Simply, if you put a BCG designed for an op-rod setup (Like from the old conversion kits) into a standard "DI" upper, would it work?
I'm thinking it actually might, like a true DI like a MAS49. Could only find people asking the opposite question though.
"Customer is very particular" -- SIG Sauer
If we are picturing the same thing, I doubt it. All of the op-rod BCGs that I've seen (Disclaimer: only seen a few) have a gas key that is different. It is designed to be acted upon by the op-rod vs being hollow and designed to carry gas into the BCG.
Correct. So instead of the gas being carried into the BCG it acts directly against the modified gas key imparting at least some force to it.Originally Posted by Jason M
"Customer is very particular" -- SIG Sauer
Don't think so. The gas going into the BCG creates pressure that forces the bolt forward (even though it is stationary) while moving the carrier backward. The carrier moving backward cams the bolt to allow unlocking, extracting, and ejecting. In a piston gun the piston smacks the modified carrier key to start the process, and the gas gets vented to the atmosphere.
That said, I don't have a lot of use for the piston ARs, the DI works for me. To quote Ken Elmore, "jet engines don't need no pistons, and neither to my M4s...."
pat
Last edited by UNM1136; 03-27-2020 at 11:27 PM.
Yes. The op-rod BCG gas key is, in the few that I've seen, shorter and solid. Its the part that the op-rod exerts all of its force against to make the BCG travel rearward. The DI gas key is, of course, hollow.
Here is what I am thinking:
https://www.google.com/imgres?imgurl...MygAegUIARD6AQ
This would not work in a DI gun.
Correct, but the gas entering the carrier and pushing forward on the bolt is not necessary to function, as an op-rod AR does not have that effect and functions just fine without it.
Yes, we're picturing the same thing. So in my head the face of the gas shoulder (that's what I'm going to call it, to avoid confusion with a standard gas key) has a divot (ref SIG 516, HK416) that would, while the rifle is in battery, abut directly with a standard AR gas tube. So the question is would the gas impacting that gas shoulder have any effect at all, and if so, enough to cycle the action?
"Customer is very particular" -- SIG Sauer
ReverendMeat, it's not a stupid question. It's a very good question because, as you say, it approaches the subject from the other direction- can an AR be converted to a direct impingement system?
Yes it can, but not with an unmodified oprod gas key. Direct impingement systems have a small cup in the carrier the gas tube fits into, or a small nub that the gas key fits over, to create a small piston & cylinder with which to convert pressure into linear motion. The oprod "gas key" has a flat strike face. Blowing gas against a flat surface won't build up enough pressure to push the carrier.
With all due respect to Ken Elmore, he was wrong about the AR not needing or having a piston. The bolt is the piston and the carrier is the cylinder. Also, the AR is not a direct impingement system. Eugene Stoner clearly states so in the patent. The AR uses an expansion chamber. A direct impingement system does not.. Another difference is the AR piston does not separate from the cylinder to vent the system. The piston of the direct impingement system must separate from the cylinder in order to vent the system. In fact, it could be said the direct impingement system is a shortened long stroke piston.
The expansion chamber is a key design feature of the AR has system. It gives the gas a larger surface to push against for greater mechanical advantage. It's the same as gearing down for greater torque.
If you wanted to convert an AR to a direct impingement system, you could do so by plugging the gas key so gas doesn't flow gas into the expansion chamber. The gas would pressurize the gas key cup and push the gas key instead of the carrier. The gas port would likely need to be enlarged to make it work.
Last edited by MistWolf; 03-28-2020 at 12:52 AM.
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I think you would have too much gas. The carrier key is hollow and mates with a corresponding hole in the carrier. When that hole plugs up gun no workee right.
Thank you! Between the MAS-49 and the Jungman/Hakim (not aware of any other DI guns out there) I thought one of them didn't have a cup for the gas tube to vent into. I figured most standard 16"/cl uppers are already overgassed and might be able to make it work.
"Customer is very particular" -- SIG Sauer
Mistwolf nailed it!
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