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Rifles, Reloading, Optics, Equipment
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Limbsaver Air Tech Recoil Pads
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<blockquote data-quote="Clark" data-source="post: 980271" data-attributes="member: 6600"><p>1) The threshold of recoil pain in a rifle I have given some thought:</p><p></p><p>Given:</p><p>The rifle weighs 10 pounds.</p><p>The bullet weighs 250 gr</p><p>The powder weighs 75 gr.</p><p>The muzzle velocity is 2500 fps</p><p>The gas velocity = 1.5 projectile velocity</p><p>The untrimmed large recoil pad is 9 sq inches</p><p>The recoil pad stiffness is 100 pounds/ 0.2"</p><p>The loss of Navcom material in Limbsaver recoil pad 75%</p><p>The threshold of pain is ~10 kg/cm2</p><p></p><p>Goal:</p><p>Calculate if it will hurt.</p><p></p><p>Calculation:</p><p>projectile momentum = mv = [[250 gr] / gravity ]][2500 fps] = 250 gr / 7000 gr/lb / 32.2 ft/sec2 [ 2500 fps] = 2.77 slug ft/sec</p><p>gas velocity = 1.5 projectile velocity = 1.5 2500 fps = 3750 fps</p><p>gas momentum = 75 gr / gravity 3750 fps = 1.25 slug ft/sec</p><p>Total forward momentum = bullet mv + gas mv = 2.77 + 1.25 = 4.02 slug ft /sec</p><p>rifle momentum = bullet + gas momentum = 4.02 slug ft /sec</p><p>rifle velocity = rifle momentum/ rifle mass = 4.02 / 10 pounds/ gravity = 4.02 / [10 / 32.2] = 12.9 fps</p><p>Rifle energy = [1/2] [m] [v][ v] = [.5] [[10] / [gravity]] [12.9] [12.9] = .5 [10 /32.2] 12.9 12.9 = 26 foot pounds</p><p>Energy not absorbed by recoil pad = rifle energy [1 - pad loss] = 26 fp [ 1 - 75%] = 6.5 foot pounds</p><p>The recoil pad stiffness is 100 pounds/ 0.2" = 100 lb / 0.0166 ft = 783 lb /.13 ft =</p><p>783 pounds compressed, 0 pounds relaxed, averages 391 pounds.</p><p>[391 pounds] [.13 ft] = 6.5 foot pounds</p><p>783 lb peak force / 9 sq in = 87 psi</p><p>The threshold of pain is ~10 kg/cm2 = 142 psi</p><p>87 psi peak pressure of recoil pad on skin / 142 psi threshold of pain = 62% of the threshold of pain</p><p></p><p>Will it half hurt?</p><p>No, because the whole shoulder moves, while 783 pounds of force may have been available to a brass statue shoulder, it was not realized. Only part of the energy goes into the recoil pad, unless the shooter is backed up against a tree. </p><p></p><p></p><p>------------------------------------------</p><p></p><p>More thought</p><p></p><p>The threshold of pain is ~ 20 psi on skin.</p><p>Some ways to keep recoil from reaching that level:</p><p>1) Lighter and slower projectile and gas</p><p>2) Heavier rifle</p><p>3) larger contact area on shoulder</p><p>4) compliant contact with shoulder to give uniform pressure over shoulder shape</p><p>5) spread the impact over time:</p><p>..a) recoil pad compresses </p><p>..b) parabolic taper hydraulic shock absorber</p><p>..c) separate recoiling mass delay line</p><p>.......1) semi auto mechanism bolt and carrier</p><p>.......2) Mercury starts in forward position and moves back in recoil.</p></blockquote><p></p>
[QUOTE="Clark, post: 980271, member: 6600"] 1) The threshold of recoil pain in a rifle I have given some thought: Given: The rifle weighs 10 pounds. The bullet weighs 250 gr The powder weighs 75 gr. The muzzle velocity is 2500 fps The gas velocity = 1.5 projectile velocity The untrimmed large recoil pad is 9 sq inches The recoil pad stiffness is 100 pounds/ 0.2" The loss of Navcom material in Limbsaver recoil pad 75% The threshold of pain is ~10 kg/cm2 Goal: Calculate if it will hurt. Calculation: projectile momentum = mv = [[250 gr] / gravity ]][2500 fps] = 250 gr / 7000 gr/lb / 32.2 ft/sec2 [ 2500 fps] = 2.77 slug ft/sec gas velocity = 1.5 projectile velocity = 1.5 2500 fps = 3750 fps gas momentum = 75 gr / gravity 3750 fps = 1.25 slug ft/sec Total forward momentum = bullet mv + gas mv = 2.77 + 1.25 = 4.02 slug ft /sec rifle momentum = bullet + gas momentum = 4.02 slug ft /sec rifle velocity = rifle momentum/ rifle mass = 4.02 / 10 pounds/ gravity = 4.02 / [10 / 32.2] = 12.9 fps Rifle energy = [1/2] [m] [v][ v] = [.5] [[10] / [gravity]] [12.9] [12.9] = .5 [10 /32.2] 12.9 12.9 = 26 foot pounds Energy not absorbed by recoil pad = rifle energy [1 - pad loss] = 26 fp [ 1 - 75%] = 6.5 foot pounds The recoil pad stiffness is 100 pounds/ 0.2" = 100 lb / 0.0166 ft = 783 lb /.13 ft = 783 pounds compressed, 0 pounds relaxed, averages 391 pounds. [391 pounds] [.13 ft] = 6.5 foot pounds 783 lb peak force / 9 sq in = 87 psi The threshold of pain is ~10 kg/cm2 = 142 psi 87 psi peak pressure of recoil pad on skin / 142 psi threshold of pain = 62% of the threshold of pain Will it half hurt? No, because the whole shoulder moves, while 783 pounds of force may have been available to a brass statue shoulder, it was not realized. Only part of the energy goes into the recoil pad, unless the shooter is backed up against a tree. ------------------------------------------ More thought The threshold of pain is ~ 20 psi on skin. Some ways to keep recoil from reaching that level: 1) Lighter and slower projectile and gas 2) Heavier rifle 3) larger contact area on shoulder 4) compliant contact with shoulder to give uniform pressure over shoulder shape 5) spread the impact over time: ..a) recoil pad compresses ..b) parabolic taper hydraulic shock absorber ..c) separate recoiling mass delay line .......1) semi auto mechanism bolt and carrier .......2) Mercury starts in forward position and moves back in recoil. [/QUOTE]
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