D
Dave Shattuck
Guest
As many of you may have read, I have been having one heck of a time with my new AZ built Theoben MFR. When I first got my new gun in April I had great expectations as to what type of accuracy the gun would be capable of, so it was easy to attribute most of my consistencys to either my inexperience with air guns, or the fact that I was still using Theoben's factory stock when shooting from the bench, and thus was having a great amount of Rocking-N'-Rolling occuring from shot to shot.
Then, to my delight, a few weeks ago my new Gary Lemons benchrest stock finally arrived and not only looked fantastic, but also eliminated 100% of the instability from the gun. Now, the problem became that I could see what was going on, didn't like what I saw, and now needed to figure out what was causing it.
Last week for the first time I tore everything other than the regulator apart and found nothing that would seem to be causing my problem, but hoped that just by doing what I had done would ...could ...might have eliminated it. Well, it didn't!
At last Saturday's match though something did become blatantly obvious as my problem appeared to be more physical than mechanical. What I mean by this is: the gun would shoot several shots in a row landing within a reasonable area of the target, then, for no apparent reason, the shots would start landing 2 or even 3 points out, but all were either in the exact same spot, or at least along the same line, for example, out at 5:30.
Upon my return home my first guess was that the Leupold 36X I had been using had turned to crap when adjusted down for 25Y and had started allowing a POI shift. Easy fix, so I swapped it off for a 6X Weaver T-6 that adjusted down to 20Y and went from there .... but with the exact same results. So, I now knew it wasn't the scope. Back to tearing everything apart again, but this time with one major difference: I also slugged the bore of my barrel!
I had shied away from slugging before because of hearing about how sensitive these barrels are to running a brush or rod through them as the metals used are so much softer than the ones used for making rimfire and centerfire barrels. Had I slugged my bore initially I would have found what I was looking for.
After running just 3 pellets (one CPH and 2 Kodiaks) through the full length of my bore it was obvious even without magnification that there was a section of shiny, freshly exposed lead being cut away between two of the grooves, and that the shaved area had simply been laid over rather than taken off. So, where was this happening?
I next just pushed a pellet far enough into the chamber to go past the port, then carefully removed it by running my rod in from the muzzle end, and sure enough, there is was. So, to make sure that it was the porting and not something in the chamber, I now pushed a pellet in only far enough for the head to pass over the port, but for the skirt to only enter the chamber ...and the head was shaved, but the skirt was not.
It seems that when my barrel was ported through to the bore no one had bothered to lap the burr that was created out of the bore, and thus this flap of metal is still sticking up into the bore and is not only shaving some of the lead from the underside of my pellets heads, but also from their skirt as well, and then leaving that piece of shaved off lead hanging from the backside of the pellet Not very good for maintaining any kind of shot-to-shot consistency as what this is doing is two-fold: it not only throws off the pellet balance, but it also will allow blow-by of the air pressure to go out in front of the pellet when the trigger is pulled, and, on top of that, will create an area of turbulance right in front of the bore for the pellet to exit into, especially if you have a shrouded barrel like mine.
So, if you are experiencing an inconsistency that you are unable to resolve ....remove your barrel and check the porting for burrs.
By the way, the barrel on my Theoben is a Walther .177 FAC 16mm Bull Barrel.
Then, to my delight, a few weeks ago my new Gary Lemons benchrest stock finally arrived and not only looked fantastic, but also eliminated 100% of the instability from the gun. Now, the problem became that I could see what was going on, didn't like what I saw, and now needed to figure out what was causing it.
Last week for the first time I tore everything other than the regulator apart and found nothing that would seem to be causing my problem, but hoped that just by doing what I had done would ...could ...might have eliminated it. Well, it didn't!
At last Saturday's match though something did become blatantly obvious as my problem appeared to be more physical than mechanical. What I mean by this is: the gun would shoot several shots in a row landing within a reasonable area of the target, then, for no apparent reason, the shots would start landing 2 or even 3 points out, but all were either in the exact same spot, or at least along the same line, for example, out at 5:30.
Upon my return home my first guess was that the Leupold 36X I had been using had turned to crap when adjusted down for 25Y and had started allowing a POI shift. Easy fix, so I swapped it off for a 6X Weaver T-6 that adjusted down to 20Y and went from there .... but with the exact same results. So, I now knew it wasn't the scope. Back to tearing everything apart again, but this time with one major difference: I also slugged the bore of my barrel!
I had shied away from slugging before because of hearing about how sensitive these barrels are to running a brush or rod through them as the metals used are so much softer than the ones used for making rimfire and centerfire barrels. Had I slugged my bore initially I would have found what I was looking for.
After running just 3 pellets (one CPH and 2 Kodiaks) through the full length of my bore it was obvious even without magnification that there was a section of shiny, freshly exposed lead being cut away between two of the grooves, and that the shaved area had simply been laid over rather than taken off. So, where was this happening?
I next just pushed a pellet far enough into the chamber to go past the port, then carefully removed it by running my rod in from the muzzle end, and sure enough, there is was. So, to make sure that it was the porting and not something in the chamber, I now pushed a pellet in only far enough for the head to pass over the port, but for the skirt to only enter the chamber ...and the head was shaved, but the skirt was not.
It seems that when my barrel was ported through to the bore no one had bothered to lap the burr that was created out of the bore, and thus this flap of metal is still sticking up into the bore and is not only shaving some of the lead from the underside of my pellets heads, but also from their skirt as well, and then leaving that piece of shaved off lead hanging from the backside of the pellet Not very good for maintaining any kind of shot-to-shot consistency as what this is doing is two-fold: it not only throws off the pellet balance, but it also will allow blow-by of the air pressure to go out in front of the pellet when the trigger is pulled, and, on top of that, will create an area of turbulance right in front of the bore for the pellet to exit into, especially if you have a shrouded barrel like mine.
So, if you are experiencing an inconsistency that you are unable to resolve ....remove your barrel and check the porting for burrs.
By the way, the barrel on my Theoben is a Walther .177 FAC 16mm Bull Barrel.
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