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Miseryguts

Head Space Head Ache

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Hi, chasing my tail here trying to make sense of head space readings!!

I have a round that will not allow the bolt to close on my Tikka T3X 223 remington rifle.

Case comparator tells me the head space dimension of the cartridge is 1.462 or 1.463. SAAMI spec says Minimum chamber size is 1.4636 with a tolerance of +0.002, and maximum cartridge head space dimension is 1.4666. So why won't the bolt close?

I have unused(brand new) FL sized cases 1.457 to 1.459

I have fired (in my rifle) FL sized cases 1.458 to 1.459

I have once fired (NOT in my rifle) FL sized cases 1.455 to 1.457

So, does this mean that the Head space on the chamber is less than spec and if not, then are my cases going to have excessive head space? I thought that one should aim for 0.002 head space difference not the 0.003 to 0.007 these cases will give me?

BTW, I have checked the digital vernier against a micrometer!

Any input gratefully accepted!

M

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New cases are usually a few thou. short of a 'go' gauge.

So, in the absence of a 'go' gauge, take a brand new out of the box (pref Lapua) case and see if it chambers.

If it does, you can then do a rough headspace check by stripping the bolt to remove the spring assembly and cutting a small circular shim from an aluminium beer can/Coke can to fit the bolt face recess. Hold it in place with a dab of grease. The shim will be about 4 thou. thick.

Now, carefully close the bolt on the new case - it will probably close with a bit of force - if so, the headspace is about right.

Once you've established that - it's up to you to size your brass to fit the chamber.

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A 'comparator' bushing is NOT a headspace gauge! As the name suggests, it is for comparing a resized case to one that is fireformed and therefore (more or less) matches the chamber. IME experience, the diameters of the Stoney-Point / Hornady case and bullet bushings sometimes vary considerably from each other of the same nominal size / calibre, so much so that if you have more than one example of a calibre bullet comparator bushing, you have to mark them and ensure you use the same one each time for a particular job.

The idea is to ideally start with new brass in your rifle which as the Gun Pimp says usually has a little too much clearance. (Not always though - the Stalking Directory forum has more than a few complaints  of factory PPU ammunition giving tight chambering through a small amount of negative headspace, usually in 243 Win and often in T3s.) After firing, the comparator is used to get a reading on the case shoulder (or properly, several case shoulders) as you've done - then it is used to set the sizer die up in the press so that the shoulder is 'bumped' back 0.001-0.002" for a conventional turnbolt bolt-action rifle. If this (measurement plus die setting) is done correctly, the case will fit the chamber easily but incur minimum case-body stretch on firing.

You need to find out why the round won't fit your chamber and sort it. Headspace is one of several possible causes and isn't guaranteed to be the culprit. You don't say for instance whether it is a case previously fired in another rifle - sometimes you cannot full-length size a case fired in a slack chamber down to fit that of a tightly chambered job, but it can be over-long COAL and other problems. A sticking case leaves witness marks on the brass or bullet - where are they on your cartridge that won't chamber?

 

 

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Hi yes the cartridge case is one that has been previously fired on my rifle. COAL is 2.230, neck dia. is 0.247, case at shoulder is 0.354, case at web is 0.373 - all of which are within spec. There are some longitudinal striations on the slope from shoulder to neck

M

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I would expect the brass that wont chamber is oversize in the body, web area.

Its possible to get incorrect headspace readings because the resistance you feel when closing the bolt is coming from the oversized web struggling to fit the chamber rather than the shoulder bottoming out at the end of it.

Try comparing brass dimensions at 0.200" from the end of the case or like this, you could also put the case in a shell holder and rest the caliber blades on there to give a consistent reading. This is my gauge which is a piece of barrel with a hole bored in it to a depth of 0.200", the flat blades of the caliber then rest on there to read in the same place on every case. Less than 0.001" can be the difference between the case sticking and giving a false impression of the shoulder touching the bottom and not. 

