Spring-Back Breech Pressure Graph
THE SPRING-BACK METHOD FOR DETERMINING
MAXIMUM BREECH PRESSURE
This describes an empirical method to determine the maximum breech pressure appropriate for a T/C Contender test barrel. The method defines the maximum powder load combination or, Load Redline appropriate for a test barrel. This method is one suggested in concept by gunsmith, Mike Bellm, then developed, refined and quantified by myself. The method involves measurement of the diameter of the fired brass case at the expansion ring just ahead of the solid case head, then documenting the “growth” of the fired cases with increasingly heavier powder charges.
As a loaded cartridge is fired, the case will expand to the diameter of the chamber. Then the chamber and the case together expand a small bit more. As the internal pressure subsides, both the chamber and the case spring-back toward their original dimensions, the case more than the chamber. The concept of this technique is based on the fact that the spring-back of the case is inversely proportional to the actual breech pressure, or the powder load level. That spring-back is measurable. In practice, the rate of growth of the fired cases is linear as a function of powder load up to a point. The heavier the load, the less the spring-back of the case until the elasticity of the brass itself has been exceeded. Then, as the stretch of the brass approaches the limit of elasticity, its rate of growth abruptly changes. At even greater increased load levels, as the spring-back of the case and the chamber become equal, or that of the case is less than of the chamber, a stuck case will be experienced. This, of course, is an unsafe condition and must be avoided. However, safe conditions do persist while the case is growing at a linear rate until it reaches the inflection point on the growth curve where it then expands at a much slower rate. This property of spring-back and linear expansion comprises the bases for the determination of the maximum breech pressure FOR THIS SPECIFIC TEST BARREL when using reformed R-P 444 Marlin brass.
Application of this concept involves measurement of the brass spring-back, which can be done by measurement of the diameter of the fired cases at the expansion ring to 0.0001 inch. The expansion ring is that portion of the case just ahead of the solid case head and it is resized for reloading. Piece-to-piece variability among the pieces of brass of a 5-shot string will show a range of case spring-back values. In order to deal with this variability, all 5 cases of the string can be measured and a mean diameter for that string can be determined (means can be determined to 0.00001 inch). Further, after all the sets of 5-shot strings of a load work-up series have been measured and averaged, the series of average values can be presented in graphical format to show the rate of growth of the cases as a function of powder charge vs. mean case diameter. This plot will show the rate of growth of the fired cases up to and past the inflection point of the curve (if the loads exceeded that level). A plot showing a typical growth curve is shown below.
CURVE OF POWDER CHARGE vs. CASE DIAMETER AT WEB
For this specific T/C Contender test barrel using reformed R-P 444 Marlin brass
IMR 4895 Average
Powder (Gn.) Dia. @ Web (in.)
For those accustomed to dealing with experimental data, this curve comprises an exceptionally good relationship between powder load and breech pressure (albeit, uncalibrated pressure). In fact, it is readily appropriate to fit straight linear curves to both portions of the overall curve so as to, more directly establish the actual maximum (Redline) powder load.
It is Bellm’s postulation that the point where the reformed 444 Marlin brass reaches its limit of elasticity can serve as an indication of the maximum pressure to which a load combination may be taken under safe conditions in the T/C Contender. This being the case, the actual breech pressure itself, becomes immaterial, although knowing it may be desirable.
Now that a load Redline has been established for the test barrel, other powder/bullet combinations can be tested. The maximum powder charge for each new combination can be set at the point where the average diameter of the case from the new fired load equals the critical value just established.
Experimental results show that in order for this method to produce a reliable curve of powder load vs. case diameter, the case, at the expansion ring, must be sized to be at least 0.003 - and better yet 0.004 inch smaller diameter than that of the chamber. That amount of clearance is needed since the method involves a measure of the case spring-back and the case must first stretch before it can spring-back. If the standard sizing die does not provide enough clearance, a “small base” die can be used. If that too does not accomplish sufficient sizing, other measures must be employed. In one such case a 444 Marlin die was modified in order to accomplish this necessary additional sizing. A 444 Marlin die was used because it has the correct diameter at the base; it has the right taper and has no shoulder. The modification was accomplished by grinding about 0.100 inch from the bottom end of the die. Then a new internal radius was ground and polished where the material had been removed. This shortening of the die allowed that the test case could be pressed farther into its taper, which in turn accomplished additional reduction of the case diameter.
© Donald L.