First review I've ever written, so forgive me if there are any mistakes/inconsistencies.
First off, a little description of the product itself; the mags come in packs of 5 and each sells for around $15 (if I recall correctly) and two boxes of these magazines + shipping costs around $50. This means each magazine costs around $5 and is no big hit to the wallet if you lose or break one of them.
I ordered two packs from Airsoft Club and they came out of the envelope looking like this:
And each pack has 5 magazines inside packed like this:
The magazines are made out of plastic and have 9x19mm IF molded near the bottom of the magazine; here's a side by side comparison of a Beta Project magazine and the MP5 short magazine that came with my TM MP5K PDW:
The plastic is somewhat strong and I have dropped one or two mags once or twice without scratching, chipping, or denting the magazine though I wouldn't trust them to survive a fall on concrete.
They are a tight fit in my TM MP5K PDW and wobble in my Aftermath Broxa, though it is important to note that even the magazine that came with my TM MP5K PDW also wobbles in it meaning that the Aftermath/CYMA MP5s have magwells with different dimensions. I have run around 3K-4K BBs through these magazines (average of 300-400BBs through each magazine up till the time this review was written) and they feed flawlessly with BB Bastard ammunition. They worked flawlessly in -15 degree Celsius weather at an SOF game and worked flawlessly in the sandy environment of Sgt. Splatters. One problem I have encountered with them is that they misfeed when loaded with Velocity Arms BBs though I believe that this problem is caused due to the ammunition rather than the magazine. If I were to unload a magazine that is fully loaded with BB Bastard ammunition back into its bag, it would take about one second, though emptying out a magazine full of VA ammo delays it to about 3 seconds.