Good question Opie.  First off mil-spec standard tolerances are +/-.015 (unless otherwise noted).  And Broken Arms tolerances are +/-.005 (unless otherwise noted).  We are a Silicon Valley machine shop holding very tight tolerances (.0001) on critical semiconductor parts.  So our expectations for machined products are extremely high.

Here are a few examples of the fuckery I have personally seen on other companies lowers.

Problem:  The mag release wouldn’t fit into the slot.
Reason:  The tips on the endmill they used must have chipped it left a big radius, rather than a square corner.  This wouldn’t allow the mag release to seat correctly.
Solution: Repaired in shop.

Problem: When putting the magazine into the magwell, it would start but get stuck halfway.
Reason: The magwell was machined, rather than broached.  The shop had machined it from both sides (top & bottom).  This is defiantly an acceptable way to machine it.  But their numbers were off and the pocket didn’t line up with itself.
Solution: Returned to manufacture.

Problem: Front takedown pin fit in hole, but wouldn’t go all the way through.
Reason: Lack of deburring skills.  Many shops throw the lower into a vibratory rock tumbler.  A tumbler will only deburr areas where the rocks can fit.
Solution: Hand deburring tools.

Problem: Front detent hole location was off by .040.
Reason: Either bad setup, or machinist loaded the part wrong.
Solution:  Modified the takedown pin. (but looks like ish)

Problem: Buffer hole thread too small
Reason: Lack of proper thread gauge, or didnt use thread gauge
Solution: Sent back to manufacture (replacement was also bad)

Problem: Takedown pin holes too small.
Reason:  Not sure, too many choices of why.
Solution: Reamed to .2505.

Problem: Rear takedown pin crooked.
Reason:  Maybe the part lifted on that operation.
Solution: Returned to manufacture.

Problem:  Grip wouldn’t screw on.
Reason:  The 1/4-28 hole was drilled in the wrong location (appx .125)
Solution:  Dremeled the hole into an oval.

Problem:  The front detent pin would get stuck at an angle.
Reason:  Manufacture drilled the detent pin hole too big.
Solution:  Made a bigger detent pin. (the rear was also bad, but didnt get stuck at an angle)



^^  These are things off the top of my head.  Ive seen it all when it comes to lowers.  We arent the only quality manufacture out there (obviously).  But with the 80% lowers, there are a high number of hack shops.

Last edited by Mike (11/07/2013 5:45 pm)

I am thinking of building one of these 80% lowers.  My question is, what jig kit would you recommend?

Quality is great!
Set screw pin for the bolt catch is a big plus!

I have ordered my jig, yours seems to be the most comprehensive out there, as well as a fantastic how to video. Any idea if you will have a 308 jig and video?
Thanks, G 

shredimaster wrote:

Question for the mods about the Broken Arms jig: will this jig work with any mil-spec lower?

After our recent modification, yes.  It fits Broken Arms, and Mil-Spec lowers.