Gunnut, the member who produced the recoilless A2 stock, hasn't been active here in a while. According to him, he had the misfortune of developing a strong allergy to the chemical components in the 2-part casting compound he used to fill the inside of a hollow A2 stock and had to abandon making them.
Having read his initial R&D into making the recoilless A2 stock, it was apparent that anyone could reproduce his result. You need an A1 or A2 stock, some kind of 2-part casting compound and a model of what you wanted to cast into the compound.
First and foremost are simple solutions. Adding weight to any rifle will have a positive impact upon perceived recoil. You can buy a simple insert molded to match the cleaning kit compartment inside an A2 stock. It adds weight, plain and simple. It is not an active component. The price is carrying more weight, which depending upon how much you weigh and your upper body strength may or may not be a factor.
Gunny's stock went beyond that solution in casting one or two chambers inside the stock that were the appropriate dimensions to allow adding one or two spring weight or mercury filled recoil reducers, sold by several sources.
My solution was to cast a hollow metal tube inside my stock fitted with a threaded end cap. The inside diameter of the tube could accommodate any number of either active or passive components. Anything from lead shot to in my case, a DIY spring weight. The length of the tube was limited by its outside diameter. The bigger it was, the shorter it had to be due to the stock being tapered toward the receiver. You could however cast several in parallel, of different lengths. I wound up using fiberglass fiber entrained
Bondo. It sticks to the inside of the stock very well, if you rough it up with some 60 grit sandpaper.
My first foray of several involved a single tube about 1 inch ID. My second experiment involved 3 tubes in parallel of smaller (I'm at work right now) diameter and of different lengths. The single tube with a large "shake weight" in it was very effective. The 3 tube solution employed 3 different smaller weights equaling the one big one and 3 different tension spring sets to go after different recoil impulse frequencies. It is also effective. Without an accelerometer to actually measure the two side by side, it's hard to say which one is best. They both have interesting points. The sky is the limit depending upon your imagination and talents.
Gunny developed an assessment tool that utilized an accelerometer mounted to the upper and feeding readings into a PC, to go beyond perceived recoil and into measurable recoil as a means of testing different experiments to the same standard. His thread in the Market section
Here. It is a long but good read. If your lack of patience eclipses your need to understand, it will be an even longer read.
Active recoil reduction techniques do work. Do they totally eliminate recoil? No, but properly tuned, they can have a significant impact (no pun intended) upon it. They go beyond floating the upper on a pom-pom spring like some reduction techniques, which also work.
Before you ask, don't...
Hoot