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Posted: 7/1/2017 3:12:36 PM EDT
| Does anyone know how much time in seconds it takes from the moment the bolt carrier goes all the way forward until the cartridge has been fired and the bolt carrier starts to move rearward on a standard trigger group/auto sear full auto AR15? I found a high quality video on YouTube but if the video really is 18,000 frames per second like they say, the rate of fire calculates to 1350 rounds per minute which seems rather high. |
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Does anyone know how much time in seconds it takes from the moment the bolt carrier goes all the way forward until the cartridge has been fired and the bolt carrier starts to move rearward on a standard trigger group/auto sear full auto AR15? I found a high quality video on YouTube but if the video really is 18,000 frames per second like they say, the rate of fire calculates to 1350 rounds per minute which seems rather high. lock time - from sear release or trigger press until the hammer strikes the firing pin and ignites the primer. bore time - from ignition until the bullet passes the gas port, allowing gas to enter the DI system impingement time - from the moment the bullet passes the gas port until the gas can start forcing the bolt carrier backwards lock time varies based on sear and trip surface relative position, as well as hammer mass and hammer spring rate bore time varies based on the powder charge, bullet weight, bore surface and gas port location/barrel length impingement time varies based on the gas system length, gas pressure and volume, carrier mass and can be made longer by losses in the tube to carrier key fit, or bad gas rings on the bolt, or oversized bolt cavity on the carrier. If you can post a link to the video you're asking about, we may be able to dissect it for you - but 1350 rpm, while a higher than normal rate of fire, is achievable in a short barrel/short gas system. A full rifle original M16 is 650 to 750 rpm, depending on ammo. M4 carbines can be 800-1050rpm, again depending on ammo and some other factors. My 9mm M16 sugbgun can routinely hit 1200rpm with standard springs and regular ammo, or 1300+ with hot ammo. |
Great answer, thank your for the definitions. I've always wondered how impingement timing really worked. Is it just a matter of enough pressure build up to push the BCG rearward? What controls how much pressure that requires? Is it just a function of the time it takes to rotate the bolt lugs off the barrel extension lugs or is it something else? I don't know and am genuinely interested/curious. Never understood what exactly controls the timing of the bolt unlock process so everything is at safe pressure by the time the chamber is open.
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I'm interested in carbine length gas with M855 or M193 and all mil spec components.
This is the video I was describing:  Failed To Load Title |
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That's an 11-inch "commando", so even shorter than a normal M4 (14.5"). 10 rounds a second is 600rpm, and would require 3 seconds to empty the 30rd mag - just over 2 seconds as in the description implies it's running around 900rpm. Totally to be expected from a shorty commando carbine.
The entire firing cycle at 900 rpm (30 rounds in 2 seconds flat) would take 66 2/3ms, and most of that is spent in counterbattery stroke, extracting and ejecting the case, then in batter stroke, stripping and chambering the next cartridge. I'd estimate about 6-7ms from trigger press or sear release until the carrier starts moving back, then the other 60ms are spent doing the mechanical housekeeping for the next shot. Is that the time duration you were asking about? |
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I counted about 96 frames from when the bolt carrier stopped to when it started moving again. The gun was moving and the bolt carrier is small so it was hard to tell. 96 frames at 18,000 frames per second is .0053s. Does that sound right? |
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That sounds close. I was actually reading a thread related to this the other day at work when comparing a SCAR vs. an M4 action speed. Quoted:
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I counted about 96 frames from when the bolt carrier stopped to when it started moving again. The gun was moving and the bolt carrier is small so it was hard to tell. 96 frames at 18,000 frames per second is .0053s. Does that sound right? |
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I counted about 96 frames from when the bolt carrier stopped to when it started moving again. The gun was moving and the bolt carrier is small so it was hard to tell. 96 frames at 18,000 frames per second is .0053s. Does that sound right? |
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1,400 rounds per minute can be achieved using an aluminum bolt carrier.
Smith Enterprises used to market their aluminum carrier for that purpose. I can also tell you that on a carbine gas system, running an aluminum carrier and doing a Beta C-Mag dump that the standard gas tube will not survive it. |
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It's pretty close - but the point at which the bolt carrier stops moving is not the point at which ignition occurs - the hammer's still swinging forward inside the action. Nor was it the start of the lock time - the sear trips nominally .100" before the carrier finishes its forward stroke. Sounds like my 6-7ms estimate was pretty good, and the real value is closer to 6ms than 7ms, to account for the lock time starting before the carrier has actually finished its forward stroke. |
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1,400 rounds per minute can be achieved using an aluminum bolt carrier. Smith Enterprises used to market their aluminum carrier for that purpose. I can also tell you that on a carbine gas system, running an aluminum carrier and doing a Beta C-Mag dump that the standard gas tube will not survive it. The M4A1 is 750-850 rounds per minute by design. |
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The best I have achieved on any of my M16's with a carbine length upper has been just a bit over 800 RPM, but there was a guy here a while back that was saying he got over 1300 RPM on one of his guns. 
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What's the point of 1400 rounds per minute? The M4A1 is 750-850 rounds per minute by design. 
This was LONG before anyone had thought of anything called an M4A1. One bit of history, the M231 firing port weapon had a cyclic rate of over 1,200 rounds per minute. The M231 differs internally from the standard M-16 design, it fires from an open bolt. |
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What's the point of 1400 rounds per minute? The M4A1 is 750-850 rounds per minute by design. With an aluminum carrier and carbine gas system I think you could push 1600rpm if the mags could keep up. Personally I've moved to all gen 3 pmags for anything that will be cycling 1000rpm+. That has cut down on my feed failures. |
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Shits & Grins.
This was LONG before anyone had thought of anything called an M4A1. One bit of history, the M231 firing port weapon had a cyclic rate of over 1,200 rounds per minute. The M231 differs internally from the standard M-16 design, it fires from an open bolt.
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