I have had 3 different gun bearers, and never have had a failure of the plastic clamping piece luckily. It is good to have options like the SG though...but also just in case you haven't tried it, there are a lot of gunbearer adjustments that can be made for riding the bike (e.g. where the gunbearer attaches on the shoulder strap, how tight the chest & shoulder straps are, where on the hip belt the gunbearer is attached, and how far up or down the portion of the gunbearer that holds the rifle butt is adjusted.
My main SG pack actually right now uses the lower SG frame connection but with a Kifaru rifle butt holder, and I have both the SG behind the shoulder strap and Kifaru on the shoulder strap upper connections. It is all pretty interchangeable.