Question for the long range precision guys

[MachIVshooter]

Most of you guys know that I do a lot of machining. Well, I recently started working with an FFL07. I was initially just supposed to be doing barrel threading, RMR cuts, stuff like that, as I never really wanted to be a gunsmith for a variety of reasons, not the least of which is that I have my own 2 businesses to run and kids to raise, so don't want to be spending countless hours in his shop working on equipment I don't like as much as my own. But I kinda got nudged into doing more, and now I'm building high dollar bolt guns on actions like Badgers, Deviants, etc. with equally expensive barrel blanks. He's ponied up for the proper PTG match reamers and bushing sets, and holding tolerances and turning out beautiful work is not a problem, but I want input on the general preferences from this crowd, as many of our customers don't seem to know exactly what they want aside from a really expensive range toy.

My 3 questions are:

How tight do you like your bolt to close (headspace)? At present, I'm treating it like a precision machinist, and that is keeping everything on the tight side, just barely closing on a go gauge.

What sort of crown do you prefer? My SOP is a .030" counterbored flat crown, and .002" bevel on groove diameter with a near-mirror finish. Fairly easy to do, looks nice, and protects the crown.

How long do you like your muzzle threads? Unless someone requests otherwise, I'm doing .625" length right now, except for rimfires due to threads longer than 1/2" bottoming out on baffles in some .22 cans. I have never seen or heard of a centerfire can bottoming on 5/8" long threads, but I certainly don't want someone coming back to us with that problem and needing a cut & re-crown or having to use shims, so if anyone is aware of a can that won't take threads 5/8" deep, I want to know about it.

Thanks in advance!

 

[taliv]

when you say how tight do you like your bolt, i would normally think you are talking about the clearance between it and the raceway or inside ofthe action. if you're really talking about headspace, that's a different question. zero is good. there's no accuracy advantage to making it where the bolt just barely closes. it's just a function of brass life and a thou isn't going to make a difference. and also leaving a little room for dirt and water and crap if it's not going to be a bench gun.

most of my crowns are 11* but i've had a lot of different ones. no difference in accuracy as far as i've ever been able to tell, just a matter of how easy it is to clean the carbon off when it builds up inside a brake or suppressor. if someone is shooting bare barrel a counterbore is good. if they're using a brake or suppressor, i just go with flat

go to zak's thunderbeast website and see his thread specs.

 

[Howard Roark]

How tight do you like your bolt to close (headspace)? At present, I'm treating it like a precision machinist, and that is keeping everything on the tight side, just barely closing on a go gauge.

Taliv is absolutely correct about headspace. The chamber should be cut so that the bolt closes on the go gauge with none to almost no resistance when chambering. This will allow for thread crush to open the headspace up to an allowable dimension when the barrel is torqued down. Actions based on the Rem 700 3 rings of steel design should have .005 clearance on the circumference of the bolt nose. .005 between the bolt lugs and the face of the barrel tenon. .005 clearance between the nose of the bolt and the face of the barrel tenon around the chamber. Actions like BAT, Barnard and Tubb 2000 with cone nosed bolts also get .005 clearance. There needs to be clearance in case of a bit of trash or gunk builds up in there.

What sort of crown do you prefer? My SOP is a .030" counterbored flat crown, and .002" bevel on groove diameter with a near-mirror finish. Fairly easy to do, looks nice, and protects the crown.

I like a flat 90* crown also, it's hard to screw up a rebated 90* crown. That said the standard seems to be 11*.

For the most part the only threads I've cut have been an AR15 service rifles and the specs for those are all over the net.

You know how a gunsmith becomes a millionaire? Start out with 2 million and wait a while!

 

[MachIVshooter]

when you say how tight do you like your bolt, i would normally think you are talking about the clearance between it and the raceway or inside ofthe action. if you're really talking about headspace, that's a different question.

Yes, I mean headspace. I worded it as I did to get input from guys who may not know the numbers, but know how they like it to feel on closing.

The reason I'm keeping them tight is that many of these people are not handloaders, so can't use fireformed brass in a looser chamber or with more headspace. They're the folks who spend big bucks on commercial "match grade" ammo. As such, my line of thinking is that we want the bolt to hold factory cases tight in the chamber.

