Measuring rifle chamber ?

[Bassfish1952]

So far I have fired 20 rounds thru my new 6 ARC barrel , Before working up a load for this rifle would it benefit me to measure my rifle chamber ? I am looking at an add from Copper Creek and wonder if it is worth sending a fired brass to them or someone else to have a modified case made for my rifle ?


Thanks

[grayfox]

I know they advertise that, and maybe it's a decent one... but it's not worth it IMO, especially if you have more than 1 rifle. Maybe for folks who have 1 real-precision type gun and want everything to be match grade quality, but...

C Creek also makes a standard-modified 6mm Arc case that works for the hornady OAL comparator. I just got their standard modified case. What you're doing anyway is measuring things like bullet fit OAL's and a standard comparator case will tell you that.

If your barrel was made by a solid outfit the chamber is most likely fine (unless you have some evidence to the contrary), unless I see something way-off I don't worry about that. Instead I need COAL's for the different bullets so I don't jam one into the barrel lands and risk all the problem-"niceties" that causes (lol!), they can differ a lot one bullet-type to another. 100 tgk, 103 eldx, 95 Nosler BT, 90 Eldx, 108's, 95 Bergers, etc. From 2.140 to 3.338 OAL's, 6Arc has large spread of these, wider than most calibers IMO.

What you're thinking about is shoulder setback, that's where a fired case comes in. A fired case has a shoulder that pretty much tells you what the "datum" point is going to be for your chamber... so you can resize and set back a few 0.001's (0.003, or 0.005 like I do b/c I don't want to recalibrate for every chamber). Saami datum point (the 0.350 diameter on the shoulder, or "B") is 1.190-0.007. So you want that resize to get you back to there but more importantly, is the 0.003-0.005 (your choice) less than fired size shoulder measurement. For that you need to measure fired shoulders of a few cases out of that chamber, and that doesn't use a modified case, rather it is the Hornady lock/load head space comparator (or something equally good), using the 0.350 diameter or "B" insert and a digital caliper. Thing is, there's no guarantee that the "B" is exactly 0.350 (it's close) but that's fine because you're using it for your barrels... just can't do a 1:1 comparison for someone else's measurement (they should be close but no guarantee of being equal). Anyway that's how you find your chamber "size" but really it enables you to set your resizing die so as to get the small 0.001's setback you need for chamber fit plus longer brass life.

[Bassfish1952]

I know they advertise that, and maybe it's a decent one... but it's not worth it IMO, especially if you have more than 1 rifle.  Maybe for folks who have 1 real-precision type gun and want everything to be match grade quality, but...

C Creek also makes a standard-modified 6mm Arc case that works for the hornady OAL comparator.  I just got their standard modified case.  What you're doing anyway is measuring things like bullet fit OAL's and a standard comparator case will tell you that.

If your barrel was made by a solid outfit the chamber is most likely fine (unless you have some evidence to the contrary), unless I see something way-off I don't worry about that.  Instead I need COAL's for the different bullets so I don't jam one into the barrel lands and risk all the problem-"niceties" that causes (lol!), they can differ a lot one bullet-type to another.  100 tgk, 103 eldx, 95 Nosler BT, 90 Eldx, 108's, 95 Bergers, etc.  From 2.140 to 3.338 OAL's, 6Arc has large spread of these, wider than most calibers IMO.

What you're thinking about is shoulder setback, that's where a fired case comes in.  A fired case has a shoulder that pretty much tells you what the "datum" point is going to be for your chamber... so you can resize and set back a few 0.001's (0.003, or 0.005 like I do b/c I don't want to recalibrate for every chamber).  Saami datum point (the 0.350 diameter on the shoulder, or "B") is 1.190-0.007.  So you want that resize to get you back to there but more importantly, is the 0.003-0.005 (your choice) less than fired size shoulder measurement.  For that you need to measure fired shoulders of a few cases out of that chamber, and that doesn't use a modified case, rather it is the Hornady lock/load head space comparator (or something equally good), using the 0.350 diameter or "B" insert and a digital caliper.  Thing is, there's no guarantee that the "B" is exactly 0.350 (it's close) but that's fine because you're using it for your barrels... just can't do a 1:1 comparison for someone else's measurement (they should be close but no guarantee of being equal).  Anyway that's how you find your chamber "size" but really it enables you to set your resizing die so as to get the small 0.001's setback you need for chamber fit plus longer brass life.

