Parashooter
04-29-2021, 10:31
One of the problems with the internet is that erroneous or incomplete information sometimes gets circulated and accepted to the point that it overwhelms the truth.
In the case of M1 rifle port pressure, the erroneous information is that port pressure is primarily related to powder burning rate and bullet weight. The truth is that these are merely secondary factors. M1 port pressure is most closely related to gas volume (technically, mass and temperature), which is directly related to powder charge weight. Burning rate and bullet weight of course have a direct influence on PEAK pressure, but this occurs long before the bullet gets to the gas port.
With light bullets, we normally use faster powders for best performance since the relative ease with which the bullet starts to move means we can use a fairly large charge of fast powder without excessive peak pressure. With heavy bullets that take longer to accelerate, charge tables tell us the slower powders will give the highest velocity with the lowest peak pressure.
The M1 rifle's gas system was designed for the port pressures generated by the volume of gas produced by a charge of about 44 to 50 grains of powder behind a 173-grain bullet at 2640 fps (M1 Ball). It also happened to work just fine with about the same charge using 150-grain bullets at about 2800 fps (M2 Ball). The burning rate that gives these velocities to these bullets is about that we find in IMR 4895 and 4064. If we use a slower powder, say 4350, we find the appropriate charge for these velocities is heavier - about 55 grains for the 173 and 58 for the 150. Such heavier charges naturally generate a larger volume of gas, but at a slower rate that keeps peak pressure in normal limits. Given that the volume of the cartridge case and bore (up to the gas port) is a fixed quantity, the larger volume of gas necessarily translates to higher pressure at the gas port.
Conversely, if we stick with 4895 but change to a 110-grain bullet, we can stuff in some 54 grains of powder at normal pressure, for a much higher velocity. Again, the heavier charge generates more volume of gas and gives high port pressure. With 200-grain bullets, on the other hand, we can get good performance with 45-50 grain charges of slightly slower powders like 4320 or 4350, giving the same gas volume and consequently appropriate port pressure.
A lot of people who haven't well understood the role of gas volume have focused on burning rate or bullet weight instead - and that's what gets them into logical difficulty. It's very true that an optimum load of the slow powders with 150-180 grain bullets will give excessive M1 port pressure, and also true that the usual best bolt-gun loads of the really slow numbers (like 4831) with 200+ grain bullets will also give excess port pressure. What's missing in the logic is that it's neither the powder burning rate nor the bullet weight that's the problem's root cause - but rather the charge weight (mass, to be more accurate) and consequent gas volume.
It's unfortunate this mistaken (or just incomplete) logic has been so widely publicized, since knowing the whole story really makes powder selection much easier. Regardless of bullet weight, powder charges below 50 grains will generally give appropriate M1 port pressure (or less). Between 50 and 52 grains is marginal. Over 52 grains we may begin to see risk of damage to the operating rod. Of course powders must be chosen that will also yield acceptable peak pressure and velocity. (50 grains of 4198 will still make a mess - thanks to excessive peak pressure - but the port pressure would be near normal.)
49156
50-grain charges of fast and slow powders yield vastly different peak pressure and velocity, but M1 port pressure is virtually the same even with these extreme examples.
There are certainly exceptions to this basic rule. Different powder compositions give off different volumes of gas for a given charge weight. But if we stick to the commonly-available rifle powders now on the market, there is surprisingly little variation in the mass/gas relationship and we're not likely to get in trouble with excess port pressure if we choose a published load using less than 50 grains of a powder that gives acceptable performance with our chosen bullet weight.
I urge anyone finding this concept difficult to stick to their existing guidelines. After all, there's little to be lost by limiting one's bullet and powder choices to the accepted standards - 150-180 grain bullets and powders close to 4895's burning rate.
In the estimates below, "P.Muzz" for a 23" barrel corresponds to the pressure behind the bullet at the M1 gas port, related to port pressure but not exactly the same. (Estimates are sorted by descending muzzle pressure to show general correspondence with charge weight.)
Cartridge : .30-06 Spring.
Bullet : .308, 150, Hornady SP 3031
Cartridge O.A.L. L6: 3.340 inch or 84.84 mm
Barrel Length : 23.0 inch or 584.2 mm
Predicted Data for Indicated Charges of the Following Powders.
Matching Muzzle Velocity: 2700 fps or 822 m/s
These calculations refer to your specified settings in QuickLOAD 'Cartridge Dimensions' window.
