Different fire control pockets

[Intimid8tor]

As I am working on my lower I keep a factory lower around for reference. I noticed that the shelf machining is different from what we see on most lowers and the mega.

I'm not talking high shelf vs low shelf, but rather the bump on the right side of the receiver.




Here is a Mega lower.

Mega lower

It looks like BCM does the same.




Here is what the patterns are for the 80% lowers and what many look like.


The reason I ask is that it would be a little easier to machine if I didn't need to leave the little bump there. Does anyone know the purpose of the bump? Is it related to RDIAS?

[jamesob]

That's why some companies do it that way, it's easier.

[Dreaded Claymore]

Personally I'd eliminate the bump in order to get a (tiny) weight reduction. But I, too, would like to know why the bump is there. If I were a betting man, I'd bet that it's something that Colt did at first in an attempt to make it more difficult to illegally convert, and everyone else did it because Colt did it, in an example of cargo cult engineering. (But that's nothing but a guess. I have no idea.)

[yari]

^^ got it.

[kcstott]

Nop
It's there to add material to support spring for the selector switch below the switch. Also for the auto sear. The auto sear is narrow and has to locate properly to sit in the groove on the selector switch. On a full auto rifle only. Which doesn't apply here.

It's on my ordinance print from 1970 so it had nothing to do with Colt trying to make it harder for it to be converted to full auto. It actually helps.

So that pocket behind the selector can be cut to .500" wide with no issues. But you do need to leave it there below the selector to have enough wall thickness for the safety spring hole. Notice the first pic posted and look below the selector the bump is still there just not all the way up to the top.

[Intimid8tor]

Nop
It's there to add material to support spring for the selector switch below the switch. Also for the auto sear. The auto sear is narrow and has to locate properly to sit in the groove on the selector switch. On a full auto rifle only. Which doesn't apply here.

It's on my ordinance print from 1970 so it had nothing to do with Colt trying to make it harder for it to be converted to full auto. It actually helps.

So that pocket behind the selector can be cut to .500" wide with no issues. But you do need to leave it there below the selector to have enough wall thickness for the safety spring hole. Notice the first pic posted and look below the selector the bump is still there just not all the way up to the top.

Thanks for the explanation. So, if I ever got lucky and bought a registered dias legally, the hump would need to be there for it to work. Otherwise, it is not "needed." above the .630 low shelf limit.

Correct?

[kcstott]

Yep.

[goldfinger]

I remove that bump on my 80%s