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6mm br precision mic


gunner

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Question for the precision reloaders , how are you guys measure your headspace / fired case / to bumped case difference? As it seems rcbs dont manufacturer a 6Br mic ! Iv read you can use 308/243 with a spacer put in which probably works but thinking its probably not ideal ...

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Question for the precision reloaders , how are you guys measure your headspace / fired case / to bumped case difference? As it seems rcbs dont manufacturer a 6Br mic ! Iv read you can use 308/243 with a spacer put in which probably works but thinking its probably not ideal ...

 

 

You can obtain reference measurements for your headspace using a comparator in your calipers in the same way you measure your base to ogive on your bullets. If you have the Hornady comparator body then all you need to buy is bushing 'B' with the 0.350" hole in it.

 

head-space-gauges-for-oal-gauge-system

 

Do as Vince suggests and remove your firing pin, extractor and ejector so your bolt face has nothing but itself to interfere with your case base. Without a case in the chamber your bolt should simply fall closed under its own weight. Bump your shoulder back until the bolt handle falls most of the way down on its own weight and requires the most gentle touch with your index finger to complete the close and then measure that case from base to shoulder.

 

When the bolt is as I describe there will be around half a thou interference 0.0005" so bump the shoulder back in your die by one thou 0.001" and you will see the bolt close again under its own weight. The measurement you then have plus half a thou 0.0005" is your exact chamber length.

 

From that length I would push the shoulder a further one thou 0.001" and call it good, every case of that size on your comparator will have one thou clearance 0.001"

 

Another way to then check this is to add a single thickness of sellotape onto the size case base and trim it, when you insert this in the chamber it will take you back to the point where the bolt doesn't full drop on its own because sellotape is typically one and a half thou 0.0015 thick so you should be back where you started.

 

In practise this whole procedure takes about 5 mins and is a lot easier than it might sound, it takes longer to strip the bolt and reassemble!

 

I have tested brass headspace in my benchrest gun and you would be quite surprised at how inconsistent headspace has an effect on ultimate accuracy. Im currently running one thou 0.001" headspace. I found brass that was harder on bolt closing due to being one or two thou longer than optimum to be quite a lot worse in terms of accuracy than properly head spaced cases.

 

Its quite amazing how many things are out there just looking to spoil those groups. :)

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  • 3 weeks later...

Thanks for advice on this , iv used the strip bolt technic to gauge the feel for headspace over the years but really wanted a tool to be able to measure the two fired and unfired case's in just a few seconds at the bench . Hey presto I spoke to my rifle builder and he made me a bump gauge which will sit nicely with the other kit ! Using the same reamer etc , now i know the difference is 3_4 thou from unfired to twice fired and now needs a bump 3 thou . If you read it thanks niel.

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Thanks for advice on this , iv used the strip bolt technic to gauge the feel for headspace over the years but really wanted a tool to be able to measure the two fired and unfired case's in just a few seconds at the bench . Hey presto I spoke to my rifle builder and he made me a bump gauge which will sit nicely with the other kit ! Using the same reamer etc , now i know the difference is 3_4 thou from unfired to twice fired and now needs a bump 3 thou . If you read it thanks niel.

 

 

Seems like a lot of trouble/expense to go to for no better results than using what is widely available as I recommended.

 

https://www.1967spud.com/shop/hornady/head-space-gauges-for-oal-gauge-system/

 

Headspace is just the measurement from the base of a case to a datum circle diameter on the shoulder, its not complicated and can be done in seconds. The gauge you are showing offers no advantage over a simple hole bored into a piece of material and used in the manner you are showing. Whatever size hole you use it will always reference on the same part of the shoulder and act as a comparator to show the difference in case length to that point on the shoulder. The exact hole diameter as available on SAAMI and CIP websites but in truth you dont need them, in the case of your 6BR its 8.5mm and that will work for lots of cases.

 

I would go as far as to say the gauge your showing here has far more likelihood of giving incorrect readings due to a bit of debris getting in there with the case than a simple comparator that touches the case only where it needs to on the shoulder but so long as you are careful with it then it will work ok. Im not trying to be clever here, resorting to reaming what needs to be a simple bored hole is just an over-complicated way of doing something relatively simple, sometimes more can in fact end up less.

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I know what your saying alan , if i had my own lathe id of made a gauge myself as its so easy really . But i just dont have room for a lathe yet . Hence asking niel to do it . On the debris yes it could hide some crud in there , the gauge is only cut just past the shoulder and is easily accsesable for a brush out or wipe with a rag . As for cost it was irrelevant as it was so little compared to some of the custom options from branded stuff and well cheaper than a rcbs mic 'if they did one' It also has another benefit as its a 269 neck my brass is fully turned its easy to test a possible new batch once the first cuts made etc by just dropping it in . And i may be able to measure the neck length by using a depth stop on my caliper so ill know exactly how long i can leave my brass overall - So im happy :) atb

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For a long time now I have used a relatively crude method of maintaining a pretty consistent headspace,,,,On a lathe I had made up suitably sized bushings for use with my hornady comparator body,,,once a new case had been fired a couple or three times OR there was a definite feeling on bolt closure that clearances were getting tighter then I would bump back with a Redding body die to achieve a headspace 2 to 3 thou witnessed by before and after readings from the bushings set in the comparator...this amount of bumping back always seems to give a good feel to bolt closure again but whether its text book or not I don,t know but have always been pleased with consistent accuracy using this method. The crude bit is that rather than fiddle about with the body die whilst in the press I just use some cut to size and roughly shape shim steel between where the body die and the shell holder meet ,,,,specific shims from 6 to 8 thou seems to cover all my calibres and give me the two to three thou of space......I find as long as I have appx two thirds throw on the press its no effort to bump,,,,no over centre stuff needed ,,,works every time.I also leave the body die to float to aid self centering,,,well that's the idea and again it must work as I have very little if any runout on assembled rounds........O..

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