 

[MachIVshooter]

Taliv is absolutely correct about headspace. The chamber should be cut so that the bolt closes on the go gauge with almost no resistance. Actions based on the Rem 700 3 rings of steel design should have .005 clearance on the circumference of the bolt nose. .005 between the bolt lugs and the face of the barrel tenon. .005 clearance between the nose of the bolt and the face of the barrel tenon around the chamber. Actions like BAT, Barnard and Tubb 2000 with cone nosed bolts also get .005 clearance. There needs to be clearance in case of a bit of trash or gunk builds up in there.

I've been running .004" on the circumference clearance, .008" between bolt nose and breech face.

The Badger I just built requires 5.6 pounds on the bolt knob to close on a go gauge.

You know how a gunsmith becomes a millionaire? Start out with 2 million and wait a while!

Yeah. Lol. That's why this is something I'm wanting to do as fill-in/spare time work. I make a lot more per hour of actual work with automotive. But if I can be making $40 or $50 per hour instead of a wasted day when I have a no-show or a vehicle that ends up not needing much, I'll take it.

 

[redbullitt]

As for barrel threading, it depends on use. For my suppressed rifles I have my smith cut them to the specific needs of the suppressor. IE a silencerco sparrow requires a smooth part be machined in front of the threads for a little rubber o ring. Most all muzzle device companies have their thread specs available. If just threading for a flash hider then it may not matter at all.

Most important part is getting the threads concentric to the bore as well... some guys take this part a bit too lightly.

As to headspace, I am in agreement with the previous comments. I like my rifles to be able to take any commercial ammo without neck turning or anything like that. Too tight and a little dirt or whatever can become an issue, then again my rifles are not bench style guns. That is probably a different ballpark all together.

 

[Varminterror]

In a benchrest rifle, I want to feel a slight resistance on closing, but not so much crush I have to run the bolt hard. My thumb and forefinger should be able to close the bolt, but my 4yr old son's fingers should not... I don't want to have to close the bolt hard enough to make my sear slip.

On a practical precision rifle, I want a touch more clearance to ensure easy closing and a secure sear during fast repeating. I still want to be able to feel the difference between loaded and empty closing.

 

[Sunray]

"...closes on the go gauge with almost no resistance..." There should be no resistance at all. No 'almost' involved. In any case, headspace is a manufacturing tolerance, not an optional thing. No offence but it sounds like you need some smithy training. Being a machinist does not make one a smithy.
"...how a gunsmith becomes a millionaire?..." Start with 2 mil, open a shop and wait a while. snicker.

 

[Howard Roark]

I've been running .004" on the circumference clearance, .008" between bolt nose and breech face.

The Badger I just built requires 5.6 pounds on the bolt knob to close on a go gauge.

Clearance numbers run all over the place and you are right in there. I would not go any farther than .008" bolt gap though. I've seen as much as .015" and I don't like that much unsupported brass.

Badger actions are weird but the PRS guys seem to like them. I get the insert for the lugs and threads, like a Stolle but removable. But it's like they tried to make the firing pin as complicated as possible and the bolt shroud is Huge!

 

[Howard Roark]

"...closes on the go gauge with almost no resistance..." There should be no resistance at all. No 'almost' involved. In any case, headspace is a manufacturing tolerance, not an optional thing. No offence but it sounds like you need some smithy training. Being a machinist does not make one a smithy.

Sunray, there are many ways to cut a chamber with different reasons for each from hunting to benchrest and in-between. None to almost no resistance has worked for me for different applications when measuring headspace while chambering.

When the barrel is torqued down to the receiver there will be .001" or so crush of the threads which opens up the headspace dimension. This allows the bolt to close with no resistance on the gauge when finished. Depending on the application as long as the final headspace isn't under go or over no-go it's good. Thread crush has to be accounted for when checking headspace when chambering.

I'm not a smithy or machinist. Just a guy who builds his own rifles.

