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Posted: 9/17/2013 4:21:17 PM EDT
| Can anyone answer this: Do piston ARs cycle slower/ faster than DI? |
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Ideally the same.
ROF (Rate of fire)/bolt speed) varies from gun to gun, design to design, ammo type to ammo type. Gas port, buffer weights, spring weights, operating system, carrier weight, ammo type, lube, how the shooter is holding it, cleanliness can ALL affect bolt speed and ROF. The bolt needs to move fast enough to strip the round but not so fast it outruns the magazine. That's ideally 750-900 RPM. Push it past 1000 and you'll outrun the magazine, less than 650 and you're not going to be able to overcome the friction holding the round in the magazine and still close and lock the bolt. The Stoner recoil design and the STANAG magazine are really what determine what the bolt speed needs to be. That said, I know of working designs that were built to operate on both ends of the spectrum. One major MFG built a proto for a foreign contract designed to run ~500-500 RPM which is slow enough to be able to shoot a gun effectively in semi-auto while on the full auto selector setting. I know another smaller MFG I worked with who built theirs to run effectively at 1100+RPM. But, to reiterate, in the stoner design 750-900 RPM seems to be the range where any gun, piston or DI works best. I prefer 775-800 for my personal guns and have my 11.5" DI and 10.5" piston running at that. It's not enough to lock back Wolf, but it shoots most .223 and 5.56 very smoothly. On a side note, I did once hog out a gasport on a 16" carbine to 2.2mm and ran it with can, which pushed it to 1200 RPM when it would feed. |
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Quoted:
Joe, I appreciate the education. The recoil on my OPS 416 and AAs feel slightly slower than my Colt A2. I have heard of something called "reciprocating mass" and I was wondering if that impact on on the BCG speed. Reciprocating mass would be the mass that moving back and forth along the recoil path. Typically it refers to the mass of the bolt carrier and buffer. For example, a JP low mass bolt carrier will cycle faster than a heavier M16 carrier due to the mass being easier for the operating system to move. (Basic physics and all that). So yes, it will impact BCG speed; the same concept applies to both piston and DI guns. Lighten the carrier/buffer and it will move faster (conversely it will exert less force when trying to strip rounds from the mag; which is one reason why a heavier carrier is considered more reliable in many circumstances). From a practical perspective from one of my DI guns: 11.5" barrel 1.74mm gas port M16 Carrier H buffer: ~800 RPM Change the buffer to a granulated tungsten buffer (both heavier and changes how the mass behaves), 736-753 RPM. So a drop of 50 RPM for a buffer change. I could probably slow it even more by adding an LMT enhanced carrier, which changes the cam pin path to a more aggressive angle, which keeps the carrier from moving rearward until the gas pressure has dropped. One possibility why your pistons feel "slower" is that you're dealing with the feeling of the push rod and not just the carrier/buffer mass. It doesn't necessarily change the speed of the carrier, but it adds additional moving mass to the feeling of perceived recoil. |
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Reciprocating mass softens recoil, not the other way around.
For a practical demonstration shoot a .45ACP revolver and M1911 side by side. The M1911, with the reciprocating mass of the slide, has a much softer recoil. Recoil pads on shotguns do the same thing only in this case the entire shotgun reciprocates. Cyclic rate, as noted above, is regulated by a myriad of things. Colt had massive problems regulating the cyclic rate of the original XM16 rifles until the ammunition was standardized and still had to play with the bolt carrier mass. -- Chuck. |
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Quoted:
Reciprocating mass softens recoil, not the other way around. For a practical demonstration shoot a .45ACP revolver and M1911 side by side. The M1911, with the reciprocating mass of the slide, has a much softer recoil. Recoil pads on shotguns do the same thing only in this case the entire shotgun reciprocates. Cyclic rate, as noted above, is regulated by a myriad of things. Colt had massive problems regulating the cyclic rate of the original XM16 rifles until the ammunition was standardized and still had to play with the bolt carrier mass. -- Chuck. ^^^Succinct and correct answer. Basic physics: F = M x Va So if you rearrange the equation, and have M (Mass) moving in the opposite direction of the applied Force or enough Mass that the force applied is not enough to upset its static state, theoretically you can reach a state where there is little to no felt recoil. Recoiless rifles are one way to approach this. The Gepard GM6's long recoil barrel assembly is another practical solution for a large caliber weapon. |
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Given the generic nature of the question, the correct answer is "it depends." It depends on the way the two rifles you're comparing are sprung, on the way the gas block and piston meter the gas, on the mass of the carrier, and many more things. With all other parts being identical, a piston system might be anywhere from a little faster to a little slower than a DI system, depending on how the block meters the gas. A well designed piston gas block should meter gas as closely as possible to the way the traditional system does (the bolt and its chamber within the carrier actually do meter gas). But how a specific piston system manages things is "implementation dependent." Osprey and Adams might be slightly different because of their design specifics. Ideally, changing a standard gas system AR to a piston system should produce a gun that functions indistinguishably from its original configuration.
Felt recoil is a different matter. The system Stoner used is designed to make ALL the reciprocating parts exactly in line with the bore of the barrel. This causes the recoil forces to be as straight as possible, minimizing the tendency of the rifle to rise with each shot. Compare that to an AK, where there is a whole lot of off-axis mass (the piston and rod, and most of the carrier are off axis), which rises no matter what. AR piston systems tend to have their off-axis parts very small and light to combat this. But the mass difference between traditional and piston ARs is so small that there shouldn't be any real perceptible difference. On the other hand, many people think that a (blowback) 9mm AR has "worse recoil" than a standard AR in 5.56mm. In the 9mm system, the bolt is very heavy, which slows the opening of the breech enough to handle the 9mm cartridge, but this also tends to draw out the recoil impulse. If you do the math (55 gr bullet at ~3000fps versus 125gr bullet at ~1100fps), there isn't a lot of difference in the recoil energy between the two, but it feels different. You might feel the recoil of a piston based rifle a little differently than with a traditional AR, but it shouldn't be much of a difference at all. |
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Quoted:
According to G3Kurz, an HK-usually reliable source, the HK416 goes at 150 RPM, and the M27 IAR (HK 416) is 300RPM. A standard M4 is 90 RPM. Your statement makes no sense. What does "goes at RPM" mean? The M4 is 750-950 rounds per minute cyclic rate. I should be able to find out tomorrow what the HK416's ideal ROF is supposed to be, but I can't guarantee it's cyclic rate isn't 150RPM. Heck I'm 250 RPM with my finger and a semi-auto trigger. |
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HK416, depending on barrel length, if it is fitted with suppressor or not and if it is with AGR or not can go in between of 700rpm to 1100rpm. Generally HK416 is rather in higher rpm rate than similar DI rifles.
M27 has rate little slower than it's equivalent HK416D16.5RS, because M27 has gas venting hole 0.2mm larger. |
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