here are my original postings:
The first attempts with rifling, the button was too tight, but after diamond filing the button down a small amount, I could get it through the barrel without damaging the gun. I am very happy with the results. What used to be a 5 inch spread at 25 yards
with pellets ( remember this is a smooth bore shotgun ) is now down to 1 inch at 25 yards, freehand. Now I can shoot relatively small groups ( for what was a shotgun, if you call it that) with pellets (no tumbling) and still use the very limited shotgun function….snakes!
To my surprise, the barrel on the Gamo shotgun barrel was at least a 1/2 inch diameter of steel. I initially thought is was lined with maybe 1/8 of steel since it was never meant to be rifled.
I put several cleaning pellets into the breech before I started to push the rifling button through so that no shavings would get into the air chamber.
https://www.ebay.com/itm/Rifling-Buttons-Spiral-Reamer-Tungsten-Alloy-Novice-Use-Prevent-Break-Style-A/282827051714?ssPageName=STRK%3AMEBIDX%3AIT&var=582185897573&_trksid=p2060353.m2749.l2649 Here's the website on Ebay, it's seller is named "riflingbutton"
I started from the muzzle end because I thought hammering with the the rifle half-cocked would put too much strain on the linkages.
I ordered a 5.5 mm button and had to diamond file it down to
5.47-5.48 to get through the barrel, the custom buttons are not returnable!
The one I bought was labeled
5.5 mm by 12, meaning one rifling rotation in 12 inches,
AND I had to file it down,
see above, to get it through the barrel, with silicone lube. But
definitely worth the trouble!
you might want to try this guy. "Baandgood.com" much cheaper! https://www.banggood.com/5_5mm-9_0mm-Rifling-Button-12-Flutes-Hard-Alloy-Chamber-Helical-Machine-Reamer-Tool-p-1315906.html?gmcCountry=US¤cy=USD&createTmp=1&utm_source=googleshopping&utm_medium=cpc_union&utm_content=2zou&utm_campaign=ssc-us-en-all&gclid=EAIaIQobChMIleirv4Xf3gIVA49bCh3_EQPFEAQYASABEgIE3fD_BwE&ID=534777&cur_warehouse=CN BUT, If you want, I can just send my stuff ( button and rods) already sized to save you the trouble and you just send it back when you're finished. You need to cover the air chamber outlet port and lube the button. The rods that you used to hammer the button easily slide out of the barrel once they are flush from hammering, then you just insert a longer rod or combination of rods, there are about 8 in different lengths.
Do you have a Dremel tool, because you will need to open up the first 1/2 inch of muzzle endto about 25 caliber to get the button started. Once the button is flush with the muzzle, you will have to use the 2 inch rod to get in further, then use the the 3 inch rod and so on then the 4 inch rod and then a combination of the rods. I will include the Dremel bits in the package I send. By the way you will probably need someone to hold the rifle as you hammer the button through. I used a claw hammer striking the the side of the hammer because hitting the rods accurately was difficult for me. Sorry to be so long-winded.
dark
and yes you can still shoot shotshells, just make sue you remove empty shotshell or pellet adapter before rifling 
