The 155 mm mag is obviously between the 140 and 170 with regard to capacity. I comfortably get 4 additional rounds in the 155. However, the capacity isn’t the selling point.. It’s the size of the mag tube that seals the deal. The mag loads faster and more efficiently for me, because there’s more room to get my hand on it when I reload. I’m probably not going to buy another 140 mag. Last but not least, I didn’t have to swap springs or followers with this mag like I typically do when I buy a new magazine. What a score! 

The mag would not feed 100% new. I left the mag full overnight to seat the spring. And still had feeding issues. The gun runs perfectly with 4 different sti mags. 

Giving 2 stars since I tried to contact MBX. They do not have a phone #, only email. So I emailed to see if they had a solution. I got no response to my emails, just crickets. So if you have a problem with their product , dont expect any customer service. 

Btw I did get the mag to run flawlessly.  I ended up having to tune the feed lips!






Response from Chuck- Glad you got them running good with your gun. Yes sometimes the variance in geometry between different mags, guns and ammo may require some tuning for your set up. Been doing ti with all makes of mags for 25 years myself. Something the new comers in the sport don't realize, they tend to expect everything to drop in a work across the board with every variable imaginable. Just not realistic. I don't think the 4 rating is fair. More of a jab at MBX for not responding to your email. I do agree sometimes it takes them some time since they are a small operation and go to matches so they are not always in the shop. 

Shot a match straight out of the bag! Runs great and zero issues.