C A U T I O N : any load listed can result in a powder charge that falls below minimum suggested
loads or exceeds maximum suggested loads as presented in current handloading manuals. Understand
that all of the listed powders can be unsuitable for the given combination of cartridge, bullet
and gun. Actual load order can vary, depending upon lot-to-lot powder and component variations.
USE ONLY FOR COMPARISON !
Powder.type..........Filling/Loading.Ratio..Charge.....Vel..Prop.Burnt.P.max..P .muzz
......................................%.....Grains ....fps.....%.......psi.....psi..
-----------------------------------------------------------------------------------
Alliant.Reloder-25.................106.7.....63.0.....2700....91.8 ....38700...12760
Vihtavuori.N560....................101.1.....61.3. ....2700....84.8....39596...12449
Winchester.WXR.....................104.2.....61.2. ....2700....87.9....39509...12344
Alliant.Reloder-22.................101.5.....60.9.....2700....88.1 ....39419...12340
IMR.7828...........................106.0.....61.2. ....2700....85.5....40833...12166
IMR.7828.SSC.......................100.3.....61.2. ....2700....85.5....40833...12166
Hodgdon.H1000......................112.2.....65.9. ....2700....85.6....43163...12158
Ramshot.Hunter......................90.6.....57.3. ....2700....92.5....40099...11741
Alliant.Reloder-19..................98.3.....58.3.....2700....89.0 ....41104...11735
IMR.4831...........................101.8.....57.9. ....2700....93.4....40395...11678
Accurate.4350.......................97.5.....55.3. ....2700....95.0....39928...11675
Winchester.760......................88.3.....55.5. ....2700....91.5....40405...11605
Vihtavuori.N550.....................90.2.....54.7. ....2700....94.8....40399...11570
Hodgdon.H4831......................106.2.....61.3. ....2700....85.7....43270...11561
Hodgdon.H4831.SC...................102.1.....61.3. ....2700....85.7....43270...11561
Hodgdon.H4350.......................93.0.....54.9. ....2700....93.0....40107...11553
Vihtavuori.N165....................105.8.....62.1. ....2700....90.7....42726...11465
Hodgdon.H414........................85.1.....53.8. ....2700....93.0....41707...11366
IMR.4350............................98.8.....56.2. ....2700....88.6....42892...11284
Ramshot.BigGame.....................89.2.....52.9. ....2700....96.6....40531...11182
Vihtavuori.N160....................105.8.....60.2. ....2700....88.5....44410...11158
Hodgdon.H380........................86.5.....52.5. ....2700....95.3....42521...11066
Hodgdon.BL-C2.......................79.9.....51.8.....2700... .98.1....41563...10949
Alliant.Reloder-15..................84.3.....50.1.....2700....96.5 ....41890...10925
IMR.4320............................86.5.....50.5. ....2700....97.1....42158...10640
Winchester.748......................77.3.....49.4. ....2700....98.9....41435...10593
IMR.4895............................84.5.....48.5. ....2700....98.4....40668...10587
Accurate.2700.......................91.8.....56.3. ....2700....92.8....46789...10574
Hodgdon.VARGET......................87.6.....50.3. ....2700....96.8....43595...10573
Accurate.4064.......................90.6.....50.3. ....2700....99.9....42289...10535
Vihtavuori.N540.....................87.7.....52.7. ....2700....97.9....44964...10526
Accurate.2520.......................82.3.....50.3. ....2700....99.5....42340...10495
Ramshot.TAC.........................79.0.....49.2. ....2700....98.6....43064...10451
IMR.4064............................88.4.....49.6. ....2700....97.1....43783...10451
Vihtavuori.N150.....................93.6.....52.5. ....2700....99.3....43787...10355
Vihtavuori.N530.....................79.1.....47.2. ....2700....98.7....43334...10315
Hodgdon.H4895.......................87.1.....50.0. ....2700....99.2....42732...10300
Accurate.2460.......................78.0.....49.7. ....2700....99.3....44230...10242
Hodgdon.H335........................74.5.....48.3. ....2700....99.8....43352...10108
Accurate.2230.......................77.0.....49.0. ....2700....99.3....44666...10088
IMR.3031............................83.4.....46.3. ....2700...100.0....41236...10055
Accurate.2495.......................84.5.....47.5. ....2700...100.0....42846...10037
Vihtavuori.N140.....................87.2.....49.8. ....2700...100.0....44444....9860
Ramshot.X-Terminator................75.3.....46.3.....2700.. .100.0....44152....9787
Vihtavuori.N135.....................91.0.....49.0. ....2700...100.0....44050....9627
Accurate.2015.......................79.7.....45.8. ....2700...100.0....46238....9195
Alliant.Reloder-10x.................77.7.....43.5.....2700...100.0 ....45761....9134
In the case of M1 rifle port pressure, the erroneous information is that port pressure is primarily related to powder burning rate and bullet weight. The truth is that these are merely secondary factors. M1 port pressure is most closely related to gas volume (technically, mass and temperature), which is directly related to powder charge weight. Burning rate and bullet weight of course have a direct influence on PEAK pressure, but this occurs long before the bullet gets to the gas port.
