I’ve got a Gen 9 M4A1 and an STD SLR.
The SLR is severely underperforming, it’s not feeding well, and it keeps just jamming up the barrel.
I have spares on the way, they will be here on Friday.
The main issue I have found is the magazine doesn’t line up well with the t piece, but changing to the Gen 9 mag is better.
What I also found is it seems like the t piece is too sloppy and let’s too many into it at once, giving you a shotgun effect.
Here’s a photo of the mag line up.