As Vince suggests I would measure the headspace with a resized case that fits and allows the bolt to close, the only difference is I use sellotape added to the bottom of the case and trimmed with scissors, its 0.0015" thick so you can build it up in layers increasing headspace each time by 0.0015" once the bolt starts to drag but still closes then you can measure it with your comparator and call that zero headspace, set your sizing die up to then bump the shoulder 0.002"-0.003" less than your zero measurement.

 

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Sorry I can't seem to upload images, the file size available to me is not enough, you can see the gauge I mention on my Facebook page in my signature.

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Big Al

Measured the cartridge that cannot be loaded as follows:

COAL 2.230; CBTO 1.875(distance to lands with this bullet, Sierra 55gn FMJBT, is 1.967); Head Space dimension is 1.462 to 1.463 measured using a Hornady comparator. Neck dia. is 0.247; dia of case at shoulder is 0.354; dia. of case 0.2in from base is 0.374

I think all these dim. are within SAAMi specs. Cartridge chambers, but bolt stops halfway down. I have no way of checking the comparator insert dimension, as it looks as though this is suspect as per Laurie's post

Gun Pimp

All the cases listed in my OP chamber without problem - will try your method to get an idea of chamber head space shortly

M(still with a head ache)

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18 minutes ago, Miseryguts said:

Big Al

Measured the cartridge that cannot be loaded as follows:

COAL 2.230; CBTO 1.875(distance to lands with this bullet, Sierra 55gn FMJBT, is 1.967); Head Space dimension is 1.462 to 1.463 measured using a Hornady comparator. Neck dia. is 0.247; dia of case at shoulder is 0.354; dia. of case 0.2in from base is 0.374

I think all these dim. are within SAAMi specs. Cartridge chambers, but bolt stops halfway down. I have no way of checking the comparator insert dimension, as it looks as though this is suspect as per Laurie's post

Gun Pimp

All the cases listed in my OP chamber without problem - will try your method to get an idea of chamber head space shortly

M(still with a head ache)

Can you take the bullet out of the case and see if you can close the bolt properly on the case, that would narrow things down to the case or the bullet. Also have you checked the brass trim length, could the case be binding at the end of the neck?

Is the round that wont chamber one you made yourself or is it a factory load?

You say earlier that the 0.200" mark is 0.373" which is within spec but the one that wont fit is 0.374"?

What spec? do you know the size of your chamber at 0.200", I doubt it, you can't rely on specs because you dont know what size reamer was used to cut your chamber regardless of where it was done.

I would want to slim the 0.374" dimension down to 0.373" or less and see if things chambered.

There isn't that many places where the problem can be, its just a case of eliminating them one by one. Forget SAAMI or any other spec as you can't verify anything in relation to your chamber. Work with what you have, size things in the key areas progressively smaller and eventually you will find which dimension was causing the trouble, it really is a simple case of elimination.

Start by making sure the trim length and loaded neck gives clearance, then the web then the shoulder length, its got to be one of them if its the case, if not then its the bullet seated too long.

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Big Al

will try that, I would not of thought the trim length was wrong, I trim all my cases every time. Removal may be difficult, it is one that is crimped, and I only have a kinetic removal tool. The duff cartridge is one of a 100 batch that I reloaded, all the rest were ok

M

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8 minutes ago, Miseryguts said:

Big Al

will try that, I would not of thought the trim length was wrong, I trim all my cases every time. Removal may be difficult, it is one that is crimped, and I only have a kinetic removal tool. The duff cartridge is one of a 100 batch that I reloaded, all the rest were ok

M

Try seating the offending bullet 10 thou deeper and see if it closes, it might just be a bad bullet in the batch that is making the CBTO different.

If you have one problem round from a batch of 100 its not much of a problem really. I would try loading 10 more the regular way and if they all chamber fine I would move on and forget it, sometimes you wont always get to the bottom of every irregularity but when its 1% or less I dont think I would kill myself trying to find it.

