Not certain what type of piston the CMMG uses...but, if it's a short stroke, carrier tilt issues have been rearing up in the piston AR's out there.
Personally, I have no issues with DGI rifles; if you clean it fairly regularly, and run the bolt wet, you shouldn't ever really have a problem. I've got a couple rifles that I've put over 500 rounds through in an afternoon, without a single problem...
I guess, to me, piston drives make sense in SBR'd and full auto AR's...
ETA--as to the which and why...
If you like the features of the Wasp, go with that; but, then again, if you are sold on piston drive, you can always add features you like (such as the rail, etc) to it later.
The research I've done seems to indicate that the vast majority of shooters will be served very well by either a piston or DI gun, provided both are a proven design, made of quality components, and properly maintained. Given that, I would (and did) save the additional cost of a piston for other things.
Are you sure you want a bull barrel? If you're doing much more than benchrest shooting, those things are heavy on your support arm. If you can, find one in a shop somewhere and handle it - see how it feels to you.
What are the prices on these guns, if you don't mind my asking?
I'd check with people who do the competitions in your area, and what kind of system most of them use, piston or gas impingement.
Now, I know that piston driven AR's are the new in thing these days, but honestly, I've never really gotten to run one. My issued 16A4 has the saame gas impingement system that has been used in 16's and AR's for since they started being made. And I have shot hundreds of rounds out of it on some days. And that was in the desert where clp is used very sparingly to avoid it attracting grit into the action. Of course, it also has a full length system, and not a shortened M4 system, but I have seen them do the same thing.
If you like the features on the gas impingement more, than I'd go with that one personally, but that is just me, and I don't really have issues with the gas system. I'm sure someone on here will be able to make good arguments as to why you should go with the piston one though.
The piston driven systems do seem to run cooler and cleaner. That being said the gas system has served us well for MANY years. I don't think the gas system is bad, the piston system seems to be good as well. The HK416 runs it, and everything I have read about them is positive. I really dont think you'll be disappointed either way with what you are planning on doing with it.
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