I currently only own DI AR rifles. I have not made the plunge and purchased a piston AR yet. I do however intend to do so in the future.
Pros of the piston vs. DI:
1. Cleaner on the gun's internals and lessens fouling due to venting of gases out the side of the gas block/piston cylinder.
2. Cooler firing for the gun's internal parts ie. BCG since the hot gasses are vented outside of the gun at the gas block and not into the actual gun itself.
3. Supposedly more durable and reliable - able to fire much more without needing to be cleaned/jamming
Basically its supposed to bring the AR closer to the AK's durability/ruggedness and ability to function in adverse conditions. Its not going to match that of the AK but it's supposed to be a good compromise.
1. Price - piston uppers are typically more expensive than their DI counterparts.
2. Parts interchangeability (or rather lack thereof) - each manufacturer is making the parts designs proprietary. As of the moment if you have a piston system from Adam's Arms then the only parts you can use to replace broken parts are strictly from Adam's Arms. Likewise with other manufacturers such as CMMG etc. I've even heard that within manufacturers their different models/lines do not all have interchangeable parts (although this is only heresay and cannot be considered correct in any way).
The only reason I'm waiting on purchasing a piston system is that I'm just researching this subject more. I'm also looking at who has what could be the best system (at least according to my opinions). At the moment its turning out to be CMMG. Price wise its not too much more than a normal DI system and their parts are designed and manufactured larger and more robust. Adams Arms, LWRC etc. all have pretty good systems too but their systems use a lot of smaller/smallish components and are extremely high priced.
I've got a buddy who shoots 3-gun competitions and he said that a bunch of guys who went with the piston AR in both Adams Arms and LWRC systems end up having a lot of trouble after 1,000 or more rounds through their guns - the springs become too soft or something to that effect and the gas tubes also gunk up quickly and as such they start to suffer short stroking. That's an issue I'd like to avoid when I convert.
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