In my opinion, the HK416 is not a good carbine. This is primarily due to its increased weight and recoil compared to the standard M4. The increased recoil, due to greater reciprocating mass and velocity, also increases the probability of damaged or loosened components. Furthermore, the already high cyclic rate of the 416 is increased further by the attachment of a silencer. As you can see in this video, the cyclic rate is simply too fast for even one of the strongest magazine springs on the market, the Lancer L5 AWM, to keep up with – resulting in a bolt-over-base malfunction (the rear of the case has not risen to the top of the magazine before the bolt face has returned to push a new round into the chamber). This is not an isolated incident, but a common occurrence with the multiple 416s I have witnessed exhibiting this problem.
If you are part of a unit or organization which must use the HK416, the best upgrade (short of replacing the entire weapon) would be to replace the receiver extension tube and buffer assembly with the Vltor A5 system (specifically the A5H4). Vltor tested the HK416 with the A5 and saw a reduction in cyclic rate from 1106 rounds per minute (stock, suppressed) to 973 rounds per minute (A5H4, suppressed).
If you cannot replace the receiver extension tube with the A5 system or get rid of the HK416s entirely, try using a Tactical Springs LLC/Springco “Red” action spring along with an H3 or heavier buffer.