Alexey S. Lavrov: Benelli Supernova. Tuning. Shooting. Science.

In this section I plan to post some information connected to using, tuning and shooting from Benelli Supernova.
TBD - To Be Done

Installing collimator holographic sight MSE AQC-1. (TBD)

Measuring recoil. A tool for comparison of muzzle brakes. (TBD)

Installing cheap 3x magnifier. (TBD)

Making 3-shell holder. (TBD)

Aluminum low drag follower

Benelli's original follower is good, but it is better to use custom one.
I designed simple follower. I made it of aircraft-grade aluminum. It performs very well.

Inspecting firing pin. (TBD)

How to increase accuracy of slugs. Briley Helix CYL. (In progress)

If you want a choke designed specially for shooting slugs you will find recommendations of two kinds only:
1) rifled choke or barrel
2) Cylinder choke (0.0 constriction)
This selection is in fact incomplete. Because there is a third option:
3) using a ported choke.
All options, of course has it's limitations. Rifled barrel very rare in Russia because it needs to be licensed as rifle weapon. As long as fully rifled barrel rifled choke works best with specially designed slugs. Bird shot and buck shot performance is also has its issues. With every shot rifled choke can screw itself tighter and tighter into the barrel so sometimes you'll gonna need lots of force to unscrew it.
Cylinder choke allows you to shoot basically anything with known performance.
Ported choke can increase accuracy of slugs. But such a choke must have really big ports and it works better with certain type of slug only.

Theory

After leaving barrel projectile is exposed to the forces, created by the high velocity flow gases. Some types of slugs like ball slug are not deflected because of their symmetrical shape. But for nose heavy slugs like Brenneke, Foster, Diabolo, Lyman this effect lowers slug's stability. During further flight slug eventually stabilizes, but it flies deflected.

This video shows how simple cylinder shaped projectile leaves barrel. Notice how long pressure behind projectile stays at great values.
What can we do to lower this "bad" pressure? We are using ported barrels or choke tubes.
Most ported choke tubes has comparatively small ports. Typically, area of all ports combined is lot greater than 1/3 of cross section of shotgun bore. Such ports is not very effective.

This video shows same scene as previous, but four ports are added. Area of the ports combined is equal to shotgun bore cross section. Pressure drop can be seen easily.

In this video barrel has 12 ports. And the area of the ports combined is 3 shotgun bore cross sections. Pressure drop is dramatic.

There is one type of commercially available choke tube with ports area close enough to shotgun bore cross section. Biley Helix choke tube has ports area almost equal to 2.5 shotgun bore cross section.

These pictures shows Briley Helix choke tube, CYL (no constriction)

On the other hand this type of choke tube has thin walls and such a great ports make it even weaker. So only CYL and IC constrictions can be used to shoot slugs.