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Topic: PT-157 Gun Sight |
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David Waples |
TOP BOSS
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Posted on: Aug 2, 2022 - 8:57am
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Greetings,
While watching the interview that Frank did with Maurice Kowal I noticed in a photo of PT-157, Aces "n" Eights, that there was something unusual at what was likely the port .50 Cal turret. I hadn't seen anything like that before but it reminded me of old anti aircraft machine gun sights or pointers.
Then a few photos later I came across this. I think it is likely the 157 boat but there's no proof unless somebody can identify the crew member. the only other distinguishing feature I can pick out is the grab handle extension off the side of the turret.
Has anyone picked up on this gun sight before on any other boats?
Hope you are all doing well.
Dave
David Waples |
Total Posts: 1670 | Joined:
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Jerry Gilmartin |
TOP BOSS
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 Posted on: Aug 2, 2022 - 6:59pm
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Wow! Eagles have nothing on you, Dave (as far as noticing small details!) I have never seen that gunsight before and it was right under our very nose! It looks like they used a rubber eyecup from a 20mm Gunsight Mk4. Here is a passage from the Manual:
There are five types of sights in use for the 20 mm Oerlikon AA machine gun. :Sights Mark 2, Mark 4, Mark 4 Mod. 1, and Mark 5 and Gun Sight Mark 14 Mod. 2.
Mark 4 and Mark 4 Mod. 1 sights can be easily distinguished by their name plates. Mark 4 sight has the rubber eyecup and the 300 mph rings.
The Mark 2 sight is identical in appearance to Mark 4, except that it has an adjustable back sight (Rubber Eyecup)
Mark 5 sight, uses a single adjustment rear pin which compensates for both range and elevation and has a simple elevation indicator.
USE OF SIGHTS MARK 2, MARK 4, AND MARK 4 MOD. 1
The Mark 2 sight has an adjustable backsight having two positions: One for 750 yards range (marked "Normal"), and one for parallel bore sighting (marked "Zero Line Up").
Mark 4 and Mark 4 Mod. 1 sights are calculated for a horizontal range of 750 yards. The backsights are fixed. The foresight horizontal bar is used for 750 yards range and there is a white mark on the vertical center bar-to indicate parallelism.
The line of sight from the backsight (set to Normal in case of Mark 2) to the center of the foresight is 24.5 minutes below a line parallel to the axis of the bore of the gun. This gives 24.5 minutes elevation of the bore above the line of sight, which corresponds to the elevation required for a horizontal range of 750 yards.
The 100, 200, and 300-knot rings indicate the approximate amounts of lead required, at average ranges, against targets whose speed components across the sight are 100, 200, and 300 knots, respectively.
Jerry Gilmartin
PT658 Crewman
Portland OR |
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David Waples
TOP BOSS
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Posted on: Aug 2, 2022 - 8:09pm
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Interesting. You think they modified a 20mm gun sight for the .50 cal so that they could elevate for a specific distance? Seems like a smart way to go about doing it given available resources. Thanks for sharing that Jerry!
David Waples |
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Jeff D
Moderator
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Posted on: Aug 4, 2022 - 6:55am
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I agree Jerry, David has a heck of a good eye for details!
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