Life is too short. :)

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True, especially in my case (lost 70+ years already) 10 mins hard bashing moved the bullet 12 thou!. Tried checking the case length with bullet in situ still, and measured that at 1.766 - which is 6 thou over max - are we saying this is the problem? I can see how 1 case did not get trimmed in the batch of 100

M

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Big Al

just pushed the bullet back in to COAL 2.191, and still cannot close the bolt - as you say it must be the case length that is stopping it.

Thanks for your time and help. Now all I have to worry about is wether all my cases are the right dimension to head space correctly in the chamber!!

M

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18 minutes ago, Miseryguts said:

Big Al

just pushed the bullet back in to COAL 2.191, and still cannot close the bolt - as you say it must be the case length that is stopping it.

Thanks for your time and help. Now all I have to worry about is wether all my cases are the right dimension to head space correctly in the chamber!!

M

That shouldn't be difficult.

Take the firing pin out and follow the sellotape method, it should not take long at all.

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I don't know if it's of any help, or even relevant given the differences that can exist between comparitor gauges but using the same Hornady gauge my 223 fire-formed cases measure 1.461" averaged from about 25 fired cases.  I FL resize them to 1.459 to 1.460.

I had a similar issue with my T3 in .308 using one fired PPU brass (reloaded by HPS....quite a few wouldn't chamber).  They needed the shoulders bumped back a little.

 

 

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OK done the sellotape test, and the bolt is starting to firm up at +3.5thou on a case which measures 1.457 on this comparator, so I suspect my chamber head space dimension is 1.461 as measured on this comparator, so I guess I should be looking to size my cases at 1.459 as measured on this  comparator.

Thanks to those who sent in suggestions and explained things.

M (without a headache)

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...which is bang on what mine measure using the same comparitor tool.

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2 hours ago, Miseryguts said:

OK done the sellotape test, and the bolt is starting to firm up at +3.5thou on a case which measures 1.457 on this comparator, so I suspect my chamber head space dimension is 1.461 as measured on this comparator, so I guess I should be looking to size my cases at 1.459 as measured on this  comparator.

Thanks to those who sent in suggestions and explained things.

M (without a headache)

Bingo! ;)

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15 minutes ago, VarmLR said:

...which is bang on what mine measure using the same comparitor tool.

Ive seen Hornady comparator bodies of the same designation giving different readings from one and other, this is down to a variance in the hole size, I've seen as much as 0.003" - of course it doesn't matter as long as you use the same comparator but it makes switching between them difficult if your not aware of this.

I make my own inserts now and usually make two or three at the same time by boring one appropriate sized hole through a piece of aluminium and then turning the bodies from it, it means I can have one at home and a couple at work. 

 

 

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Quite so Al-it's unrealistic to expect a mass produced item (esp sub £30) to hold perfect very fine tolerances in manufacture.

This measureably is so for  comparators,but also for human digit sensitivity (also been  measured.)*

There are therefore two sources of potential 'measurement' dispersion between two measurers with the same comparator,let alone two differnt examples of the 'same make/model' comparitor. Within measurer (same comparator and same sensitivity) should be ok-as a comparison,but not an absolute measure.

* and quite widely anatomically,for those with more exotic hobbies.    :-)

gbal

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16 hours ago, Big Al said:

Ive seen Hornady comparator bodies of the same designation giving different readings from one and other, this is down to a variance in the hole size, I've seen as much as 0.003" - of course it doesn't matter as long as you use the same comparator but it makes switching between them difficult if your not aware of this.

I make my own inserts now and usually make two or three at the same time by boring one appropriate sized hole through a piece of aluminium and then turning the bodies from it, it means I can have one at home and a couple at work. 

 

 

I've got 5 different sets of .223 headspace gauges, and they're all different

Don't trust anything, verify!

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