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Thread: Rifling type and style effecting pressures

  1. #1

    Rifling type and style effecting pressures

    OK recently we have some posts which are mention pressures and starting loads so it got me thinking about the different designs in rifling. Not twist rates but the actual design of the lands and grooves.

    Wide grooves and narrow lands will obvious develop different pressures to rifling with lands and grooves of equal widths as will lands that are wider than the grooves.

    The depth of the grooves must also have an influence as well as it will increase bearing surface and so increase friction. This is one reason why loads in different manuals will differ from one another. They are not using the same barrel to fire their test loads. Speer for instance buy actual rifles and develop loads in those. Hodgdons use a pressure gun, it's pictured in the #26 manual I have, others do the same and a lot have special pressure barrels made and makers like Walther make these pressure barrels for labs and powder/bullet makers.

    As an example of what can happen I'll relate this one. Now I have several rifles chambered for the .270 Winchester cartridge and so try to develop loads that will shoot well in them all however one particular rifle has a much tighter bore as I found out the hard way. This tight bored rifle was the very first one I brought and I have handloaded for it since soon after acquiring it without problems. Years later I acquired another .270 of the same make but different model, an earlier model in fact, then #3 came along again the same make but different model.

    About this time I started to experiment with the Vectan powders as they were cheaper to buy and on picking up the Nobel Sport vectan loading leaflet I noticed that in the .270 loading I had everything to make the load mentioned. The same make case, exactly the same bullet, same make and type of primer and of course the powder. So I assembled 20 rounds of this recipe and on the next range trip tried them out. The first round was fine except the bolt lift was slightly harder than normal, a fact I put down to not having the die screwed right down. I was sizing just enough so that the cases would chamber in all the rifles.

    The second round showed a stiffer bolt but no pressure signs were evident so it was again attributed to the sizing. Round #3 however was definatly wrong. I felt something hit my nose and on opening the bolt the primer was discovered laying on the magazine shell lifter. The primer pocket of the Norma case was obviously greatly expanded. These Norma cases were on their very first re-load and had been sized and trimmed all the same. I called a halt and put the ammuntion away the rifle was undamaged by the way .

    Some time later after some serious thought and consideration one of the other .270's was taken to the range with the 17 rounds left and the first carefully tried. The case extracted normally and not pressure signs were evident even measuring with a micrometer the pressure ring of the case showed normal pressures. The difference is the rifling the later rifle has a hammer forged barrel and the earlier one a cut rifled barrel and there is enough difference in bore size and bearing surface area to make this load un-safe in one rifle and perfectly safe in another. Now I am aware of this I keep the loads down a bit more and have even started using Nickle plated cases for that one rifle so I can instantly tell they are for this particular rifle.

  2. #2
    Without direct chamber pressure measurement -and subsequent pressure curve tracking- you're going to be in the land of conjecture I think. I don't give it much thought myself. I have two groove Springfield 30-06's and 2 groove .303's and there has never been given any particular deference to these barrels over my six groove 30-06's nor my 6 groove Savage #4 MkI .303.

    Additionally pressures in general are constantly decreasing after forcement (entering the rifling) is completed. I am not sure that rifling depth or form -unless carried to some ridiculous extreme- is of much import, pressure wise. JMHO, of course.~Muir

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