A friend of mine is a class 2 manufacturer(sp?) and wants to design cans for the AR (5.56). The hang up is that he can't find any subsonic ammo to test his designs with. Thus bringing me to my question, does anyone make bullets heavy enough to cycle reliably this slow? I was thinking he could use an aluminum bolt, light buffer, and a weak recoil spring, but this would defeat the purpose since firing normal ammo through it would damage the gun, right?

Anyways, thanks for your help.

I thought the ammoman (ammoman.com) used to offer subsonic (blue tip) 5.56mm.

No, firing regular ammo will not damage the gun.  In fact, a recently Weapons and Ammo magazine tested a suppressor and they used Black Hills and another brand of ammo that was subsonic.  I think the DB difference was like 2db or something miniscule.  Regular ammo will not damage the gun, but back pressure on full auto will wear out the throat of a barrel pretty fast.  Single shots spaced a second or more apart will not damage the gun.  Faster shooting just wears it out faster.

www.botachtactical.com

They did have the Israeli military ammo made for this very purpose. Might give them a try. I have no need for it, but maybe your friend can put the stuff to good use.