Project Thumper - Overview

lmar

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I've wanted a short barrelled shotgun ever since I saw a witness protection shotgun by Scattergun Technologies in the 90's.
Just something about the abbreviated barrel appealed to me.

However, lack of availability, cost, and other more important purchases got in the way over the years.

Fast forward to a couple of weeks ago.

Needed another project SG to work on so I purchased a 12.5" Dominion Arms Grizzly over the weekend. Almost immediately, I had buyers remorse as I all ready owned a 12.5 but couldn't justify the 8.5 version. After giving it some thought I figured out my reluctance to purchase the 8.5" version was due to the "minuscule" 2 +1 round capacity. Basically, going from the 12.5 to 8.5 version resulted in a loss of 2 rounds (2-3/4). If I used it like I did 30+ years ago when camping in what is now the "Great Bear" rain forest I had empty chamber with full magazine. Should Yogi or Boo Boo appear it would just be a matter of pumping the action and chambering a round. With 4 round capacity in the tube I didn't feel under gunned.

With only 2 rounds in the magazine using the 8.5" version I considered the shotgun more a "toy" than a purpose built shotgun. This bias was further reinforced with the use of a forward folding (Serbu) or fixed pump handle on the forearm. Never understood the need to reduce the length of a shotgun only to increase the height - negating any advantage of the compact nature of the firearm. As I understood the sales information the gun was capable of firing 3" shells - meaning I might have some wiggle room to get 3 rounds of 2-3/4 into the magazine tube.

I ended up changing my order the following Monday and by Friday that same week I received the 8.5" Dominion Arms Grizzly. I immediately hauled out the dummy 2-3/4 test rounds and determined I need at least 1/2" of extra length in the magazine to get 3 rounds in (over compressing the spring will be fixed later). Work got busy so I wasn't able to try anything until last night.

The magazine cap retention system uses the same Remington Express divot with plastic spring retainer/mag lock. First thing I did was remove the factory follower and replace it with a shorter one I had in my spares box. Secondly, I deep sixed the plastic retainer which resulted in being 1/8" short of my third round. I then proceeded to back off the magazine endcap one full thread at a time until the shell fit into the magazine - success. The downside was I only had one thread holding the magazine cap onto the SG. Determined the sling swivel backing washer used up a lot of room in the Dominion
Arms supplied endcap.

I had a factory Remington Endcap which used the detente ball system to retain the tension on the endcap. The shape and depth appeared at first glance to give me the room I might need. Surprisingly, swapping over the part resulted in being able to tighten down the barrel and fit 3 rounds to test my spring compression. There was none.

Thought about installing a 1 round extension but I wanted to keep the magazine tube from projecting past the front of the barrel.
Gave Joseph a call @ Dlask and I have a 2 part Happy Face Cap being sent to my house. I figure I can modify the centre "cap" to provide the necessary depth needed to prevent full compression of the spring. I have 3/16 to 1/4 of extra length to build into the cap if I want to have the mag tube flush with the barrel.

I now will have to test the reliabiilty of the system to now hold 3 rounds in the magazine. I feel this is bringing it closer to functional shotgun than a "toy".

Stay tuned for the next step which is a choked barrel (may have to have a Dlask version made).
 
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Sweet shorty, but it looks like your under 26". If that's the case, you have to call the CFO and register it. Let us know what your total length is.

If I attach it to the gun it will be 20" OAL and in the restricted category as you pointed out. Currently it wears a Hogue stock. Deciding if I am going the registration route or not.
With only two rounds I was going to go Restricted. With 3 rounds I am considering keeping it over 26".

L
 
Was able to model up the front end of the shotgun using my CAD system.
The model is a very accurate representation of what is actually happening in the gun.

The front end of the follower is butting up against the magazine tube dimples (not shown in drawings). Spring is compressed using stock endcap. Do not need to remove the dimples as they act as a natural block to prevent over compression of the spring (unless the slots in the follower line up with the dimples and then over-compression is possible. Added spring guide/limiter to prevent this from happening should this occur.)

Ended up designing a custom endcap that does not compress the spring as much as a stock endcap. Was trying to keep the endcap shorter than the barrel but was unable to do so. Will take the slightly longer length to maintain reliability of the spring.

Added a UHMW spring guide to minimize binding/bending of the spring during maximum compression (3 rounds in tube).
Used an uncle mikes sling swivel stud that threads into a tapped hole on the spring guide. Holds the guide centred as well as allows for the stud to swivel.

New Components:



Slightly different view.



To minimize amount of machining I am looking at modifying the centre disk of a Dlask Happy Cap to accomplish the same thing as the full custom endcap.
Might also just cut down both ends of the the Express Spring retainer (becomes a cylinder) and use this for the spring limiter instead of the UHMW post.
Will depend on how much real estate I have with the Dlask cap.

L
 
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Update - I was able to get the clearance I needed using an off-the-shelf Dlask Happy Face Magazine Cap

http://dlaskarms.com/collections/shotgun-parts/products/remington-870-magazine-happy-caps

While you do lose the front swivel mount capability in trade-off to reliability - you gain a #####ing happy face logo at the end of your magazine.

The recess at the back of the happy face cap matches the diameter of the magazine spring and the thinness of the cap gives enough room for the spring.

All this and more for a price of $13.00 from Dlask. Next up is to drill the barrel for the detente ball used to secure the non-Express endcap.






 
Least with the no swivel at the end you don't have to worrie about blowing it off. Seen that happen once or twice with these. Interesting build, also like seeing the over molded front hand stop on these short flush barrels.
 
Update - I was able to get the clearance I needed using an off-the-shelf Dlask Happy Face Magazine Cap

http://dlaskarms.com/collections/shotgun-parts/products/remington-870-magazine-happy-caps

While you do lose the front swivel mount capability in trade-off to reliability - you gain a #####ing happy face logo at the end of your magazine.

The recess at the back of the happy face cap matches the diameter of the magazine spring and the thinness of the cap gives enough room for the spring.

All this and more for a price of $13.00 from Dlask. Next up is to drill the barrel for the detente ball used to secure the non-Express endcap.







Cool job - install a sling plate for a single point sling or bungee cord, use a wave washer to secure the end cap if you want to avoid any drilling...
 
Cool job - install a sling plate for a single point sling or bungee cord, use a wave washer to secure the end cap if you want to avoid any drilling...

Sling plate up front is a no-go. Front forend covers half of endcap in the furthest forward position.

L
 
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