With light bullets, we normally use faster powders for best performance since the relative ease with which the bullet starts to move means we can use a fairly large charge of fast powder without excessive peak pressure. With heavy bullets that take longer to accelerate, charge tables tell us the slower powders will give the highest velocity with the lowest peak pressure.
The M1 rifle's gas system was designed for the port pressures generated by the volume of gas produced by a charge of about 44 to 50 grains of powder behind a 173-grain bullet at 2640 fps (M1 Ball). It also happened to work just fine with about the same charge using 150-grain bullets at about 2800 fps (M2 Ball). The burning rate that gives these velocities to these bullets is about that we find in IMR 4895 and 4064. If we use a slower powder, say 4350, we find the appropriate charge for these velocities is heavier - about 55 grains for the 173 and 58 for the 150. Such heavier charges naturally generate a larger volume of gas, but at a slower rate that keeps peak pressure in normal limits. Given that the volume of the cartridge case and bore (up to the gas port) is a fixed quantity, the larger volume of gas necessarily translates to higher pressure at the gas port.
Conversely, if we stick with 4895 but change to a 110-grain bullet, we can stuff in some 54 grains of powder at normal pressure, for a much higher velocity. Again, the heavier charge generates more volume of gas and gives high port pressure. With 200-grain bullets, on the other hand, we can get good performance with 45-50 grain charges of slightly slower powders like 4320 or 4350, giving the same gas volume and consequently appropriate port pressure.
A lot of people who haven't well understood the role of gas volume have focused on burning rate or bullet weight instead - and that's what gets them into logical difficulty. It's very true that an optimum load of the slow powders with 150-180 grain bullets will give excessive M1 port pressure, and also true that the usual best bolt-gun loads of the really slow numbers (like 4831) with 200+ grain bullets will also give excess port pressure. What's missing in the logic is that it's neither the powder burning rate nor the bullet weight that's the problem's root cause - but rather the charge weight (mass, to be more accurate) and consequent gas volume.
It's unfortunate this mistaken (or just incomplete) logic has been so widely publicized, since knowing the whole story really makes powder selection much easier. Regardless of bullet weight, powder charges below 50 grains will generally give appropriate M1 port pressure (or less). Between 50 and 52 grains is marginal. Over 52 grains we may begin to see risk of damage to the operating rod. Of course powders must be chosen that will also yield acceptable peak pressure and velocity. (50 grains of 4198 will still make a mess - thanks to excessive peak pressure - but the port pressure would be near normal.)
49156
50-grain charges of fast and slow powders yield vastly different peak pressure and velocity, but M1 port pressure is virtually the same even with these extreme examples.
There are certainly exceptions to this basic rule. Different powder compositions give off different volumes of gas for a given charge weight. But if we stick to the commonly-available rifle powders now on the market, there is surprisingly little variation in the mass/gas relationship and we're not likely to get in trouble with excess port pressure if we choose a published load using less than 50 grains of a powder that gives acceptable performance with our chosen bullet weight.
I urge anyone finding this concept difficult to stick to their existing guidelines. After all, there's little to be lost by limiting one's bullet and powder choices to the accepted standards - 150-180 grain bullets and powders close to 4895's burning rate.
In the estimates below, "P.Muzz" for a 23" barrel corresponds to the pressure behind the bullet at the M1 gas port, related to port pressure but not exactly the same. (Estimates are sorted by descending muzzle pressure to show general correspondence with charge weight.)
Cartridge : .30-06 Spring.
Bullet : .308, 150, Hornady SP 3031
Cartridge O.A.L. L6: 3.340 inch or 84.84 mm
Barrel Length : 23.0 inch or 584.2 mm
Predicted Data for Indicated Charges of the Following Powders.
Matching Muzzle Velocity: 2700 fps or 822 m/s
These calculations refer to your specified settings in QuickLOAD 'Cartridge Dimensions' window.
C A U T I O N : any load listed can result in a powder charge that falls below minimum suggested
loads or exceeds maximum suggested loads as presented in current handloading manuals. Understand
that all of the listed powders can be unsuitable for the given combination of cartridge, bullet
and gun. Actual load order can vary, depending upon lot-to-lot powder and component variations.
