Purpose of Rectangular Hole Under Security 380 Rear Sight?

[SCW]

I spent a frustrating several hours banging out the rear sight of my wife's new Security 380 so I could install a Galloway Precision optic mount for a Holosun 507k. Finally managed to get the sight out, luckily without marring the slide itself, only to get the optic plate halfway in before giving up on it. Sent the slide into North Carolina Engravers to have it milled for a direct mount of the Holosun.

But my question, which I can't find the answer to anywhere, is this: Underneath the rear factory mounted sight is a mysterious rectangular hole. This hole isn't necessary for drifting the slide left or right, and you can't see the hole unless you remove the rear sight. Anyone know why it's there?

 

[NikA]

Could very well be a slide lightening cut. I have seen such cuts on 380 models to allow similar external dimensions to 9mm models while maintaining appropriate slide weight for reliability.

 

[SCW]

Could very well be a slide lightening cut. I have seen such cuts on 380 models to allow similar external dimensions to 9mm models while maintaining appropriate slide weight for reliability.

Interesting! That's a great guess. Makes perfect sense. The slide already has the side ports in front that serve only to lighten the weight; the Security 9 doesn't have those ports, and the Security 9 also doesn't have that rectangular hole under the rear sights. Thank you.

 

[Fox Mike]

Probably to lock the rear sight down via the center screw in the original sight after sight adjustment.

 

[Rcustom88]

That rectangular hole under the rear sight of Ruger Security 380 is almost certainly a fixture point from the manufacturing process.

 

[SCW]

Probably to lock the rear sight down via the center screw in the original sight after sight adjustment.

Unfortunately, I didn't take a photograph of the top of the slide before I sent it off for milling. However, the rectangular hole is neither to lock the rear sight down via the screw or a fixture point in the manufacturing process. The hole doesn't go all the way through the slide. It's most of the width of the top and doesn't go all the way through the slide; it's more of an intentional, rectangular depression that is completely hidden until you remove the sight. It's farther back than the set screw hole. The sight on the Security 380 is really long, and the rearmost portion - what you're actually looking through if you're using the fixed sight - is what completely covers the hole. Nothing is affixed to the hole.

I have to go with the suggestion that it's there simply to reduce weight.

 

[s4s4u]

I have to go with the suggestion that it's there simply to reduce weight.

For .0034 ounces?

 

[Fox Mike]

I just installed a red dot on my Security 380 and remember that there was a 'hole' or notch but honestly do not remember if it was a rectangular or not. This is from the instruction manual and is what I was speaking of in my first post. Also look at the parts page:
https://ruger-docs.s3.amazonaws.com/_manuals/security380.pdf
REAR SIGHT ADJUSTMENT
The SECURITY-380® pistol is equipped with a rear sight which is adjustable for windage.
To adjust for windage, after the rear-sight lock screw has been loosened using a 5/64” Allen
wrench, the rear sight can be drifted laterally in its dovetail on the top of the slide by
tapping it with a wooden- or plastic-headed hammer or similar implement (see Figure 17). Move the rear sight in the direction you want
the shot to move on the target. After the rear sight is adjusted, tighten the rear-sight lock screw

 

[Dry154]

That rectangular depression under the rear sight on the Security380 slide is a slide lightening cut. Ruger milled it to reduce rear slide mass so the lighter .380 recoil impulse cycles reliably keeps the same external dimensions frame as the Security 9 but without needing a heavier slide that could cause malfunctions.

The Security 9 doesn't have it or the side ports up front because 9mm needs more mass. It's fully hidden by the long factory rear sight, no functional purpose once assembled. Common on lighter caliber variants sharing platforms. Your direct mill will cover fill it anyway smart move ditching the plate.

 

[Fox Mike]

That rectangular depression under the rear sight on the Security380 slide is a slide lightening cut. Ruger milled it to reduce rear slide mass so the lighter .380 recoil impulse cycles reliably keeps the same external dimensions frame as the Security 9 but without needing a heavier slide that could cause malfunctions.

The Security 9 doesn't have it or the side ports up front because 9mm needs more mass. It's fully hidden by the long factory rear sight, no functional purpose once assembled. Common on lighter caliber variants sharing platforms. Your direct mill will cover fill it anyway smart move ditching the plate.

Just how much 'mass' was reduced, and where did you get this information from? As far as weight (mass) goes the 9 is 2.1 oz heavier than the 380 and less than 3/4 inch longer

 

[TestEngineer]

Why the either/or mentality? It's very probable that the notch is for both - tooling and lightening. Ruger is famous for designing with multiple purposes in mind.

I have a lot of examples, but here's an obvious one:

Look at the right/bottom side of a mark pistol bolt. There's a notch cut into the rail that the disconnector moves up/into to engage the sear. The notch is ridiculously large considering that the disconnector only needs to move up about 0.050" to engage the sear. It's like digging the Grand Canyon when you really only need an irrigation ditch! Why would Ruger make that notch so big? Simply to reduce the weight of the bolt for reliable cycling. They could have reduced the weight somewhere else, but they chose that location because the process wouldn't require any additional steps to the manufacturing process.

 

[NikA]

Just how much 'mass' was reduced, and where did you get this information from? As far as weight (mass) goes the 9 is 2.1 oz heavier than the 380 and less than 3/4 inch longer

Comparison should be to Security 9 Compact, which is same size as Security 380. Still 2 oz heavier, probably all in the slide; guessing the frames are identical and the magazines have a spacer to ensure proper 380 feed.

If I remember this afternoon, I'll take a picture of similar cuts in a Sig P290. They are a necessity for .380 locked breech pistols built on 9mm platforms.

 

[NikA]

Below, comparison of Sig P290 slides for .380 (silver) and 9mm (black). Lightening cuts clearly visible between feed lug and slide rails of .380 slide make it 2.3 oz lighter (7.1 vs 9.4 oz), paired with a shorter recoil spring to allow the .380 slide to function properly on same lower frame as 9mm.

I suspect the Security 380 and Security 9 Compact have more or less the exact same relationship.