Best datum area to use for Headspace measurement?

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Best datum area to use for Headspace measurement?

Post by cirles » 04 Mar 2019, 2:40 pm

Hi there,
When measuring for ‘Headspace’ in a firearm, should the ‘datum’ measuring point be taken near the shoulder case end (as opposed to the other shoulder neck end) or approximately half way along the shoulder?

Am totally new to reloading rifle or handgun shooting. Will begin to start reloading for 6.5 Creedmoor cartridges. So the above question regarding using a datum dial comparator device to measure Headspace. The device manufacturer (Forster) indicated that I use the .400 hole for this purpose (which measures the datum area just above/below the shoulder area (angle) of the case end (as opposed to the neck end shoulder). Now when I looked up a Hornady device used for this purpose (youtube – not a rep.) he indicated to find the correct datum hole size to use, 1st measure the neck diameter at the shoulder area & then also the case diameter at the other shoulder area, add these two figures together and then divide by 2 (to get a measure of approximately half way down the shoulder). This final figure is used to determine which hole size to use by choosing the closet one to your arrived figure. When I did this for the 6.5 cartridge, the figure came to exactly .375, thus another datum hole, which when eyeballing the cartridge this measures to the half way area on the shoulder.

Which is the best for use of determining the (relative) measure for headspace (I understand that as long as you remain consistent in what you use that perhaps there is no issue?).
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Re: Best datum area to use for Headspace measurement?

Post by SCJ429 » 04 Mar 2019, 5:32 pm

Are you making this measurement so that you can bump the shoulder when reloading? What are you using to make this measurement? Yes you are right, the datum line is halfway down the shoulder unless the case is straight walled or belted.
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Re: Best datum area to use for Headspace measurement?

Post by bladeracer » 04 Mar 2019, 8:02 pm

cirles wrote:Hi there,
When measuring for ‘Headspace’ in a firearm, should the ‘datum’ measuring point be taken near the shoulder case end (as opposed to the other shoulder neck end) or approximately half way along the shoulder?

Am totally new to reloading rifle or handgun shooting. Will begin to start reloading for 6.5 Creedmoor cartridges. So the above question regarding using a datum dial comparator device to measure Headspace. The device manufacturer (Forster) indicated that I use the .400 hole for this purpose (which measures the datum area just above/below the shoulder area (angle) of the case end (as opposed to the neck end shoulder). Now when I looked up a Hornady device used for this purpose (youtube – not a rep.) he indicated to find the correct datum hole size to use, 1st measure the neck diameter at the shoulder area & then also the case diameter at the other shoulder area, add these two figures together and then divide by 2 (to get a measure of approximately half way down the shoulder). This final figure is used to determine which hole size to use by choosing the closet one to your arrived figure. When I did this for the 6.5 cartridge, the figure came to exactly .375, thus another datum hole, which when eyeballing the cartridge this measures to the half way area on the shoulder.

Which is the best for use of determining the (relative) measure for headspace (I understand that as long as you remain consistent in what you use that perhaps there is no issue?).


Headspace is a measurement in the firearm, it has nothing to do with your brass.
I'm thinking you are using a comparator to set your shoulder when sizing your brass?
I would simply adjust the die bumping the shoulder slightly until it just chambers fine, then put your comparator on that case and use that as your setting, regardless of the position on the shoulder.
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Re: Best datum area to use for Headspace measurement?

Post by marksman » 04 Mar 2019, 8:53 pm

When measuring for ‘Headspace’ in a firearm, should the ‘datum’ measuring point be taken near the shoulder case end (as opposed to the other shoulder neck end) or approximately half way along the shoulder?

as long as the datum you are measuring is not sitting near the body/shoulder or shoulder/neck junction the actual one you choose has no bearing on the results you are trying to obtain. use the .400 or .375 whatever is closest to midway :drinks:
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Re: Best datum area to use for Headspace measurement?

Post by cirles » 05 Mar 2019, 10:21 am

Thanks for the reply SCJ429, bladeracer & marksman.

Yes, using the measurement for sizing the brass at the shoulder (to maintain the same measurement for following brass), but will mainly be neck sizing the brass to get as many reloads as possible (less stressing of the brass & a whole other subject area I will have questions on). My understanding is the Headspace measurement is pretty critical within a certain amount of measurement range (0.006 max?) to stop the potential of case separation. So at the moment, want to know how much change there will be regarding virgin brass for reloading (understand initial virgin brass measurement not critical, just for my own knowledge) & what it stretches out to when it has been fired (fireformed), and use that final fireformed measurement (for bumping back) for all following brass fired (of the same make).