 

[MachIVshooter]

As for barrel threading, it depends on use. For my suppressed rifles I have my smith cut them to the specific needs of the suppressor. IE a silencerco sparrow requires a smooth part be machined in front of the threads for a little rubber o ring. Most all muzzle device companies have their thread specs available. If just threading for a flash hider then it may not matter at all.

Much of the time, these guys don't know themselves what can they'll be running (if any), so I'm cutting threads as "general purpose", so GH2 with fore & aft undercut unless otherwise specified.

Most important part is getting the threads concentric to the bore as well... some guys take this part a bit too lightly.

Agreed. My Max total (diameter) runout is .0005", which is definitely more perfect than needed, but doesn't really take much longer to dial in.

"...closes on the go gauge with almost no resistance..." There should be no resistance at all. No 'almost' involved. In any case, headspace is a manufacturing tolerance, not an optional thing. No offence but it sounds like you need some smithy training. Being a machinist does not make one a smithy.

I asked for personal preferences, not opinions stated as fact. Tolerances are exactly that, allow one to choose from a range of relative tight to loose, based on preference.

I am trying to glean from this crowd what sort of features they like best so I can have a good "standard" based on consensus of the experienced in this application when our customers don't/can't articulate some of the specifics.

 

[Jim Watson]

I expect a stripped bolt to close on a Go gauge with fingertip pressure, maybe even by that 4 year old.
5.6 lbs sounds like a lot.

 

[taliv]

personally, i would be concerned about the liability of making tight chambers for factory ammo. it is demonstrably more dangerous, makes the operation of the bolt less smooth, and it won't make it any more accurate.

lose, lose lose proposition.

 

[MachIVshooter]

I expect a stripped bolt to close on a Go gauge with fingertip pressure, maybe even by that 4 year old.
5.6 lbs sounds like a lot.

It's not a stripped bolt.

It closes on loaded factory ammo with barely more resistance than an empty chamber. Bearing in mind, this build was a .300 Win mag, so it's spaced on the belt. My Starrett 445BZ-9RL depth mic tells me that I am at .2226" from bolt face to the edge where the belt seats in the chamber. Go is .220", no-go is .225".

The last build was a .308 on a Deviant, was still snug, 2.2 lbs bolt handle effort on go gauge, barely detectable effort with factory ammo, but would not close on fired cases from my .308 AR.

personally, i would be concerned about the liability of making tight chambers for factory ammo. it is demonstrably more dangerous, makes the operation of the bolt less smooth, and it won't make it any more accurate.

lose, lose lose proposition.

What do you feel makes it more dangerous? We're not talking about beating the bolt home, just a distinctly different feel between empty and loaded, still a thumb and index finger close. On that note, if you have found no difference in accuracy between tighter and looser headspace, I'll take that into consideration. My life is certainly easier if I'm not holding tenths on headspace.

 

[Walkalong]

A go gauge should "chamber" without resistance. You do not want to "crush fit" your loaded round in the chamber. It's bad for function and bad for accuracy. The round needs some head clearance. Not much, but some. Cut the chamber so a go gauge has maybe .001 or .002 head clearance. After that is it up to the competitor to make his loaded rounds fit. Factory ammo should chamber without resistance if made right, but not have more head clearance than needed/wanted/optimal if you cut the chamber to near minimum as far as headspace goes.

 

[taliv]

if it's not stripped, the pressure from the ejector is interfering with your measurement

because all kinds of things can work their way into the chamber, like shavings of brass, or carbon build up, that can cause spikes in pressure. not just water and dirt

 

[MachIVshooter]

if it's not stripped, the pressure from the ejector is interfering with your measurement

because all kinds of things can work their way into the chamber, like shavings of brass, or carbon build up, that can cause spikes in pressure. not just water and dirt

Not to be argumentative, but from an engineering standpoint, a couple thou less room for a case to expand during firing will make no appreciable difference in chamber pressure. Less so, in fact, than the disparity from one load to the next. .308, for example, has an internal case volume of about .21 sq. in. with a bullet seated. For the sake of this calculation, let's just call it exactly .21000 sq. in., and assume that peak chamber pressure is generated in that volume (it isn't, so the figure based on this assumption will exaggerate the actual difference). Anyway, if a load measures 60,500 PSI in a chamber with .003" wall clearance and .002" headspace, .002" wall clearance and .0005" headspace allows 0.55% less volume increase, should roughly translate to 60,830 PSI. If the chamber were so tight the case couldn't expand at all, the increase would be 1.5%, so 61,400 PSI. In reality, the disparity in volume due to tighter chamber will be less, since the bullet is into the bore when peak pressure is reached, decreasing the % change in volume, ergo also the % change in pressure.

