Any short-barrelled 5.56mm on full-auto isn't going to last a fantastic amount of time IMO. I keep making this point to all the police units I see who buy the things. It's handier to have something with a 10-inch barrel, but these things aren't MP5s using pistol rounds, 5.56mm is MUCH higher pressure and the smaller the gun the faster the wear and tear.
I see people mentioning quantities of ammo like 5,000 rounds, which is nothing really for something used as an SMG. Go to a rental range in the US that has an MP5 and they typically put through 150,000 rounds through their guns before any major parts replacement.
The shorter the gas piston, the bigger the gas port, you've got more energy being transferred to a smaller mass to produce the same effect in cycling the gun. So it's going to break more easily, and forget about the barrel, no 5.56mm barrel will last anywhere near as long as something in 9mm. Just simple physics. Look at all the problems SOCOM have been having with their M4s, putting silly amounts of ammo through them. They're flipping carbines, i.e. guns for support personnel, not LMGs!
I do think the 551 is probably a wiser choice unless you absolutely MUST have something smaller. Ditto for an AR-15, the shorter the gap between the muzzle and the gas port, the less pressure build up there is at the gas port, so the gas port has to be made larger. This can lead to erratic cyclic rates and other nastiness. So then you have to use more sophisticated buffers, etc. to compensate.
Just use barrels that are a few inches longer. This is the best option, IMO. I know it's not always possible but I think it's more possible than a lot of people make out for CQB. It's like people who buy Glock 26s instead of 19s because the 26 is "more concealable". It may be easier to hide, but it's harder to shoot. In reality with minor effort the Glock 19 is easily concealed.
Moreover, if the barrel is longer, this means higher MV, and that will translate into more effective terminal ballistics also.
