This has been the part that most freaked me out to think about. It was the part with the least info on (at least in my searches). I could find NOTHING in the way of pictures how people did it. Only one sentence lines about "I made a spacer".
Connecting the SPAS grip to the 870 sliding arm
through the SPAS cage is the part I have dreaded a long time, and it did not disappoint.
While a simple spacer is actually all it is, the level of precision needed here was enough to drive me mad.
First up is the base 870 arm. Once you remove the grip from the slide, you are left with this arm and gigantic tube. You actually have a lot of wiggle room for where you want to cut, but I thought leaving about an inch of tube past the main ring would give me some extra anchoring points and I was glad I did it.
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Next up is arguably the most important part, because this is the part you
see. A little dremel work, a little patient and slow utility knife work, and a tiny bit of drilling and you can work out your rectangle recessed area on your grip. This is also where you will be deciding your bolts go. Try not to mess it up!
Where you put the holes will also determine where your forward position is on the finished product.
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(The steel rectangles were shaped from leftover 14g steel from the stock project. 14g feels thick and sturdy but wasn't prohibitively huge needing a gigantic recess in the grip, so I liked it)
Now to make the actual spacer. I cut this from a 3/16 chunk of steel (the SPAS slots are 15/64" wide, 1/4" is too big) and cut the notch in the middle so it would straddle the main ring. I think some material in front and behind that ring will only help structurally so it isn't the bolts alone bearing the force during racking.
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Once you like the 'fit' of the spacer on the 'arm ring', you drop the whole thing into place and see how the height works. I grinded it down to just enough where the slide could barely go over it so it so it was nice and snug.
When I had a good height and was happy with the fit, I dropped a fine tip sharpy down the holes on the grip and marked where the holes on the spacer will need to be drilled out. I think it is important to put one of the bolts right through the center of the main ring on the arm for strength reasons. That is a tough steel anchor point there (as I will explain shortly).
To get a good clean hole drilled then tapped, I couldnt have the spacer bouncing around, so I did decide to weld it in place. This was a nightmare. 5 tries. Too high, too low, too crooked, you name it. Actually on the 3rd try I got the spacer exactly where I wanted it. Drilled the holes exactly where I wanted them. Then when I tried to 'tap' and thread the holes... the f'n tap broke off IN THE HOLE the instant it hit the "main ring"!!!
I had to dremel off the block entirely and trash that spacer. I had to start all over again making a new spacer, drilling, and tapping (this time more carefully. Steady pressure when tapping the steel! No quick movements and always remember to back it out occasionally so the shavings can settle into the grooves!)
So please...be kind and ignore the welding job here.
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So if you measured everything correctly, you now have a "Grenade Launcher" grip that bolts directly too the slide arm of the 870. If you welded it in place correctly, it slides freely back and forth in the extra long slit that was cut into your SPAS cage.
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One other bit of potentially useful info: I used a 6-32 threaded flat head hex bolts on this. The heads do look a bit small to me compared to some of the images of other pulses, but anything bigger than that will simply not fit in the 3/16" steel spacer. I assume its a metric thing where a spacer made from 5mm steel (just a hair larger than 3/16, but still fits in the 15/64"/6mm slots of the SPAS) might be able to take a bigger bolt.
* I have 3/4" length bolts pictured which proved too long. A 5/8" length, 6-32 thread bolt proved perfect.
**Ignore home depot and lowes. They don't have ANYTHING useful for this project in the way of bolts. Find a mom-and-pop or an ACE True Value.