Hello to the group here. My first post after just joining. I am not new to guns at all but just picked up a used SU16a for a song and it's in great shape. Do some of these rifles have tighter mag wells? I have a large selection of 10, 20, and 30 rd.mags to choose from and it's hit of miss as to whether they drop free when the release is pressed. Is there some procedure to relieve this such as sanding/filing/etc.? I read through the FAQ section and saw several mentions of early vs. later versions of the SU16 series rifles. Does anyone here know the approx. serial number separation? Thanks in advance!