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Posted: 5/17/2012 7:15:16 PM EDT
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So I already asked this in the general discussion and didn't get any useful responses. It seems like maybe the retro folks would have a better idea of where to find this info. I'm looking for data on how fast a BCG travels in an AR with a standard rifle length gas system and rifle buffer, when firing standard milspec ammunition (M193 or M855 or whatever, I'm not picky on this point). Specifically, the speed the BCG travels at when it moves to the rear and also the speed when returning forward. I feel like I've seen some old Colt document that gives these stats, but now I can't find it anywhere.
Also, I've already been told that it's "F A S T" so no need to repeat 
Thanks |
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Figuring most any M-16 is capable of 650 rds per minute the cycle rate for one round will be .092 sec. At 750 rds per minute figure .08 seconds. I'd say split it in half and allow for dwell time (basically it stops) as it returns forward and again as it closes for next shot. Same as comparing a carbine system to a rifle system where both cycle 650 rds per minute basically the main reason for different weight buffers. Still the carbine system is harder on parts. It's not the speed it's the sudden stops.
Actually pretty hard to capture but mathmatically possible to figure. It's very fast! Not bullet fast but still fast. I believe average reaction time for a human being brain to hand or foot is much slower (.2-.25/sec) without anticipation factor as in drag racing or hand slapping, punching, ducking etc. Cycle rate of 900 rds per minute isn't out of the question with a modified M-16 rifle/carbine for this purpose but one may encounter a few problems with too many variables mostly being excessive heat, ammo and mag issues. Open bolt works better for higher cycling rates and belt feeds. M-60 modified will do 1100 rds per minute pretty easy. Never tried a SAW but I'm guessing they run pretty fast. MG-34 will burn also. |
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Hmm ok so I typed in your numbers and came up with this. If anyone wants to check my work that would be great.
.092sec (rate of one cycle) The BCG travels ~3.8in one direction so 7.6in total (back and forth) 7.6in = .633ft .092sec x 10.87 = 1sec .633ft x 10.87 = 6.88ft SO.... the bcg travels at an average rate of 6.88fps (although in fact a bit faster when moving towards the rear and slower when returning) ??? Are those numbers right? It seems like that's too slow... |
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There also might be some way to figure it out if you could find some high-speed video shots of the bolt cycling. If you knew the number of frames per second that were captured in the video, you could probably get the exact amount of time it takes the bolt to travel fully to the rear, dwell, then return back into battery. Then if you figured out the total distance the bolt traveled, someone with some math skills could probably put those numbers to use to answer your question.
John Thomas |
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Do I remember somewhere a formula for bullet weigh x speed vs carrier weight equals theoretical carrier velocity? Bolt unlocking, buffer weight and spring action all serving to delay and/or slow carrier speed. Muzzle velocity and bullet weight are used to determine foot pounds of energy. Could you divide that by carrier weight to get an approximation of your carrier speed? I'm just throwing this out there. Lots of back pain all week and too many pain meds don't make for clear thinking. |
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Quoted:
Do I remember somewhere a formula for bullet weigh x speed vs carrier weight equals theoretical carrier velocity? Bolt unlocking, buffer weight and spring action all serving to delay and/or slow carrier speed. Muzzle velocity and bullet weight are used to determine foot pounds of energy. Could you divide that by carrier weight to get an approximation of your carrier speed? I'm just throwing this out there. Lots of back pain all week and too many pain meds don't make for clear thinking. This would only work for a blowback or recoil operated design. Mass x Velocity of bullet = Mass x Velocity of carrier. Mass and speed of bullet and mass of carrier are known... solve for speed of carrier. Since an AR is gas operated and the bolt is actuated by gas pressure, this would not hold true. |
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Quoted:
Quoted:
Do I remember somewhere a formula for bullet weigh x speed vs carrier weight equals theoretical carrier velocity? Bolt unlocking, buffer weight and spring action all serving to delay and/or slow carrier speed. Muzzle velocity and bullet weight are used to determine foot pounds of energy. Could you divide that by carrier weight to get an approximation of your carrier speed? I'm just throwing this out there. Lots of back pain all week and too many pain meds don't make for clear thinking. This would only work for a blowback or recoil operated design. Mass x Velocity of bullet = Mass x Velocity of carrier. Mass and speed of bullet and mass of carrier are known... solve for speed of carrier. Since an AR is gas operated and the bolt is actuated by gas pressure, this would not hold true. Maybe we could get closer to using this formula given the following info: pressure on the bolt carrier key... 20" M16, 100psi..... 14.5" M4 125psi (if that's accurate). However that's probably right when the gas first reaches the key. So then the unlocking process would add all sorts of complicated forces that would slow the BCG. Then you'd have to factor in the overall friction that the BCG/buffer encounters when traveling inside the rifle. You'd also have to factor in the spring force involved which varies from 5.8lbs when relaxed to 10.9lbs at full compression... too complicated. Probably the high speed video approach would be easiest. Andrew at the Vuurwapen blog has a bunch of high speed video, including shots of 20" ARs firing with different buffer weights. He seems like the type who'd know how to figure the speed of something based on the frames per second. unless.... these mystical Colt documents that I seem to remember actually do exist somewhere... |
Agreed. That's why I said 'theoretical' speed of carrier. Internal ballistic event would be divided between time of bullet in barrel before and after passing gas port. Couple that with the decreasing pressure of combustion gases during that time. Rifle recoil begins at ignition. There is going to be some pretty convoluted mathematic gymnastics to take into account for those variables. I'm beginning to like 'really fast' as the answer.
