Checking for signs of pressure.
Posted: Thu Jul 12, 2012 12:14 pm
I have been prompted to post this after reading handloading threads here where people are discussing loads and say that you should start low and work up "looking for signs of high pressure" or words to that effect.
This seems to be used as a throw away line as part of the liability disclaimer rather than a helpful instruction to load development. Very few people say what they mean by that and as such the words are useless and flippant.
Bearing in mind that safe pressure within the limits of the cartridge and gun is what safe handloading is all about "looking for pressure signs" should carry a lot of weight but is seldom explained or understood by those who use it.
I have read people advocating that primers should be checked for signs of high pressure, this one of the most unreliable methods I know as primer flattening and flow can be caused by things other than high pressure, also "the bolt being hard to lift" along with "excessive recoil" is described as "sign" this usually means that if you have reached that condition then you have reached high pressure and should have stopped before you got there.
For the average handloader access to pressure barrels, strain gauges and such like is beyond most of us on an everyday basis, although you can book time with people like Kynoch who have such equipment on their range and hire it out. The proof houses will also test your ammunition at a cost. So if you are looking at a top of the pressure range load then these are the places to test your load's pressure and find out if your load is safe for your rifle, you and those who shoot alongside you.
When we fire a rifle we are letting go a pipe bomb next to our face contained by a metal chamber and a mechanical action which have their limitations, guessing a load's pressure is not just good enough.
When handloading ammunition we need better signs that primer deformation, recoil or bolt stiffness. Fortunately we do have some better indicators available to us but how many of us use them with rigour in our handload development?
First and foremost is good data, that should come from a printed source that has done pressure checking for the loads they provide. That gives us some tested parameters to work within and gives us a ball park for our loads. Printed data usually gives minimum (start) and maximum loads complete with the caveat that these results were from their barrels and actions and good sources will list what the loads were tested in. Internet loads should always be developed from a printed source and be wary of any one offering loads without providing the source used for their development, things are apt to change in the telling.
However that in itself is only a starting point and we need more information as we test our loads within these parameters with our firearm. One of the most helpful things that a handloader can acquire and I think it is essential, is a chronograph. Good chronographs are cheap and reliable nowadays and no hand loader should be with out one. Chronographs measure velocity not pressure but as we all know there is a relationship 'tween pressure and velocity and a chronograph used intelligently alongside our published data can be a good proxy for pressure.
Thirdly I use case base expansion measurements to help me monitor pressure as I work up handloads. This is an old method and has its flaws but used alongside data and a chronograph it gives me more input to help me monitor my loads pressure build up. This requires a factory loaded round preferably of the same make of case as you will be using in your hand loads. Measure the base of the case of an few unfired factory rounds, make a note and take an average. Then fire the rounds over your chronograph noting the velocity so that you will have some idea of how they are performing in relation to your published data. After firing measure and note the size of the fired cases at the base and take an average. The case will have been expanded by the pressure generated by the factory round and as factory rounds are pressure tested that expansion range with that brass is known to be safe. You now can measure your own brass, which needs to be full length resized. As you work through your loads note the case expansion and when your cases expand to near the diameter of the fired factory brass you have another indicator as to where your pressure could be in relation to maximum.
These measurements used alongside your chronograph and published data will give you a far better indication of the pressure climb in your handloads, than primer, recoil or god forbid stiff bolts.
Not as good as pressure testing but better than nothing at all.
All this will only work if you take care at the bench and put up loads correctly within the data you are using, no amount of measurement will stop an excessive over charge turning your rifle into a pipe bomb.
So please, next time you write here advising about loads please say what you mean by looking for high pressure signs and always quote the printed source that your own data is based upon.
This seems to be used as a throw away line as part of the liability disclaimer rather than a helpful instruction to load development. Very few people say what they mean by that and as such the words are useless and flippant.
Bearing in mind that safe pressure within the limits of the cartridge and gun is what safe handloading is all about "looking for pressure signs" should carry a lot of weight but is seldom explained or understood by those who use it.
I have read people advocating that primers should be checked for signs of high pressure, this one of the most unreliable methods I know as primer flattening and flow can be caused by things other than high pressure, also "the bolt being hard to lift" along with "excessive recoil" is described as "sign" this usually means that if you have reached that condition then you have reached high pressure and should have stopped before you got there.
For the average handloader access to pressure barrels, strain gauges and such like is beyond most of us on an everyday basis, although you can book time with people like Kynoch who have such equipment on their range and hire it out. The proof houses will also test your ammunition at a cost. So if you are looking at a top of the pressure range load then these are the places to test your load's pressure and find out if your load is safe for your rifle, you and those who shoot alongside you.
When we fire a rifle we are letting go a pipe bomb next to our face contained by a metal chamber and a mechanical action which have their limitations, guessing a load's pressure is not just good enough.
When handloading ammunition we need better signs that primer deformation, recoil or bolt stiffness. Fortunately we do have some better indicators available to us but how many of us use them with rigour in our handload development?
First and foremost is good data, that should come from a printed source that has done pressure checking for the loads they provide. That gives us some tested parameters to work within and gives us a ball park for our loads. Printed data usually gives minimum (start) and maximum loads complete with the caveat that these results were from their barrels and actions and good sources will list what the loads were tested in. Internet loads should always be developed from a printed source and be wary of any one offering loads without providing the source used for their development, things are apt to change in the telling.
However that in itself is only a starting point and we need more information as we test our loads within these parameters with our firearm. One of the most helpful things that a handloader can acquire and I think it is essential, is a chronograph. Good chronographs are cheap and reliable nowadays and no hand loader should be with out one. Chronographs measure velocity not pressure but as we all know there is a relationship 'tween pressure and velocity and a chronograph used intelligently alongside our published data can be a good proxy for pressure.
Thirdly I use case base expansion measurements to help me monitor pressure as I work up handloads. This is an old method and has its flaws but used alongside data and a chronograph it gives me more input to help me monitor my loads pressure build up. This requires a factory loaded round preferably of the same make of case as you will be using in your hand loads. Measure the base of the case of an few unfired factory rounds, make a note and take an average. Then fire the rounds over your chronograph noting the velocity so that you will have some idea of how they are performing in relation to your published data. After firing measure and note the size of the fired cases at the base and take an average. The case will have been expanded by the pressure generated by the factory round and as factory rounds are pressure tested that expansion range with that brass is known to be safe. You now can measure your own brass, which needs to be full length resized. As you work through your loads note the case expansion and when your cases expand to near the diameter of the fired factory brass you have another indicator as to where your pressure could be in relation to maximum.
These measurements used alongside your chronograph and published data will give you a far better indication of the pressure climb in your handloads, than primer, recoil or god forbid stiff bolts.
Not as good as pressure testing but better than nothing at all.
All this will only work if you take care at the bench and put up loads correctly within the data you are using, no amount of measurement will stop an excessive over charge turning your rifle into a pipe bomb.
So please, next time you write here advising about loads please say what you mean by looking for high pressure signs and always quote the printed source that your own data is based upon.