Trying to get as much accuracy out of my Tikka TAC, without going to extremes (Bench Rest/ Long range reloading process) for 1000 (+) meter shooting of 4 litre water container's and also 500 meter 'Fly' shooting.

Using the 'Forster Datum Dial Ammunition Measurement System" and a vernier caliper.

marksman - The .400 hole has the cartridge measurement datum sitting very close to the body shoulder while the .375 hole is halfway down the shoulder so sounds like the .375 hole is the one. This seems king of weird because I would have thought all 6.5 creedmoor brass should then be using this hole.

bladeracer - sounds like the appropriate way to set for sizing measurement. How many thousandths back would you start bumping back (0.001, 0.002 ?) to find when cambering fine?
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Re: Best datum area to use for Headspace measurement?

Post by marksman » 05 Mar 2019, 5:56 pm

I have a similar situation with my dashers, the recommended bushing to use for the blue box dasher cases is .350 but I use the .330 bushing as it sits midway on the shoulder of my go gauge and my fireformed cases

incipient headspace separation can become a problem because of over full length sizing, usually over a period of time although I've seen win 303 cases separate on first firing, you wont have to worry about it when neck sizing only. after you have fireformed your cases properly bumping back 2 thou is the go when the bolt get sticky to lift or you could buy some redding type s match bushing die that full length size but use a bushing for the neck so that you control neck tension, expensive but the best you can get IMHO
http://www.brownells.com.au/index.php?r ... t_id=22332
anyway I think it is really something you will only have to worry about occasionally if you neck size :thumbsup: :drinks:
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Re: Best datum area to use for Headspace measurement?

Post by SCJ429 » 05 Mar 2019, 6:25 pm

The main cause of case failure are due to neck splitting due to overworking the neck or excessive pressure loosening the primer pocket. The cure for neck splitting is bushing dies and annealing. You can use FL bushing dies or if you use neck dies with a bush you can give the body a hit occasionally with a small base body die.

What about match case preparation do you not want to do? The big gains are around constant neck tension, concentrate your efforts there.
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Re: Best datum area to use for Headspace measurement?

Post by cirles » 05 Mar 2019, 6:47 pm

marksman & SCJ429 - thanks for the information & yes will be using a bushing (288 size bushing recommended for the Lapua Brass to be used) for the neck sizing die once have established the headspace measurement. I understand the induction route is a option for relieving stress to the neck area but is a step too far for me.

SCJ429 - can you please expand on the 'excessive pressure loosening the primer pocket'. Are you talking about when you get rid of the used primer of a cartridge using the Press for when starting the reloading process?
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Re: Best datum area to use for Headspace measurement?

Post by SCJ429 » 05 Mar 2019, 7:03 pm

If you really lean on your loads, that is try and get as much speed as you can via extra pressure the head of the case expands and eventually you cannot seat a primer anymore. It can be pushed in and out. This can be done with only one firing or after many firings. You first notice that the cases are hard to put into your shell holder.

I neck size only with Redding bushing dies, with Lapua brass I neck sized only for over 20 reloads before I used a body die to bump the shoulder. That said a lot of Benchrest shooters FL size their brass every time and still get excellent case life. Just work the neck the least amount possible.
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Re: Best datum area to use for Headspace measurement?

Post by cirles » 05 Mar 2019, 7:14 pm

SCJ429 - gotcha, loading up max. capacity powder for a cartridge. One thing with the Lapua brass in 6.5 creedmoor is that they have a small primer pocket hole, so I thought that should increase the strength around that area for longevity. I have heard nothing detrimental using small rifle primers for the 6.5, just have to decide which ones to use now. Thanks for the real world info. regarding when you bump the shoulder for the Lapua brass, I have heard good longevity for their stuff.
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Re: Best datum area to use for Headspace measurement?

Post by SCJ429 » 05 Mar 2019, 7:22 pm

You are exactly right, the small primer in the Creedmoor puts extra brass around the primer pocket making it more resistant to it being enlarged. They copied the 6BR in this design. To get the most life out of your necks you really need to anneal them or they will work harden and you will run out of tension.
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Re: Best datum area to use for Headspace measurement?

