Gunsmithing Measurements prior to threading / chambering

wnroscoe · Jul 27, 2011

As requested............

There are several key measurements that need to be taken prior to fitting a rifle barrel to any receiver. Of course, if were talking about a trued Remington, Winchester, Savage etc. these merasurements need to be taken after all receiver work has been completed. Prior to any machine work being performed the barrel blank should be indicated in to .0002" or better. This barrel was indicated in to .0001" TRI.

First, as I see it, the key measurements or information that is needed are as follows;

1. Headspace (include recoil lug with this measurememnt)
2. Tennon diameter
3. Tennon Length
4. Threads per inch (TPI)
5. Bolt face depth from bolt nose rim
6. Bolt lug face's from bolt nose rim (front of lugs)

The headspace on this receiver is actually 1.272". The tennon diameter is 1.0625" x 16 TPI (my signature receiver) The tennon length is determined by the relastionship between the rear of the breech just ahead of the bolt nose recess (outer ring) and the and bolt lug faces. I allow .005" clearence between breech rear and bolt lug faces. When measured, the bolt face and bolt lug faces are the same measurement from the bolt nose rim on my signature receivers. This isnt always the case with other receivers and why this is another measurement you need prior to fitting a barrel.

Getting headspace (1.272")

Getting bolt nose to bolt face and lug faces (.160")

OK, at this point I have all I need in order to fit the barrel. I just need to load the barrel in the lathe, face it off and mark my tennon for the first cuts. I set a dial caliper to my headspace measurement of 1.272" at this time and load the barrel. I use the depth measurement portion of the caliper to get enough barrel sticking out but not too much. Sometimes, I scribe a line on the tennon as you ses here. look close.

Here's a better way to mark the tennon length

Now, set tool center and releife. With machine running touch off on the workpeice and zero the dial, turn up Godsmack and away we go.

Bam, done...........1.061"

wnroscoe · Jul 27, 2011

Re: Measurements prior to threading / chambering

Cont.........

Now, check for final tennon length and face off as required. I use a travel dial during the final facing cuts for length after I measure and touch off.

1.340" is too long. I faced off to my final desired length of 1.268" and threaded.

After final facing operation and ready for threads. Insert Threading post here.....

2156SMK · Jul 27, 2011

Re: Measurements prior to threading / chambering

I made up this handy worksheet that I use in my shop to log dimensions.

www.700barrels.com/stuff/700dimensions.pdf

I have been able to re-barrel recievers with these dimensions, but measure the headspace each time as there is some wear when the lugs and receiver settle together and each new barrel can be a thousandth or more different in headspace length...

6.81star · Jul 28, 2011

Re: Measurements prior to threading / chambering

Hey William thanks for the help that's exactly what I've been looking for. It was very generous of you to take time out of your day you are a true gentlemen. This is a big help thanks again.

eddief · Jul 28, 2011

Re: Measurements prior to threading / chambering

Hopefully William won't mind me adding a little here.

Take measurement (A) from face of receiver plus lug thickness to top of the bolt lug, minus .010". This will give you your tennon length measurement (B).

Next take measurment (C) from receiver face plus lug, to the bolt nose. Then take measurement (B) minus measurment (C) plus .010" and this will give you measuremnt (D) which will be your counter bore depth for the bolt nose.

Next measure your O.D. of your bolt nose, this will be measurement (E) plus .010" and this will give you measurment (F) which will be the diameter of your tennon counter bore.

Measure receiver face plus lug to the bolt face for measurement (G) minus measurment (B) minus .002" = headspace

If your headspace dimension is a positive (.001") this means your go-gauge will protrude .001" past measurement (B) tennon length.

If it's negative (-.001") the go-gauge will be in .001" from measurement (B) tennon length.

So hopefully I didn't confuse any one.

(A) minus .010"= (B)
(B) minus (C) plus .010"= (D)
(E) plus .010"= (F)
(G) minus (B) minus .002"= headspace

trigger time · Jul 28, 2011

Re: Measurements prior to threading / chambering

Good post as always William. Quick question for anyone. What is a good depth mic that will go between the the go gauge and bolt nose relief to check headspace? My old Starrett's pin is to thick.
Thanks
Tony

2156SMK · Jul 28, 2011

Re: Measurements prior to threading / chambering

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: trigger time</div><div class="ubbcode-body">What is a good depth mic that will go between the the go gauge and bolt nose relief to check headspace? My old Starrett's pin is to thick.</div></div>

The only measurement that usually needs a thin depth mic is the lug face depth from the receiver face.
I have a smaller Starret depth mic that has thin blades.
It reaches down there real easy compared to any of the round depth mics.

