TargetTalk

I wanted to share a project I cooked up over the weekend for my Benelli MP90S. While the gun is quite stable under recoil, rapid fire can benefit from a bit of recoil damping. Benelli makes a "rocking weight" which replaces the weight carrier, but it's unobtainable. Pardinis have six sprung weights up front, which inspired me to try this. It's a prototype for now but I'm planning to execute it in aluminum bar stock once I get my head wrapped around the details.

It's quite simple - starting with some hobbyist brass tubing, some small springs and a set of tungsten weights, the components look like this:
The tubing is 13/32" o.d. brass (see product info in pic), which has in i.d. of just about 3/8". I threaded the ends with a metric M10x1.5 tap to accept the end plugs which are M10 grubscrews, which are just larger than 3/8" and the grubscrew style means they have flat ends, good for the purpose here.

The weights are tungsten for Pinewood Derby cars, about 3/8" diameter and 1/2" long and weigh 14.4 grams each (1/2 oz). The springs are 1/2" long and .020" wire. The center keeper is just a 3/8" setscrew drilled through for the fixing bolt and to isolate the two sides, it could use some improvement perhaps.

Assembled, it has a nice heft and thumping it on the table gives a definite damped feel. Here it is, showing a total weight of 65 grams in its present configuration:
Finally, here it is on the gun:
I shot it today and the effect is *quite* noticeable. In sustained fire, the gun stays more level and the recovery is truly minimal, not that it was all that high before. There is a tiny bit of pogo-stick sensation, but not much and I think I can tweak it away. Overall it feels really good, just basically more controllable and even. As a side benefit, the brass doesn't fly around quite as much. Not sure I expected that, but it makes a bit of sense that if the gun moves less, the ejection is more consistent.

There are some things to work out still:

The number and position of the weights. The set I bought comes with three 1/2 oz, two 1/6 oz, and two 1/12 oz to make 2 oz total. I could put them all in, plus one spring in each pocket, but that seems extreme. For now I'm going with three weights and three springs. I put one weight in the front pocket and two in the rear so the gun isn't too front-heavy.

The position of the springs makes a big difference too. In each pocket, a spring in front of the weight is good for absorbing the recoil of the shot (as gun goes back, weight slides forward into spring). On the other hand, a spring in back of the weight absorbs the forward rebound of the bolt as it goes back into battery. It's less than the force of the shot, so I am attempting a compromise for now as follows:

Front pocket: spring - weight - spring to absorb both recoil and rebound
Rear pocket: spring - weight - weight to absorb primarily recoil

It occurs to me that the weights are moving in a rather tight chamber, and the movement of air in there may be damping things a bit. Which may be a Good Thing. But I may drill some holes in my tubing-style design to experiment. First I'll play with springs and weights. Maybe find a softer spring or two, the ones I have are slightly stiffer than I might prefer.

I'll be shooting it again this week and will report more. Possibly this weekend I'll have some aluminum bar stock and an hour on my friend's Bridgeport and I can show a nicer version! Comments welcome in the meantime.

Nice work! Keep us posted.

Played with some changes at the range. Right now, I have to say my original setup is distinctly best!

For the record, that is

Front pocket: spring - weight - spring to absorb both recoil and rebound
Rear pocket: spring - weight - weight to absorb primarily recoil

I tried several changes and gauged the "feel" of each including an estimate of muzzle movement:

More front weight: heavier feel and greater muzzle movement (likely due to springs being more compressed)
Less front weight: "pogo stick" feel and greater muzzle movement (springs less compressed?)
Less rear weight: Lighter feel and slightly greater movement

Decided I was seeing a trend and put it back to the original, but slightly tightened the grub screw at the front to increase the spring tension. This successfully removed the slight pogo-sticking I felt yesterday. I guess the springs may be about the right stiffness after all!

I'm going to shoot it again in a match tomorrow evening and not mess with it too much. I'm hoping for a point or two in rapid, like, maybe a nine or two becomes a ten. That would be nice!

I'll definitely re-do this in aluminum soon. Or maybe a third attempt in hard plastic if that seems too heavy. I've been shooting it with either 25g or 50g and I definitely feel the increase to 65g in the prototype, we'll see if I can keep it to 80g.

> I tried several changes and gauged the "feel" of each including an estimate of muzzle movement:

If you have an Android phone and a way to secure it to the pistol, you can install the app "Science Journal" (free) and record the phone 3-axis accelerometer while you shoot.

More rounds this evening - pretty remarkable results. I left the springs and weights in the same configuration.

