Reloading Tips According To Dicehorn #15

[dicehorn 38]

Shoulder Datum

 

Bit of a grey area for many reloaders. We accept that when fired a case expands and its limitation of not blowing itself to pieces is that it is confined by being within the thick walls of the rifle's chamber. However, even all chambers in the same calibre have different tolerances as long as they fall within SAMMI spec. So we full length and fire the case and it expands a little outwards, we then neck size and fire again and repeat. At some stage we are going to have too much resistance when closing the bolt (this is where I have pity on the "I make 100 rounds up a time" brigade) and there is a risk of crushing the case and still worse unable to open the bolt.

 

Well usually we full length before that happens but when we full length we are actually putting the case right back to where we started and most of this full lengthing of the whole case is not needed. What has in reality happened with the stiff bolt closure is simply the shoulder of the case has moved forward and gradually not sprung back far enough - so when you cycle a round the shoulders are the first to make contact with the chamber. Would'nt it be good if there was a tool that would only push your shoulders back to allow easier bolt closure but the rest of the case would still be fitting snuggly in the chamber. Terah..... there is - its the Redding Shoulder bump or body die which comes as calibre specific. This little item that looks like a small die will push your shoulders back sufficently to let you cycle a round that would not fit. The added bonus (for the 100 brigade) is that you can even use this tool safely with the round loaded.

 

 

Now if you are the type of reloader that uses a comparator to take your ogive readings, you will be aware of the calibre specific bushings you use. Well there are also bushing to measure your shoulder datums ( see picky below) Just as an example taking my 22.250 cases. When I measured the shoulder datum on the new cases, they measured 1.562", originally, I did not know how far I could let them grow before I encountered 'stiff bolt ' closure. Eventually it did happen when they reached 1.571" so I put the cases in the body die and using the comparator in the vernier gauge to measure, pushed the shoulder back to 1.569" this gave a nice bolt closure with only the faintest hint of resistance - now I had a case that fitted the chamber perfectly - another aide to accuracy. This little body die helps me not having to over work the brass and so helps with its longevity

 

 

 

[zx10mike 137]

thanks for a nice lay mans explanation on the body bump dies ,today i have learnt something.