Thanks
In their add the modified cases are out but they advertise sending in one of your own , thought I would ask before spending money on something I won't need

[grayfox]

What you're doing with shoulder setback is changing (usually raising) the depth of your resizing die in the rock chucker (or whatever your model is).  Most resizing press mfrs say to set the die so that it just hits the seating collet on a full-up position.  That can set a shoulder back to "Saami" but probably sets it too far back for a given guy's rifle.  You want the fired shoulder to tell you where your chamber leaves that shoulder after firing, so  you can tweak it back a few thou's from there, so it works for the next round of loads, but also stretches and sizes the brass the smallest amount for best brass life.
You can start by putting in your fired case (you already had measured its shoulder) into the resizing press and resize down to whatever the mfr setting will give you (by setting the die to seat flush against the holding collet).  Then measure the shoulder setback.  For example the fired case might be 1.193 and resizing brings it to 1.180 - a 0.013 delta.  Lots of brass-working going on there!  You want less than that but still Saami and workable in your gun.  Let's say you want close to 1.185 final (delta of 0.005 is where I go and lets you account for some springback and still be ok). 

So the resizing die and the press usually have a thread pitch like 7/8-14, the 14 is what you need.  Math-wise a 14 pitch means 1 turn moves the die 1/14th of an inch, or 0.071428"... 0.003 can be found then by (just follow my math here) 10deg= 10*0.071428/360 or 0.00198.  So if you back off from the mfr setting by 10 deg, you have moved it basically 0.002".  15deg = essentially 0.003.  So take a sharpie and make a match-mark set on the locking ring+die body at a convenient point, then make a 30 or 90deg mark on the ring, then cut it down to 45 or 30 to get to ~15 deg.  30 deg is easy to mark and would leave setback at 1.187 (~0.006 from fired).  The die body will move ccw relative to the locking ring so your 30deg mark on the locking ring should be in that direction, then you can loosen the ring and move the die body out, matching up your "base" mark on the body to the "30 deg" mark on the ring.  Measure another fired, then resize with this new setting to see where you are shoulder-wise. Tweak and adjust from there.  Do a sharpie final match-mark set to document where your setting is. 
This gives you a logic and a method for marking so you can get to a setback you want, plus if you ever loosen the locking ring you can get back there with your final match-mark set.

---

Sportsmans.com and a couple other places have a hornady LNL modified case, 6-9 bucks plus whatever shipping... throw a couple other things in there too if you want...

[Old Bob]

I've been using the Redding Competition Shell Holders to set back the shoulders of my 6 ARC brass. Use #12 holders for 6ARC.

https://www.creedmoorsports.com/product/...ll-holders

[CZ527 Guy]

Here is a simple yet somewhat crude means of measuring your headspace setback if you are at all curious. Take an unfired round & place a thickness of scotch tape or masking tape on the base of the case. Load the round with the thickness of tape on the  base in the chamber & close the bolt. Expect this will close with no resistance. Then repeat the process with a second & third layer of tape if needed. Any factory ammo may be a bit inconsistent though this is an inexpensive yet a bit crude way to evaluate your head space. My hope or expectation is there would be no need for the third layer of tape as the second layer offered resistance to closing the bolt.

I have the gauges to test this headspace setback, purchased from Whidens for 6mm ARC only If you are at all serious about checking this . For a limited time only Ill check a case for you for the cost of a case. That is I will not return the case. Send me a case or a box of cases I'd measure the case  or cases & compare it to my PTG go Gauge. & respond with my findings.

Reality is I see no reason for concern though if you are wondering, here are a few options. My Drop in barrel from Pacnor had roughly .004" head space clearance with the PTG Go gauge with no modifications whatsoever. I was impressed, Not sure if this was luck or if this is common! Nonetheless I was pleased!

Id be curious to see what Pacnor did with your custom project. Though Ill repeat that I doubt you have any reason for concern.

[Bassfish1952]

Here is a simple yet somewhat crude means of measuring your headspace setback if you are at all curious. Take an unfired round & place a thickness of scotch tape or masking tape on the base of the case. Load the round with the thickness of tape on the  base in the chamber & close the bolt. Expect this will close with no resistance. Then repeat the process with a second & third layer of tape if needed. Any factory ammo may be a bit inconsistent though this is an inexpensive yet a bit crude way to evaluate your head space. My hope or expectation is there would be no need for the third layer of tape as the second layer offered resistance to closing the bolt.

I have the gauges to test this headspace setback, purchased from Whidens for 6mm ARC only If you are at all serious about checking this . For a limited time only Ill check a case for you for the cost of a case. That is I will not return the case. Send me a case or a box of cases I'd measure the case  or cases & compare it to my PTG go Gauge. & respond with my findings.

Reality is I see no reason for concern though if you are wondering, here are a few options. My Drop in barrel from Pacnor had roughly .004" head space clearance with the PTG Go gauge with no modifications whatsoever. I was impressed, Not sure if this was luck or if this is common! Nonetheless I was pleased!

Id be curious to see what Pacnor did with your custom project. Though Ill repeat that I doubt you have any reason for concern.

Right now I am just curious and didn't know if I needed to do it before I do load development . I have 20 fired cases now  am going next week to shoot more with the new scope and hope to have 100 once fired cases by Dec. 1st then I'll load the 90gr GMX's and see what can be worked up for January Hunting. 
Think I will pass on sending the case,  They seem to be in short supply still .