But I digress. If you guys with the experience behind the rifles say a tighter headspace makes no difference in long range performance, I'll happily give myself a break not trying to hold within .0002". The difference between thousands and tenths translates pretty close in time, is why parts that are +/- .002 cost twice as much as +/- .005, +/- .001 costs more than +/- .002 and so on. Working in tenths is tedious and exhausting.

 

[Jim Watson]

Bearing in mind, this build was a .300 Win mag, so it's spaced on the belt.

You mean all the people who have been telling me for years to ignore the belt and resize for headspace control on the shoulder are wrong or out of date?

My Starrett 445BZ-9RL depth mic tells me that I am at .2226" from bolt face to the edge where the belt seats in the chamber.

Given your "belt space" at .2226" and with a MAXIMUM cartridge belt of .220", you have a guaranteed minimum .0026" clearance or headSPACE if you are letting a round chamber until the belt stops it.
Up to .0106" clearance if you get hold of some minimum brass with a belt of .212", because spec is .220" +0, -.008"

I am not a gunsmith and do not know for sure what level of precision is important to target rifle performance, but I rather suspect you might be about a half a decimal point too fine.

 

[MachIVshooter]

You mean all the people who have been telling me for years to ignore the belt and resize for headspace control on the shoulder are wrong or out of date?

This is not a discussion about handloading with belted magnums, wherein neck sizing can mitigate the belt's function.

Given your "belt space" at .2226" and with a MAXIMUM cartridge belt of .220", you have a guaranteed minimum .0026" clearance or headSPACE if you are letting a round chamber until the belt stops it.
Up to .0106" clearance if you get hold of some minimum brass with a belt of .212", because spec is .220" +0, -.008"

I am not a gunsmith and do not know for sure what level of precision is important to target rifle performance, but I rather suspect you might be about a half a decimal point too fine.

Are you suggesting that I turn the belt off the go gauge and build a rifle that may not chamber some rounds at all?

 

[taliv]

it's not really a function of volume. just like moving the bullet from ten thou off the lands, to ten thou into the lands actually creates a minuscule amount of more volume in the case, but can spike pressure. and when you're near the edge, even a thou of clearance around the neck can make a big difference in pressure.

 

[MachIVshooter]

it's not really a function of volume. just like moving the bullet from ten thou off the lands, to ten thou into the lands actually creates a minuscule amount of more volume in the case, but can spike pressure. and when you're near the edge, even a thou of clearance around the neck can make a big difference in pressure.

Except there's a big difference between freebore that gives a bullet some momentum before it engages rifling and reduced case expansion due to tight chamber. The former is an entirely different and far more complex physics equation, which we can oversimplify and make analogous to a trying to push a nail into a board vs. beating it in with the same hammer. The latter is really pretty simple math, even if you go as far as accounting for other variables like temperature and the force required to stretch the brass.

 

[Howard Roark]

All ammo (factory, boutique loaded and handloaded) will have some variance in the dimensions of the loaded rounds. Those dimensions will vary between lots as well as manufacturers, even handloaded. Even if the set up of the dies are the same, dies have variance just like headspace gauges and we know not to mix gauge manufacturers. Obviously, the way to have the greatest control over those dimensions is to handload for your chamber and throat.

If a person buys some high speed high dollar ammo to shoot in his $5K rifle and the bolt wont close on the minimum headspaced chamber because the factory let the shoulder dimension slip a few thousandths but still within max spec length guess who the guy is mad at. It probably wont be Lapua. They may try to whack the bolt closed and break it or worse, crimp the neck and cause a pressure spike and a possibly a catastrophic failure.

