http://www.snipershide.com/shooting...customs-surprising-results-2.html#post2734328

Hopefully your disappointment can be assuaged somewhat.

Um, so you're saying that an ejector plunger's forward force at 12:00 on the cartridge head will lever the case downward in the chamber and tip its front end , in turn pressing the front of the bolt upward in the bore of the receiver with enough force to completely reverse the bolt tilt (rear up, front down, to produce bottom lug only engagement in that condition) and result in top lug only engagement?

No, all that does is analyze our way into a situation where smaller forces up front (compared to upward at the rear with the sear/cocking knob interface) where the entire bolt is pressed against the top of the receiver bore.

The only ways I can imagine THAT resulting in top lug-only engagement are:

1. The receiver bore's top line is tilted upward, from rear to front; or

2. The two bolt lugs just don't engage the receiver lugs evenly.

Number 1 can cause number 2.

Whether and how much that affects accuracy depends on how far off the two are, and whether the caseheads are more than .002 out of square, how large a diameter the casehead is, and probably 2-3 other variables.

I still must conclude that the only attempt to address the top lug-only engagement fell short.

So I stand corrected and must apologize to E. Bryant for not counting his response from 8 days ago (dated from his later post). Sorry about that, and some of us don't read the Board here every day.

 

[Wannashootit]

This topic is another reason why I like the Savage action with its floating/pivoting bolt head. Contact may not be perfectly flush/even with the cartridge base, but it is equal; providing a feature or two from a custom action without the custom action's price tag.

Greg

Which is why I've been hoping an aftermarket shop like PTG would design a bolt assembly for the 700 with a floating bolthead like Savage...
I'd buy it.

 

[smokebomb]

I find this discussion fascinating. Obviously it has very minor gains overall accuracy-wise...but that's what pushing the envelope is for.
Unfortunately there are simply too many variables to calculate with your testing method. 1: manual leverage by a human working a bolt handle 2: the angled sear engagement providing upward thrust on the REAR of the bolt body when in battery 3: bolt lug and receiver face truth 4: ejector and extractor tension on case rim 5: ANY variance in a piece of brass...etc

That said, is there merit then to a standing ejector and claw extraction? What about the prior controlled round feed but based on a 3 lug receiver? Theories are fun.

I mean, if we're splitting hairs here.

 

[Killswitch Engage]

Which is why I've been hoping an aftermarket shop like PTG would design a bolt assembly for the 700 with a floating bolthead like Savage...
I'd buy it.

Pretty sure it exists. SAC was or is doing rem bolt conversions to floating head. You may inquire to him about it.

 

[Surgikill]

That said, is there merit then to a standing ejector and claw extraction? What about the prior controlled round feed but based on a 3 lug receiver? Theories are fun.

I mean, if we're splitting hairs here.

I have never seen the benefit from an extraction plunger pushing on the brass angling it in a chamber. That's why i like mauser type claw extraction actions.

Sent from GS3 Synergy

 

[xs hedspace]

This whole discussion illustrates what I think is the correct method of bolt lug lapping/truing. That is the two lugs should be in contact when the rifle is cocked, and a round is in the chamber. That way, when the chamber pressure pops to 55Kpsi, the bolt will not move to cause more vibrations as the bullet exits. I lap lugs with the striker in and cocked, and sear pressure forcing the back end up. I also put a 1/2" dia. pad of Brownells force 44 solder at the rear of the bolt body, where the receiver bridge is, just enough to cam the bolt down when it closes, ala Borden bump. This solder is virtually the same as Babbit tin bearing metal, and wears very well. You just have to shave the solder down until the bolt barely closes, and keep grit off of it. Target rifles, not tactical.

 

[Deadshot2]

Also you could cut the case in half and put a spring in it (making it longer) to provide some "bolt thrust."

Or run a cleaning rod through the bore from the muzzle and have someone push on it when cycling the bolt. If all alone just stand the rifle on end resting on the cleaning rod handle and open the bolt.

Just a small caution. Make sure you aren't trying to pick fly shit out of ground pepper. If one bolt lug is not marked that doesn't tell you how much clearance there is. "Sharpie" ink, when dry, has an extremely thin film thickness but it does have a thickness. Is that lug on the other side only two or three microns off the lug? Also take into consideration that you'r bolt has to have some slop in it's diameter or you'll have an extremely sticky bolt. One or two thousandths movement could cause the ink on one side to be marked and not the other.

Just for grins, try building up some more film thickness with a sharpie. Paint the lugs up, let dry, quickly paint again with your sharpie (not too much scrubbing or the solvent will dissolve the previous layer) and repeat. THEN check the lug contact. If one side is wiped clean and the other is slightly marked on the surface then is there really an issue. You're talking "microns".

If course you could always just get a Savage with their floating bolt face.

 

[2Shots]

Very interesting reading, I never considered all the forces on the bolt, sear engagement, ejector spring etc. prior to the round going off that might influence the lug engagement and or squareness to the receiver. Obviously you can't build a reliable gun if you take all the play out and try to run too tight a tolerance, one stray grain of powder and it locks up!. I wish one of the custom builders would do and accuracy test for us with lapped lugs and then one lug only touching, maybe a before and after? Same everything else. I'm pretty sure the accuracy comparison between the two would be negligible.

 

[Gene Poole]

This whole discussion illustrates what I think is the correct method of bolt lug lapping/truing. That is the two lugs should be in contact when the rifle is cocked, and a round is in the chamber. That way, when the chamber pressure pops to 55Kpsi, the bolt will not move to cause more vibrations as the bullet exits. I lap lugs with the striker in and cocked, and sear pressure forcing the back end up. I also put a 1/2" dia. pad of Brownells force 44 solder at the rear of the bolt body, where the receiver bridge is, just enough to cam the bolt down when it closes, ala Borden bump. This solder is virtually the same as Babbit tin bearing metal, and wears very well. You just have to shave the solder down until the bolt barely closes, and keep grit off of it. Target rifles, not tactical.

I disagree here. If you're lapping with the bolt cocked, then it is NOT the same conditions as when the bullet ignites because it is necessarily NOT cocked at the point of ignition.

 

[damoncali]

Very interesting reading, I never considered all the forces on the bolt, sear engagement, ejector spring etc. prior to the round going off that might influence the lug engagement and or squareness to the receiver. Obviously you can't build a reliable gun if you take all the play out and try to run too tight a tolerance, one stray grain of powder and it locks up!. I wish one of the custom builders would do and accuracy test for us with lapped lugs and then one lug only touching, maybe a before and after? Same everything else. I'm pretty sure the accuracy comparison between the two would be negligible.

Harold Vaughn did precisely that in his book. He never truly finished the investigation, but he observed a substantial change in POI with only one lug bearing. This leads me to think it matters.

 

[A10XRIFLE]

300Sniper is accurate with his statements. And has posted it the best way in words to describe why only one lug would make contact in the test the OP did. Lugs are normally lapped with the bolt stripped with a spring loaded fixture that places as even as possible pressure on the face of the bolt. The lug contact is checked with Dykum (layout dye) for contact.

As soon as the rifle is assembled, the ejector, Sear, The amount of play (tolerance) between the bolt and the receiver raceway will all affect the outcome of what the OP did in his test.

The real test would be indicating and measuring the bolt from the face to the rear of each lug and indication of the receiver face to the lug seats. If this measure shows the lugs square and the seats equal . Then lapping with 400-600 is to smooth out the machining and seat the lugs.


Hope this helps.