308 lower receiver dimensions for machining

[Socaliblaster]

Hey, this is Don from southern cal.  I’m still aiming to put pictures up of the two LR-308’s that I’ve built during the last couple months, coming soon.  They’re awesome.

In my introductions post (still there, “Another new guy building a 308”), I attached some marked-up drawings for machining the 308 lower from an 80% lower.  These are mark ups of ‘the only thing I found out there’.  (I do have a full set of validated AR15 prints.)  There are two dimensions on the second page that I crossed out from the original sketch, indicating that they were not correct.  I’m not sure about that now.

The source of confusion is the mag catch hole horizontal location relative to the center of the bolt catch slot.  If you use the dimensions of the original sketch, it says that the mag catch hole is .036” further towards the mag well than is the bolt catch slot.  However, if you look how the sketch is drawn, and if I look at both my actual lowers (these features already machined into the 80% lower), it sure looks like those holes are either aligned vertically or pretty close to that.

To add to the confusion, if you look at the dimensions of the Colt AR15 lower, you’ll see that for the AR15 the mag catch hole is .0045” further away from the mag well than is the bolt catch slot.  (It’s too late to yell at the designer for that, but I really can see no engineering reason not to have aligned them instead of offsetting them RCH.)

Technically, these dimensions don’t matter to me, as the way I do it is to set my zero at the center of the bolt catch slot.  Everything you need to machine between the bolt catch slot and the buttstock is the same between the AR15 and the 308 (you don’t need to machine in the takedown pin hole in the 80%), except one depth which is annotated on the sketch.

So, although I doubt anybody would use those two dimensions (unless you’re hogging one out from a pure slab of aluminum), I wanted to clear up the fact that I am clueless about those two dimensions.  All the other dimensions that I put on there are good; validated thru machining two of them that passed functional fine and both shoot great (with Pmags of course, but that’s a different subject).

Does anybody know anything more about those relative hole locations?  I have a hard time believing the sketch when I look at my lowers, but ???