OnyxSkyDV · Jul 29, 2011

Re: Measurements prior to threading / chambering

Great post! It answered / clarified several things for me.

Thanks!

Onyx

nzgrip · Jul 29, 2011

Re: Measurements prior to threading / chambering

tag

JFComfort · Aug 4, 2011

Re: Measurements prior to threading / chambering

I followed along with everything and I understand the way you are doing it. But I for some reason think on a different wavelength and wanted to run this by you to make sure I'm correct.

I take my tenon measurement and subtract 0.010" to account for thread crush and bolt nose/lug clearance. When I’m placing the shoulder and turning down to the major diameter I want my; shoulder to back for barrel length to equal tenon length minus 0.010”. Example, If the tenon length is 1.5” I want 1.49”

For headspace I take my measurement and want to account for 0.002” or so for thread crush. So if I’m using the barrel shoulder and the back of the go gauge to measure headspace I would cut it 0.002” deeper? Take my head space measurement and add 0.002” that way when the barrel is torque on to the action I will end up at the correct headspace?

Sorry for the noob question, I’m very new to this and still working on getting things right.

bohem · Aug 4, 2011

Re: Measurements prior to threading / chambering

I don't take "thread crush" into account that much. 0.002" is a very large crush with well cut threads, many many ft-lbs of torque. I usually measure for the fat side of zero (half a thou or less).

For a "0" chamber I cut it until my depth mic gives me the exact protrusion value plus about 1 line thickness on the micrometer (works to be a couple tenths). My 6.5CM I considered the new factory brass to be "0" and then added 0.0015" to that, which ended up being just about perfect on the actual "0" gauge. The rifle shoots phenomenally.

I think you're right on except for that detail.

If you want a "0 plus 2" headspace then a "perfect" protrusion match (lets assume it's 0.115" as my BigHorn was) you would cut the chamber so the go gauge sticks out 0.115 and then go an additional 2 thou. Therefore the protrusion is 0.113" (chamber is 2 thousandths deeper than it needs to be to close).

wnroscoe · Aug 5, 2011

Re: Measurements prior to threading / chambering

The headspace measurement for me is to help determine tennon length. The final deciding point for me and headspace is a go gauge during chambering. When I'm almost finished with the chamber, maybe .020" to .050" short I'll insert the go gauge and screw the receiver on the tennon with the bolt closed. The bolt face will hit the go gauge and keep the receiver from locking up against the barrel shoulder/recoil lug. I use feeler gauges to measure the gap between receiver face and recoil lug to determine how much chamber I have left to cut. I then cut that amount out +.001 and re-check headspace. If the bolt closes on the go while you can barely feel it call it good. If the bolt closes with slight resistance, take another .001" out of it. If you cut too much and your on the plus side of go just touch the barrel shoulder with your tool to remove .001". At this point it's more about the velvet trim hammer and not the 15# sledge although I do know several trim / frame carpenters that call their sledge the “Trim Hammer”

JFComfort · Aug 5, 2011

Re: Measurements prior to threading / chambering

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: wnroscoe</div><div class="ubbcode-body">The headspace measurement for me is to help determine tennon length. The final deciding point for me and headspace is a go gauge during chambering. When I'm almost finished with the chamber, maybe .020" to .050" short I'll insert the go gauge and screw the receiver on the tennon with the bolt closed. The bolt face will hit the go gauge and keep the receiver from locking up against the barrel shoulder/recoil lug. I use feeler gauges to measure the gap between receiver face and recoil lug to determine how much chamber I have left to cut. I then cut that amount out +.001 and re-check headspace. If the bolt closes on the go while you can barely feel it call it good. If the bolt closes with slight resistance, take another .001" out of it. If you cut too much and your on the plus side of go just touch the barrel shoulder with your tool to remove .001". At this point it's more about the velvet trim hammer and not the 15# sledge although I do know several trim / frame carpenters that call their sledge the “Trim Hammer”

</div></div>

That's a really good idea! I'll write that one down, thank you for the tip!