Post by cirles » 05 Mar 2019, 7:31 pm

SCJ429 - Am I being realistic in expecting 20 reloads from the Lapua Brass without annealing (probably will be going close to or around maximum loads)?
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Re: Best datum area to use for Headspace measurement?

Post by SCJ429 » 05 Mar 2019, 7:55 pm

It would depend on the caliber, your chamber and your load and loading technique. If it is a 300 WSM then unlikely but a 223 it is possible. After twenty reloads and no annealing the work hardened brass is going to make neck tension all over the place. Better to anneal or buy new brass heaps earlier. I get a guy to anneal for me, at 30 cents a case it is well worth it.
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Re: Best datum area to use for Headspace measurement?

Post by cirles » 05 Mar 2019, 8:22 pm

SCJ429 - again, thanks for the input. Haven't done the sums yet for costing so cannot make a judgement call on this. This reloading business certainly has far more to it than I had originally thought but have no other choice because store purchased ammo is just not an option. I purchased 300 cartridges so should see me out for quite some time as am not a big shooter at the moment.
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Re: Best datum area to use for Headspace measurement?

Post by bladeracer » 05 Mar 2019, 10:26 pm

cirles wrote:bladeracer - sounds like the appropriate way to set for sizing measurement. How many thousandths back would you start bumping back (0.001, 0.002 ?) to find when cambering fine?


As I said, you just want the case to chamber neatly, 1-2 thou is all you need.
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Re: Best datum area to use for Headspace measurement?

Post by cirles » 05 Mar 2019, 11:20 pm

bladeracer - thanks for the confirmation.
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Re: Best datum area to use for Headspace measurement?

Post by SCJ429 » 06 Mar 2019, 7:31 pm

cirles wrote:SCJ429 - again, thanks for the input. Haven't done the sums yet for costing so cannot make a judgement call on this. This reloading business certainly has far more to it than I had originally thought but have no other choice because store purchased ammo is just not an option. I purchased 300 cartridges so should see me out for quite some time as am not a big shooter at the moment.


To start reloading, I would make sure the case chambers, you have enough powder to make it go bang and a pill that will make it to the target. After that you can improve incrementally. To be frank you could dent the neck on some once fired brass and that would be all the sizing needed. It would still shoot fine.

What caliber are you looking to reload for?
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Re: Best datum area to use for Headspace measurement?

Post by cirles » 09 Mar 2019, 4:11 pm

SCJ429 - apologies for the late reply to this. Will be loading for 6,5 Creedmoor, Lapua Brass, waiting on Berger Hrybrid Target 140gr bullets to come back into stock. Thinking ADI AR2209, AR2208 or AR2206H powder (a friend just getting into this caliber said he was advised AR2206H was the cleaner burning b/w it and the AR2208 he will be experimenting with) but can't make up my mind yet. Primers also will be something I have to decide on. Will be taking this very slowly.

You mention 'could dent the neck on some once fired brass and that would be all the sizing needed'. Are you referring to the method for getting seating measurements for the bullets/pills (slightly dent case neck so that bullet is held/gripped slightly, cover bullet with black marker then chamber round slowly & then extract. Marking's on bullet should indicate when the bullet had contacted the 'Lands' [do this several times to get an average], measure this & adjust seating die measurements 2 thousands back from this)?
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Re: Best datum area to use for Headspace measurement?

Post by SCJ429 » 09 Mar 2019, 8:04 pm

I was saying that you were worried about all the technical aspects of resizing and how to set up your dies, and not to worry about this too much. If you did no sizing to your once fired brass and dented the neck so that it would hold your pill, you could shoot some impressive groups.

If you are looking for a COAL to start out with, I would use one that fits into your magizene. If you want to work out what is jam for your rifle, then back it off at least 20 thou.

I have never loaded for a Creedmoor but if I were to have a go I would use 2209 or 2208. The speed looks to be faster using 2209 but you may be able to cram a bit more 2208 in the case to make up the speed. For primers I would get some CCI BR4 if you are using small primer brass.
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Re: Best datum area to use for Headspace measurement?

Post by Bills Shed » 10 Mar 2019, 7:24 am

What SCJ429 is trying to say is, keep it simple to start with. You could drown in information reference reloading. Get a primer, powder and projectile suitable for the cartridge and learn the basics. You will progress from there.

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Re: Best datum area to use for Headspace measurement?

Post by cirles » 10 Mar 2019, 6:43 pm

SCJ429 & Bills Shed - Thanks guys for clearing that up & sounds like good advice not getting too information bogged at the beginning.
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