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What xcr/barrel combos are considered low gas that you would want to move the piston?
Hell, I don't know. I've never designed moving parts before. I'm coming at this like a monkey with a rock.

I'm glad you brought this up though because I have been thinking about the XCR-P lately. RA seems to have made the XCR a long stroke and a short stroke piston system before, and I suspect they will try to make the XCR-P a long stroke piston gas system. The thing about the Sig MPX is that the placement of the short stroke piston basically makes it a direct impingement gas system kind of in the same way the AR-15 is, thus making the internals as gassy and dirty as it is. I was hoping RA would be able to do this with the gas block away from the chamber and not get gas into the upper, but what I'm seeing so far is that there isn't much difference in terms of gas port location between short- or long- stroke piston designs. I'm putting a lot of trust in Sig's engineering on this, and it seems the best place to get gas to cycle on a semi-auto 9mm cartridge is right on the damn barrel extension.

The gas port location on the .300 BLK Micro barrel is the shortest gas length that I know of on the XCR and I don't think that's short enough, it's like 5 inches from the chamber. I also don't know if the lugs on the XCR's bolt would create enough timing to allow for gas to build up in the barrel and produce enough pressure to push the piston to the system can cycle. I also don't know what kind of pressure would be coming out of that gas port and how much is needed to push the piston to cycle the system. My hope is that a good port location is about 1/2 inch in front of the taper on the barrel, just after the chamber ends; so a gas dial can be reasonably reachable and so there is enough length to put gas vents so less gas can get inside the upper.

What we really need is an engineer to figure out the math to find this stuff out; I wouldn't know where to begin. If I had to do it, it would be a lot of trial and error with a 5.56 bolt and extractor modified for 9mm, a 9mm barrel blank with XCR-L barrel extension, a welder and a machine to make lots of pistons. I'm reminded by the R&D work CMMG put into the Banshee before it came out: they went through several interations of a gas cycling system that became increasingly impractical before they went with the radial delay blowback system with the bolt. I'm taking that story to heart, as I'm seeing how challenging it could be to figure out a gas cycling system on a 9mm PCC. For that matter, I'm not sure if Robinson Arms knows or even has plans drawn. How long have we been waiting for an XCR-P? Since 2015? An engineer worth his salt would have had that figured out in a year or so and production could go on. I'm starting to suspect that it's an idea that isn't on any drawing board yet.
 

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Monkey with a rock LOL I would go the delayed system somehow. Much simpler than a gas system. I have a few 9mm pcc and the only one that's smooth at all is the cmmg i put together with the barrel/bolt combo they used to have for sale. I'd say it's on par with the mpx or VERY close. Way close enough to justify saving almost half the cost.....
 

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Monkey with a rock LOL I would go the delayed system somehow. Much simpler than a gas system. I have a few 9mm pcc and the only one that's smooth at all is the cmmg i put together with the barrel/bolt combo they used to have for sale. I'd say it's on par with the mpx or VERY close. Way close enough to justify saving almost half the cost.....
That's fascinating. I wish I knew someone who was a ballistics engineer, then I could have a better idea how a gas-operated PCC would work in more specific terms. It sounds like direct blowback, even for something like 10mm Auto, is smooth shooting in a PCC/subgun size-rifle. It's been awfully telling how large companies like CZ, CMMG and KRISS, with all their R&D budgets and time, have seen fit to come out with blowback guns for their 9 and 10mm guns, respectively. Considering the engineering and mechanical issues that Sig saw with the MPX, maybe going gas-operated with rounds that have such little gas volume isn't the best idea?
 

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That's fascinating. I wish I knew someone who was a ballistics engineer, then I could have a better idea how a gas-operated PCC would work in more specific terms. It sounds like direct blowback, even for something like 10mm Auto, is smooth shooting in a PCC/subgun size-rifle. It's been awfully telling how large companies like CZ, CMMG and KRISS, with all their R&D budgets and time, have seen fit to come out with blowback guns for their 9 and 10mm guns, respectively. Considering the engineering and mechanical issues that Sig saw with the MPX, maybe going gas-operated with rounds that have such little gas volume isn't the best idea?
The gas system is exactly like the Mcx except the port is right off the chamber for the pressure needed to run the short strike piston. Same basic design as any short stroke piston system out there with some minor technical differences I assume because of the pressure difference.
 
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