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Gamo Swarm Magnum precision: 5" at 50 yards?

Yes even after good cleaning. I did use some JB. bore paste. However just little only cause I wasn't sure if these barrels are choked at the end so I kinda doing it by feel. Even the lands in a certain spot had a burr in it cause I felt it when I inserted a jag and sure enough it jammed about halfway in. So I'm pretty convinced that these a simply mass produced and like any cutting tool towards the end of its life begins to make dirt cuts which would look like I have and to include an almost wavy cut in the LAND portion of it. I mean I looked through my RWS barrel and everything was perfectly SQUARE . I DONT know their process but it seems kinda sloppy compared to some others. 
 
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So I managed to polish it up just a tad, and the photo dont do any justice. But the burrs are gone and the barrel finally showed it's nice flat lands and the channels are nice and clean. I finished it up with some FLTIZ polishing cream it ialmost gave a mirror finish but not quite, and what a difference. So soon I'll get to take out and see how it shoots. My Gamo Magnum Swarm will be at doorstep tomorrow.i can't wait to see what the condition of the barrel will be like. Hopefully it don't need nothing.
 
So i got the Swarm Magnum Today. And actually the barrel looked really good. So I think I was right about mine. Like some people say when their Swarm Maxim some shoot good and some shoot terrible. I really think it's just the barrel. Some come out good and some just stink. I believe their QUALITY CONTROL IS getting too laxed for the sake of profit in my opinion. But the Magnum I got today was a refurbished one . It was new to say the least. But the rifling in that barrel was actually pretty clean . I'm not gonna polish that one until I take out and shoot off a few rounds. I'm kinda tinkering with a new toy like a .17 WSM ? Damn that thing is fast hahaha. 
 
I have a Gamo Mach 1 Whisper Fusion with the IGT gas piston. These are my recommendations based on what I have learned from shooting mine for two years and sending over 20,000 pellets down the barrel. Get rid of the scope and get an air gun rated scope with an adjustable objective. The adjustable objective can be used as a rangefinder when hunting as well as removing parallax. If you shoot closer ranges over the winter indoors an AO is a must. Keep the lock up parts well lubed with a silicone oil or grease and when locking up do not slam the barrel closed but close it fully by hand and make sure it is tight. If the lock up plunger is hanging up in any way there will be play in the barrel breech fit. Crosman Premier 14.3 gr. pellets are not what you shoot out of this rifle at 50 yards expecting good groups even after you are experienced. The weight varies as well as head size and they are not round. While they shoot pretty well out of my Mrod with a choked barrel there will be many fliers out a Gamo break barrel. That being said they are excellent to learn how to shoot a break barrel at a much closer range. I actually measured many thousand cphp with a dial calipers and shot pellets 5.5mm and up through the Gamo and the smaller through the Mrod. I tried H and N FTT 5.53mm head size and they shot very accurately but leaded the barrel quickly. The pellet that shoots best in mine is the JSB 14.3 gr. At 13 yards 5 shots make a hole about 3/8" in dia. and 3/4" at 32 yds. I honestly have never tried groups at 50 yards. I also lube my pellets with 1 drop in a tin with 25 pellets and just roll them in the tin. I use bicycle chain lube that I get at Walmart. It is about $5 a bottle and is called WHITE LIGHTNING and is the stuff for dry chains. You will have to find what hold works for you but I use an artillery hold. It is mandatory to place your hand at the same spot on the rifle every time. I NEVER shoot the gun off directly of a rest. The gun is in my hand and my hand goes against the rest. For a rest I use a camera tripod that I fitted with a yoke to accept the back of my hand comfortably. These guns are super hold sensitive so I suggest you start by holding the gun differently for various shots and see how the poi changes. You do not have an easy challenge ahead of you but if and when you get to the point that you can pick up your rifle and put the first shot exactly where you are aiming you will know why so many guys that have Pcp air rifles still pick up their break barrel when ever they can.
 
I feel your pain. I did almost exactly the same as you. Bought what looked like a bargain on a whim and should have bought a Diana 34 but here we are.

I can almost guarantee a big part of the problem is your hold. Search for videos on the "Artillery Hold" That solved 80% of my difficulties.

The Gamo shoots too hard. 8-10 gr pellets in a .177 usually will get it under the speed of sound. Better accuracy.

I'm presently using mine to shoot airgun silhouette and will upgrade my scope. And practice.

Good thing is in .177 and good velocity I'm not doing much sight adjusting from chickens to rams.
 
I had a similar problem with my new RWS 34. It was shooting all over the place at 25 yards even after I put well over 500 shots through it with various pellets. I wound up getting a tune kit and gave it a good lube while it’s apart. 
made a HUGE difference and I got sub MOA groups immediately. Shot like a different rifle and much quieter. I was amazed at the difference and it’s an absolute tac driver now.

As others have said a good scope designed for air rifles is also a must. I got a UTG 3-9*40 Hunter and it works great for me. Experiment with different pellets and holds. It can be frustrating at first especially if you are a seasoned shooter with live ammo.