You're bound to read more of them. I hate to say it but I think that the days of a Mac style RR being a starter gun are numbered. The market value of the Mac style RR is reaching a point where such things as the Max-11/15 are viable. With more innovation comes higher market value. But the innovation will not be cheap. Ten years ago very few Mac style RR owners would consider spending $3,000 on any innovation no matter how ground breaking. That was about market value for the RR. Now there's 900 on the list waiting to buy.
It is my understanding that a M4 configured M16, a MP5, and a full size AK are the most popular machinegun rental models. I think that all three of those mechanisms could be modified for use with a Mac style RR. Would those "uppers" be cheap? I don't think so. But with a M16 RR at $18,000 to $25,000, a HK sear at$32,00 to $35,000 plus a host to run it in, and a RR AK in the mid $20,000 range there are collectors that could spend $10,000 on a Mac style RR and $3,000 to $5,000 on a conversion of those mechanisms for half the market value of a transferable. I don't see market value of those transferables coming down, so there will be pressure for alternatives. I think that the Mac style RR will be that alternative.
Scott