I'm building an AR with a Seekins SBA15 Billet lower.  None of my Magpul Gen 3 magazines will fit.  They will not insert far enough for the recessed area on the magazine to fully line up with the release mechanism.   I've read online where some others are having this problem, but the question is:   Is it a big issue to simply file down the nub on the rearward part of the magazine so it will seat further?  I test- filed one, and it now fits, but I'm curious if any of you have experience with doing this, and if there are any other issues with the fit/safety..etc, that I'm unaware of.   Thanks!

You have the answer already

Magpul added over insertion tab to the magazine body, filing the body to fit you non standard receiver is the less expensive fix. Just dont slam the magaine in place with the bolt locked to the rear. Very small chance of the magazine overriding the magazine catch and not allowing the bolt to go forward into battery if that happens.

Thanks!!

In case anyone was curious, Seekins cleared up the issue:

Thank you for contacting us. We are aware of the Gen 3 Pmag issue and have made modification to all current and future lower receivers by adding a relief cut to accommodate this change in mag design. We can modify your lower for you free of charge. All you would need to do is print and fill out the attached RMA form and include it with the lower shipment. We will ship it back to you at no additional charge. It will be coated with an industry accepted coating (NOT Anodizing), there is no color match guarantee. We will be unable to provide an ETA on turnaround time as we will need to schedule it with the machine shop. An alternative option is to lightly sand down the over travel stop on the mags as this will allow proper fitment with the Gen3 Pmags, if you prefer to not send it in.  Please let us know if there is anything else we can do to assist you.

Great customer service from Seekins!

Heck yeah! That's amazing!

Wow talk about amazing customer service. Kudos to Seekins!