1SMALLJOHNSON · Aug 12, 2011

Re: Measurements prior to threading / chambering

ping

FullCurlEarl · Sunday at 11:08 AM

bohem said:
Re: Measurements prior to threading / chambering

I don't take "thread crush" into account that much. 0.002" is a very large crush with well cut threads, many many ft-lbs of torque. I usually measure for the fat side of zero (half a thou or less).

For a "0" chamber I cut it until my depth mic gives me the exact protrusion value plus about 1 line thickness on the micrometer (works to be a couple tenths). My 6.5CM I considered the new factory brass to be "0" and then added 0.0015" to that, which ended up being just about perfect on the actual "0" gauge. The rifle shoots phenomenally.

I think you're right on except for that detail.

If you want a "0 plus 2" headspace then a "perfect" protrusion match (lets assume it's 0.115" as my BigHorn was) you would cut the chamber so the go gauge sticks out 0.115 and then go an additional 2 thou. Therefore the protrusion is 0.113" (chamber is 2 thousandths deeper than it needs to be to close).

I’m resurrecting an oldie here! But I am curious, most actions we have measured to find out tenon length and headspace using essentially the same formula above, are always plus or minus 1 or 2 thou. That’s Remington accurized, as well as custom actions.
But a couple days ago an AnTiX came in at plus .013.
You talk above about a Bighorn that’s plus .115 but that’s way too much protrusion, as the Belt on a magnum case would be half exposed. I must be missing something.

bohem · Sunday at 5:21 PM

FullCurlEarl said:
I’m resurrecting an oldie here! But I am curious, most actions we have measured to find out tenon length and headspace using essentially the same formula above, are always plus or minus 1 or 2 thou. That’s Remington accurized, as well as custom actions.
But a couple days ago an AnTiX came in at plus .013.
You talk above about a Bighorn that’s plus .115 but that’s way too much protrusion, as the Belt on a magnum case would be half exposed. I must be missing something.

Impossible, a Defiance that was out of spec? Better call Defiance so they can tell you that your measurements are wrong because they don't make mistakes </sarcasm>

The action I posted about back in 2011 is pre TL2/TL3/Origins, so that 0.115 number is old but let's use TL3 numbers:

A TL3 is nominally 0.125 bolt face depth.
We use 0.010" dirt clearance, so that means 0.125+.010=0.135 total protrusion. This gives you 0.01" between the face of the extractor and the face of the tenon when it's closed on a GO gauge.

Plus the chamfer to break the case mouth, which is typically 0.025" so that puts you at 0.160 total unsupported case
The belted mag case is 0.220 GO
0.220-0.160=0.060 of the belt is down inside the chamber. Plenty for proper headspacing on the belt and the belt is half exposed.

The only reason the belt isn't sticking out of a Remington that far is because the counterbore in the tenon face is typically 0.150 deep and when you account for the counterbore as unsupported material you have around 0.165" of unsupported case even with the tiniest little chamfer at the mouth of the chamber. If you were to cut off the counterbore flush to the mouth of the chamfer you'd see about 0.060' of belt in the chamfer and 0.160" or so sticking out.

Incidentally, a Remington tenon can be cut without the counterbore and it will function just fine. We don't make them that way for obvious reasons but I've seen it done. I've also seen Bergara installs made from converted Remage barrels by simply cutting off the counterbore and screwing it into the front of the Bergara action.

FullCurlEarl · Sunday at 9:16 PM

Ok now I understand. When i think protrusion I was assuming past the end of the tenon, so on a Remington with a nose counterbore I was imagining about .300 + unsupported. It sounds like my experience with this Defiance is not out of the norm. Returning the action for .011 seems unneeded then. Guess like many, I expected better. I appreciate your help bohem, thank you.

bohem · Tuesday at 5:01 PM

FullCurlEarl said:
Ok now I understand. When i think protrusion I was assuming past the end of the tenon, so on a Remington with a nose counterbore I was imagining about .300 + unsupported. It sounds like my experience with this Defiance is not out of the norm. Returning the action for .011 seems unneeded then. Guess like many, I expected better. I appreciate your help bohem, thank you.

I got an email about one this morning that was 0.003" shorter than it should be.

FullCurlEarl · 2025-01-08T00:50:42-0500

Right. This one’s .011, I’ll take that 3 short!
I got an email back from them with an odd reply from sales who was not all that up on how the measurements work. Since I’m north of the Border I’m gonna have to go ahead with the customers build, just to much to return. It’s Nit coated but will have to machine the lugs back on action or bolt, or live with the .011

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