USE ONLY FOR COMPARISON !
Powder.type..........Filling/Loading.Ratio..Charge.....Vel..Prop.Burnt.P.max..P .muzz
......................................%.....Grains ....fps.....%.......psi.....psi..
-----------------------------------------------------------------------------------
Alliant.Reloder-25.................106.7.....63.0.....2700....91.8 ....38700...12760
Vihtavuori.N560....................101.1.....61.3. ....2700....84.8....39596...12449
Winchester.WXR.....................104.2.....61.2. ....2700....87.9....39509...12344
Alliant.Reloder-22.................101.5.....60.9.....2700....88.1 ....39419...12340
IMR.7828...........................106.0.....61.2. ....2700....85.5....40833...12166
IMR.7828.SSC.......................100.3.....61.2. ....2700....85.5....40833...12166
Hodgdon.H1000......................112.2.....65.9. ....2700....85.6....43163...12158
Ramshot.Hunter......................90.6.....57.3. ....2700....92.5....40099...11741
Alliant.Reloder-19..................98.3.....58.3.....2700....89.0 ....41104...11735
IMR.4831...........................101.8.....57.9. ....2700....93.4....40395...11678
Accurate.4350.......................97.5.....55.3. ....2700....95.0....39928...11675
Winchester.760......................88.3.....55.5. ....2700....91.5....40405...11605
Vihtavuori.N550.....................90.2.....54.7. ....2700....94.8....40399...11570
Hodgdon.H4831......................106.2.....61.3. ....2700....85.7....43270...11561
Hodgdon.H4831.SC...................102.1.....61.3. ....2700....85.7....43270...11561
Hodgdon.H4350.......................93.0.....54.9. ....2700....93.0....40107...11553
Vihtavuori.N165....................105.8.....62.1. ....2700....90.7....42726...11465
Hodgdon.H414........................85.1.....53.8. ....2700....93.0....41707...11366
IMR.4350............................98.8.....56.2. ....2700....88.6....42892...11284
Ramshot.BigGame.....................89.2.....52.9. ....2700....96.6....40531...11182
Vihtavuori.N160....................105.8.....60.2. ....2700....88.5....44410...11158
Hodgdon.H380........................86.5.....52.5. ....2700....95.3....42521...11066
Hodgdon.BL-C2.......................79.9.....51.8.....2700... .98.1....41563...10949
Alliant.Reloder-15..................84.3.....50.1.....2700....96.5 ....41890...10925
IMR.4320............................86.5.....50.5. ....2700....97.1....42158...10640
Winchester.748......................77.3.....49.4. ....2700....98.9....41435...10593
IMR.4895............................84.5.....48.5. ....2700....98.4....40668...10587
Accurate.2700.......................91.8.....56.3. ....2700....92.8....46789...10574
Hodgdon.VARGET......................87.6.....50.3. ....2700....96.8....43595...10573
Accurate.4064.......................90.6.....50.3. ....2700....99.9....42289...10535
Vihtavuori.N540.....................87.7.....52.7. ....2700....97.9....44964...10526
Accurate.2520.......................82.3.....50.3. ....2700....99.5....42340...10495
Ramshot.TAC.........................79.0.....49.2. ....2700....98.6....43064...10451
IMR.4064............................88.4.....49.6. ....2700....97.1....43783...10451
Vihtavuori.N150.....................93.6.....52.5. ....2700....99.3....43787...10355
Vihtavuori.N530.....................79.1.....47.2. ....2700....98.7....43334...10315
Hodgdon.H4895.......................87.1.....50.0. ....2700....99.2....42732...10300
Accurate.2460.......................78.0.....49.7. ....2700....99.3....44230...10242
Hodgdon.H335........................74.5.....48.3. ....2700....99.8....43352...10108
Accurate.2230.......................77.0.....49.0. ....2700....99.3....44666...10088
IMR.3031............................83.4.....46.3. ....2700...100.0....41236...10055
Accurate.2495.......................84.5.....47.5. ....2700...100.0....42846...10037
Vihtavuori.N140.....................87.2.....49.8. ....2700...100.0....44444....9860
Ramshot.X-Terminator................75.3.....46.3.....2700.. .100.0....44152....9787
Vihtavuori.N135.....................91.0.....49.0. ....2700...100.0....44050....9627
Accurate.2015.......................79.7.....45.8. ....2700...100.0....46238....9195
Alliant.Reloder-10x.................77.7.....43.5.....2700...100.0 ....45761....9134