I just bought an S&W M&P 15 Sport and thoroughly enjoy it. However, I am using a Ciener .22 conversion kit with a suppressor when shooting around the house. Does anyone have some firsthand experiences concerning this use? Will it damage the gun? Does the lead create a problem? Will simply firing some 5.56 ammo clean out the gas tube? I can’t complain about the rimfire accuracy in the 1:9 barrel; I hit what I’m aiming at out to 100 yards. Since I am just plinking I don’t want to invest in a dedicated upper, but I don’t want to ruin the gun either. Any thoughts will be appreciated.

It doesn't hurt anything, just gets dirty is all.

X2 .22LR can be a pretty dirty round. Try to use plated bullets if you can to minimize lead fouling. Not a big deal, just more cleaning with bare lead bullets.

The only negative I can say is based on experience with military rifles that had the conversion. They weren't the most reliable but since they were specifically held to the side for firearms qualification, they didn't care much to take as good of care of them as they do for a issued personal weapon.

If you're shooting a suppressor, it seems to me you'd want to shoot rounds that stay subsonic for best performance (in terms of noise). Do subsonic or standard velocity loads cycle the conversion reliably?
If not, that might be reason enough to pick up a dedicated 22lr semi rifle, many of which are just about the price of conversion kits anyway.

Thanks to everyone for your suggestions and advice. I'll keep using the Ceiner unit since it does not damage the rifle.
Yes, I use CCI SV and it cycles fine and is fairly accurate. The only thing I hear is the bolt going forward and the impact of the bullet down range.
Thanks again