Over the last couple of years there have been different designs of tuners that have come onto the market as well shooters that have the tools at home that have knocked up their own versions that work just as well.
At the moment it seems that tuners are only being used on rimfire rifles but there are a few tinkering around with the idea of one for an air rifle, but nothing substantial at the moment has been reviewed.
Over time this question is been like "why do we have gravity". But this is the most common answer and until such time that a positive view that's excepted by all gunsmiths, mathematicians and shooters, what a tuner does is help tune the barrel or harmonics if you like so that the bullet flight is fluid. It's been kind of proved that the barrel vibrates as the round travels down and out, why, good question, if you find the answer then let us know, but it's just one of those laws of physics.
It's also be talked about in depth about were this all happens on a barrel and the strong view which is being called the node is about 4 inches from the crown of the barrel which has also been called the damp spot. Now if it's always there then why not just cut it off, well all you would be doing is moving the node/damp spot back another 4 inches as it's always there on a stripped barrel
So the theory is that you need to move the damp/node forward of the crown, that's where the tuner comes in.
There are two forms of tuning, one is a weight that place over the node which then moves the node forward of the crown of the barrel. The second is one that is fitted at the crown end of the barrel and both of these now reduces the vibration and allows the round to travel extremely accurately to the target.
You may be asking why are not all barrels on any rifles have this kind of system fitted, well not all target shooting is trying to hit a pin head at 50m 25 times, most targets are a lot larger and some that is close to this is making the smallest groups possible and some do you tuners to help with this.
The mid-barrel tuner is a weight that can be of 5oz's and move along the barrel. By shooting 5 shot groups you should be able to see if the groups are getting tighter or not and that's what you looking for the tight group. With some mid-barrel tuners you can add a couple to the barrel if its long enough as shown in one of the pictures, some people have mid-barrel tuners as well as one on the crown, it's all down to works for you.
There are fair few different crown tuners around the world that is made that you can buy off the shelf and the Harrel tuner is probably one of the best known. The actual main body of the tuner can be screwed forwards about ½ inch and by dialling the weight using what is called the Hopewell Method you can find the sweet spot with about three boxes of ammo and then you just leave it set up like that.
The tuner has not changed much in design but what you can do is add more weights that are different sizes that screw onto the tuner, maybe because the different size barrels and makes need more than just the tuner to do the job as well as it can.
The one other thing that can help tune you barrel is a bloop tube, now these are used in a lot in different kinds of target shooting like with iron sights and there are a few that just screw on or is actually part of the tuner. The Harrel can have the JJ Pappas Noodle fitted to it as it has the same thread and recently there is a noodle that is made by Harrel.
Call these gadgets, tools, mods whatever you want but you won't see many rifles these days without some kind of tuner on the barrel or at the end of it. There are some very well machined DIY jobs out there that do just as good and win at matches. With some people and in some countries, the way that your rifle looks can be a stylized statement to look cool and as well shoot well just as some look very normal and are up there on the podium, so it's really whatever floats your boat if it's cool silver or loud PINK! Anything goes these days.