Just make sure you meet the minimum 26" overall length when stock is collapsed. To measure without the actual installation, measure the length of the receiver from the base of the barrel, length of barrel, and measure the collapsed stock from the base of receiver. These should add up to 26". I got a R597 HB, 18" barrel, + receiver when installed on a similar A556 kit, clears the 26" even when stock is collapsed. I am sure a lot of shooters has done this conversion. As long as you are not cutting the barrel or replacing it with shorter aftermarket barrel (shorter than 18") I do not see any problem.