First thing that happened was funny. Note, the prototype is just a tube and it is slimmer than the pistol but pivots in the center on the single fixing bolt. Guess what happens when it does this... the gun jumps left or right. Ok fixed that with some folded paper stuffed in the gaps.

At that point the feel is again straight-on. During rapid fire, if I kept a good hold, the red dot actually doesn't leave the black after the shot! Ok, liking that. Otherwise like any change, it is taking some getting used to. But thumbs-up so far.

I've discovered I have some Delrin pieces as well as aluminum, I'm going to make it in both materials this weekend.

Good suggestion on the accelerometer, will look into doing that.

"Benelli makes a "rocking weight" which replaces the weight carrier, but it's unobtainable. "

I have one - which I ordered through Larry's Guns. I guess that you are right it being unobtainable in the USA but I'm sure that, with a bit of googling, you can find a European supplier who will ship to you

Leon wrote: ↑Wed May 15, 2019 10:16 pmI have one - which I ordered through Larry's Guns.

I'd love to see a photo...

-TT- wrote: ↑Thu May 16, 2019 9:10 am

Leon wrote: ↑Wed May 15, 2019 10:16 pmI have one - which I ordered through Larry's Guns.

I'd love to see a photo...

Sure - but I've got something better to show you. A diagram that the Benelli factory sent me - this will be more illuminating than a picture I think... PIcture is not of mine - but it shows the weight upside down. You can work it out though...

Instructions from Benelli on the rocking barrel weight. Refer to diagram above -

We attached this following picture to simplify the explanation:
the screw number 2, adjusts the range of movement of the moving portion of the weight.
The other one, the number 1, is used to hold fixed the moving part to the receiver.
The screw number 3, is used to fix the mobile part to the entire weight, but for adjusting it, you should disassemble the weapon because is fixed from inside.

So, the spring just to the front of screw #3 holds the weight in the downward position, and it rocks upward during the shot? Is the green thing above screw #2 a rubber bumper, so the weight rests on screw #2 until it rocks, at which point it hits the bumper?

Interesting, if so. It almost seems backwards, I'd expect the design to drop the weight in response to muzzle rise. But maybe I don't understand it.

Does it work well? At 100g+, it seems heavy, that might help al by itself.

Ill get mine out and take some pictures of the weight internals for you - it wont be for a couple of days though

Ok, I executed it in black Delrin and pretty pleased so far! I'll be shooting it soon.

Sorry about the poor depth of field on these, my Nikkor Micro lens is really sensitive.
Some things to note...

- Exact same design as the brass tube version, just in Delrin.

- This was made from a 1/2" x 3/4" block, so it's a little bit wider and a little bit thicker than the Benelli weight carrier. This is good here because it gives more material to work with to cut the hole for the sliding weights.

- Minimal weight of the material, around 20g. Would be perhaps 35g in aluminum, which I may try anyway.

- I drilled the weight chambers at 23/64" which is barely a clearance fit to the tungsten weights. I used a flapper on the end of a dowel to relieve the diameter without opening up the threads at the end.

- The little holes on the top surface are definitely needed to allow the weights to move readily. I need to carve a channel to let the air pass since it's tight to the underside of the frame. For now I am shimming with a washer.

Not a Benelli but I added the Walther SSP pivoting recoil absorber to my Walther KSP200 - does a good job!

I bought the Walther pivoting weight identical to yours dino911, and to be honest could determine no reduction in recoil. It became heavier as a result of the 140 grams weight.

Glad yours worked.

Do those Walther devices pivot at the front or the back?

140g, wow.

[edit] Found this: https://www.carl-walther.com/products/s ... 01502.html

The operation looks to be similar to Leon's.

It pivots on the rear pin - opens at the front ,which is adjustable via the grub screw below the front pin. Spring is adjustable via rear grub screw.
140g is ok as it is used for rapid fire - times are not too long !

Just a quick update after shooting the new version - it's even better than the prototype. I think the fact that the weight channels are such a better fit to the diameter of the weights, and the air holes allowing better movement, make things tighter and much more effective. I'm super-happy with the result right now.

Leon wrote: ↑Thu May 16, 2019 10:38 pm Ill get mine out and take some pictures of the weight internals for you - it wont be for a couple of days though

Here we are -

Ok, I get it now - the earlier photo shows it deployed, but normally the spring holds it up to the underside of the frame. It blips down under recoil, and the spring pushes it back up. I guess that seems obvious now. :-)

Thanks for the pics! Given the success I'm having with my version right now, I'm going to shoot it for a while and not try mimicking the Benelli design.