Where is the crack on your Type B feed lip, if you don't mind me asking? I've noticed "lines' on some of my magazine feed lips, but given that none of them have been shot yet (haven't had time), and only one has been loaded, I'm wondering if it's less a crack and more just a noticeable mold line since those that have the marks are all in the same place.
Also, you mentioned the Type Bs are the ones that came with the carbine, so I'm assuming they're 20 round magazines. If possible, could you get a measurement of the spring length? As I've mentioned elsewhere, I'm wondering if the mold revision is only the most immediately noticeable of multiple potential changes to the magazine design. If so, they could still be using 30 round springs in 20 round mags, which is causing pressure that the improved feed lip design is not enough to offset on its own.
The hairline crack on the B mag is on the right feedlip, 1/16" from the front of the feedlip and 1/8" long running along the curved portion of the feedlip (left to right, right to left).
it may be a sign of premature failure or it may be nothing. the reason i'm not too worried about it and will use all my mags without bias until they eventually fail (if that day ever comes).
yes, these are all 20 round mags. i can't give you a spring length because they all vary by how long they've been in rotation. i mean, i didn't start with 14 mags, just slowly picked up a couple every so often (just got 2 more in mail today, type A) and added them to the rotation. i can give you a coil count though, 18 tip to tip. same springs in A and B mags, only difference is the mag body, follower and baseplate. how many coils are on the 30 round mags? how many are on the 10 round mags? if i have both of those numbers, i may just split the difference and cut half way between the 2.
now i can understand the crack on the right feed lip since that's where the last round rests. what i don't understand is why half of my crack mags (hairline crack, just a reminder) have this on the left feed lip. and they are cracks because if you look directly it, invisible, but as you rotate the mag, you can see the light travel through line. maybe i could feel it from the inside, but the outside is smooth. really don't feel like dumping all my rounds, gutting the mag, cleaning it and trying to squeeze my pinky in there at an odd angle to feel for something (that just sounds so wrong). guess i could use a dental pick to feel in there.
ahh, i'm just going to make a youtube video