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The elusive quick-change forend


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I know this has come up for discussion before, usually referencing the gun used in Road to Perdition, and that at one point several forum members were looking at trying some kind of similar modification. So far I haven't been able to discern that any of the theories have panned out and that the last discussion was 2005 or thereabout.

 

I ask because I've begun to make some of the aftermarket improvements to my 27A1 (FA lower, tool-less disassembly, Philly Ordnance blank barrel) and because mine gets used for reenactments and historical demonstrations the ability to swap back and forth between a vertical and horizontal forearm without additional tools appeals to me.

 

I have an idea on the drawing boards as to how this could be managed, but before I sink too deep into the engineering process I'd like to hear any other experiences in similar endeavors - what worked, what didn't, major hurdles, and so forth.

 

I'm also toying with a similar project that would allow switching between a ring sight and a compensator...that one likely involves making the ring sight from scratch, so more on that later.

 

Any horror stories or words of advice?

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Here's what I've gotten so far. This is the third attempt and the one that results in the least amount of metal removed. Of all the ideas thus far this is alters the gun's appearance the least, though some external cosmetic change is inevitable. It's also nothing I'd recommend doing to a gun of any marked historical value.

 

Having examined the process at some length I can't say that I'll actually follow through with this modification (easier to carry a flathead screwdriver and make the swap in the field) but if anybody thinks they can make use of the theory and the attached drawing they're welcome to try. If so, I would appreciate pictures.

 

For those who find the text undersized I'll copy it to the message body here.

 

***


Proposed mechanism for allowing the tool-less removal of the forearm of a Thompson submachine gun or semi-automatic clone.

A keyhole slot is machined into the rail on which the forearm is mounted. The forward hole is of uniform diameter, whereas the rear hole is stepped.


Replacing the attachment screw is a length of round steel stock with a threaded shank on the lower end. The upper end has a relief cut below the head so that it may seat on the step of the rear hole. To the threaded shank is affixed a convex round head with a single slot cut in the visible side.

The screw channel is opened at the lower end sufficient to admit a length of spring. The spring pushes against a washer and provides downward tension to keep the upper end of the pin snugged into the stepped recess. Installation of the convex head completes the assembly.

To operate, pressure is applied against the convex head, dislocating the upper pin so that the relief cut may engage the narrow cut of the key slot. The grip is slid to the forward limit, where the second hole allows the pin to clear and the entire grip to drop free.

Notes:
Drawing not to scale
No dimensions provided
Shown assembled with pin out of engagement

post-69380-0-22687600-1453016344_thumb.jpg

Edited by J.F. Bell
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Not knocking your idea, but It doesn't seem like there would be enough downward pressure on the grip bolt to keep the grip from being wobbly. What about reengineering the removable butt stock lock system into the fore grip somehow?

Edited by tray burge
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Guest 4WHLDRFTN

Not knocking your idea, but It doesn't seem like there would be enough downward tension on the grip bolt to keep it from being wobbly.

it won't work .......... it'll need a rail system to slide the grip on and the pin would be a keeper

Edited by 4WHLDRFTN
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Like I said, chances are I won't do anything with it; as it stands finances have put a hold on buying much in the way of spare parts and I'm unwilling to use my current gun for testing. I might take a crack at building a mockup in the shop, depending on how much free time I get this semester.

 

It may happen. It may not. A prototype would help (and maybe sink the idea altogether).

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  • 2 weeks later...

I finally watched this movie the other day and captured a few more screenshots to go with the ones above. In the first image, you can see the rail running underneath the grip mount, similar to the rear of the lower for the stock. In the second and third images, you can clearly see how much taller the vertical foregrip is to account for the rail system.

 

As for how it locks into place, however, I cannot say. Maybe a button on the rear of the foregrip? Could always hunt down the prop master from the movie and ask :)

 

I did find it kind of odd that he didn't build in a place for ammo in that case, even just a stick mag...he always had the drum sitting outside very conspicuously...

 

post-260337-0-47869700-1454282327_thumb.jpg

 

post-260337-0-34853200-1454282349_thumb.jpg

 

post-260337-0-44757000-1454282360_thumb.jpg

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  • 3 weeks later...

As for how it locks into place, however, I cannot say. Maybe a button on the rear of the foregrip? Could always hunt down the prop master from the movie and ask :)

 

For those interested, the prop master for Road to Perdition was Doug Harlocker (http://www.imdb.com/name/nm0363331/); you have to reach him through his local: https://local44.org/

 

Rob

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  • 1 year later...

I vaguely remember a movie older than Road to Perdition where some mobster or soldier assembled a Thompson and slid on a foregrip also. I always thought that's the way they are until I started studying up on the Kahr semi-autos (and finally bought a TA5 pistol last week!) I'll look through my movie collection to see if I can figure out what that other movie might have been. dokkerdam

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  • 5 months later...

I came across a Quick Disconnect Barrel system years ago, that I use to switch from Live fire to Blank Fire Barrel.

It involved removing the Grip Mount, drill and tap with the same threads as the Grip Mount.

The Grip Mount must be smoothed to to slide in and out the Grip Mount Slot. The Barrel when screwed in Place holds the Grip Mount in Place.

 

Ok to "Road to Perdition" type or something similar. Remove the Square Nub on top of the Grip Mount, that sits behind the Barrel Shoulder against the Receiver Nose.

This will allow the Grip Mount to move in and out without disturbing the Barrel.

Here is the part that has Stumped Me, but somebody with Machine Shop skills can figure this out.

Some sort of Ball and Detent needs to be made in the Grinp Mount slot much like locking a Socket on a Socket Wrench.

If this could be done you have a Similar Vertical Grip Mount that is Removable and locks in place when needed with no tools needed.

I need a Screw driver to take the Grip Mount off of mine.

Edited by jojeep
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  • 4 years later...

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