Because of the above I would chamber for factory ammo at .001 -.002" above min. headspace. I would save the .0005" measuring for centering the bore in the chuck.

A chamber at max headspace will shoot just as good as a min one. You just have to load for that chamber. What is aggravating is when you have two or more rifles in the same caliber but different headspaces. Keeping up with each and each's brass is a hassle.

 

[Bart B.]

Regarding barrel tenon and chamber tolerances for belted cases....

There’s some confusion in terminology in this thread. In my remarks in this post, I’ll use these terms (SAAMI glossary) for case and chamber fitments:

Chamber headspace: distance from bolt face to the chamber point that stops cartridge case forward movement. This is when the firing pin has driven the round against that stop.

Head clearance: distance from a cartridge case against its stop in the chamber from its case head to the bolt face.

And these terms, which seemed good according to the SAAMI rep I discussed them with some years ago, but they don’t officially list:

Case headspace: distance from bottleneck case head to shoulder reference diameter.

Case belt headspace: distance from case head to front shoulder on its belt.

Shoulder clearance: distance from chamber shoulder to case shoulder on a belted case stopped against the chamber belt shoulder.

(Muzzle) face: muzzle surface from barrel diameter to groove diameter. SAAMI terms this “crown” but often conflicts with so many ‘smiths term for the 40 to 50 degree angled surface from bore diameter across the lands to groove diameter plus a few thousandths that cleans up the feathered edge of the rifling.

Crown: that angled (or slightly cupped) surface from bore to groove diameter plus a few thousandths at the front end of the barrel.

My 3 questions are:

How tight do you like your bolt to close (headspace)?

Without any force at all. Standard test is to chamber a round in a horizontal held rifle, push a stripped bolt forward then let the weight of the handle and knob close the bolt.

Three things needed to ensure this happens. One is to lap the bolt lugs flat and in full contact with the receiver lugs with the complete bolt in cocked and in-battery position. This usually puts the bolt head somewhere at the top of its limit to the receiver ring. On my Winchester 70's, that’s where the bolt head goes when cocked. Another is to square up the bolt face while the stripped bolt is held tight in its in-battery condition. All cases need their head’s squared up with their long axis. That’s best ensured to happen when all new cases are first shot with squared up bolt faces behind them. Otherwise, out of square case heads cause 1/8 to 1/3 MOA accuracy degradation ‘cause they impact the bolt face at different points around its edge causing inconsistent barrel whip and vibration directions. Finally, the bolt must have enough clearance to the barrel tenon that lets it go into battery the same when it’s hard back against receiver lugs when the bullet leaves the case and goes through the barrel. .005" to .010" is usually enough.

What sort of crown do you prefer?

I like an 11 degree muzzle face; 79 degree if measured from the bore axis. Too many people testing super accurate stuff from free recoiling rifles see best accuracy with this. But the crown at the edge of the rifling has to be somewhere in the 40 degree range made with a lapping ball of the right diameter charged with fine lapping compound. As the gas pushing the bullet out is much lighter, it squirts past the bullet upon its loss of contact with the groove diameter and causes much turbulence in front of the bullet. If that mass of moving gasses is consistent from shot to shot, bullet’s won’t be deflected from the axis they left the bore at.

PS:
Folks shooting belted magnums getting best groups and scores at long range preferred new cases over neck only sized ones. Otherwise, full length sizing them setting shoulder back a couple thou lets the case headspace on its shoulder. There's a bit of "belt clearance" to the chamber belt shoulder. New cases have everything forward of the belt quite square with the case belt,, so, when it slams into the chamber belt the cartridge is quite well centered in the chamber and bore.

 

[Bart B.]

If a person buys some high speed high dollar ammo to shoot in his $5K rifle and the bolt wont close on the minimum headspaced chamber because the factory let the shoulder dimension slip a few thousandths but still within max spec length guess who the guy is mad at. It probably wont be Lapua.

If one compares chamber to cartridge dimensions in SAAMI's documents and does a little math, they will see bolts may not easily close on cartridges at maximum specs in chambers at minimum.