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Quoted:
Hmm ok so I typed in your numbers and came up with this. If anyone wants to check my work that would be great. .092sec (rate of one cycle) The BCG travels ~3.8in one direction so 7.6in total (back and forth) 7.6in = .633ft .092sec x 10.87 = 1sec .633ft x 10.87 = 6.88ft SO.... the bcg travels at an average rate of 6.88fps (although in fact a bit faster when moving towards the rear and slower when returning) ??? Are those numbers right? It seems like that's too slow... That's what I got. .092 sec/cycle gives you 10.87 cycles per second; that times 7.6" and then divided by 12 to convert inches to feet gives you 6.88 FPS. Of course, there is accelleration from standing start and then decelleration to full stop at the beginning and end of travel both ways, so what you/we have calculated is an average speed. At most any given point in travel, the BCG is traveling faster or slower than that. And, part of that time is ate up with the hammer dropping, etc.. I would bet that high speed photography would show this to be a relatively herky-jerky operation. |
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Quoted:
Hmm ok so I typed in your numbers and came up with this. If anyone wants to check my work that would be great. .092sec (rate of one cycle) The BCG travels ~3.8in one direction so 7.6in total (back and forth) 7.6in = .633ft .092sec x 10.87 = 1sec .633ft x 10.87 = 6.88ft SO.... the bcg travels at an average rate of 6.88fps (although in fact a bit faster when moving towards the rear and slower when returning) ??? Are those numbers right? It seems like that's too slow... Where did the 10.87 come from? |
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I would bet that high speed photography would show this to be a relatively herky-jerky operation.
Kinda like watching the pounding a human budy takes running around the block in slow motion or a skydiver bouncing on impact when a chute doesn't open and every bone in there body is crumbling from impact like dropping a glass on the floor. Even though the eye can't capture it in real time I've seen them bounce 10-15 feet after impact on video and I've also seen them sink into the medium like in the old cartoons when I was very young. Go Acme and Humpty Dumpty! If a top fuel funny car engine turns 8000 rpm and runs the quarter mile in 4 sec the engine only turns 533 rpm. If it runs 1000 ft in 4 seconds it still only turns 533 rpm. Interesting that two cars can also run the same elapsed time in an equal distance but one will run 10 mph faster than the other and they aren't running into a brick wall twice during each rpm and rate of accelaration is obviously different for same result (variables of gearing clutch slippage, HP, etc.). With different closing rates it's amazing what one extra foot can do to the result also. Games of inches (distance and speed). Carbine gas system / rifle system. Two different systems creating the same result (650 rds per minute) but the carbine system is harder on the parts as a general rule while accomplishing the same result. More than one way to pet the neighbor's kitty. Should OP's question be as fast returning to GD as it was leaving and will the distance traveled be equal? |
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Quoted:
Quoted:
Hmm ok so I typed in your numbers and came up with this. If anyone wants to check my work that would be great. .092sec (rate of one cycle) The BCG travels ~3.8in one direction so 7.6in total (back and forth) 7.6in = .633ft .092sec x 10.87 = 1sec .633ft x 10.87 = 6.88ft SO.... the bcg travels at an average rate of 6.88fps (although in fact a bit faster when moving towards the rear and slower when returning) ??? Are those numbers right? It seems like that's too slow... Where did the 10.87 come from? 10.87 is just the number required to multiply .092 seconds into one second. Then to keep the ratio equal you multiply the other number (.633) by the same thing. Oh but I just realized I think I have the distance that the BCG travels wrong. I was just subtracting the length of a buffer from the internal length of a buffer tube to find this distance, but in actuality the buffer protrudes slightly into the receiver at the receiver extension junction, so this distance will really be slightly longer than the one I have. Also, thanks to everyone for the help! |
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Quoted:
.092sec (rate of one cycle) The BCG travels ~3.8in one direction so 7.6in total (back and forth) 7.6in = .633ft .092sec x 10.87 = 1sec .633ft x 10.87 = 6.88ft SO.... the bcg travels at an average rate of 6.88fps (although in fact a bit faster when moving towards the rear and slower when returning) ??? Are those numbers right? It seems like that's too slow... Mag dump in 3 seconds sounds a little slow to me, but based on the numbers you have the math is correct. You are moving 6.88ft sec... with 3600 seconds in an hour = 24,768 ft. Divide by 5280 (feet in mile) and you get 4.69 MPH. Not really moving so fast. That said, I believe your cyclic rate a little low in your original calculations... which would raise the speed a little. It also isn't really adjusting for dwell time. I'd bet the carrier is actually moving much faster, then sitting still all the way back for a good portion of the cycle, before that energy is dispersed and the spring actually pushes it back forward... high speed film might be the key.. youtube it?? |
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https://www.youtube.com/watch?v=575Q0O41u5s
Pretty good high speed vid with different buffer weights and gas system lengths. Not full-auto. Andrew at vuurwapenblog.com is who posted this video, I'm guessing he could look at the frame count for the duration of one cycle in order to tell us, since it's 1000fps that should be pretty easy. |
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