Any of you get barrel fouling when trying certain ammo?

[ctshooter]

I am asking this as I've noticed a trend in a couple of airguns.

• Bantam Sniper XR - hole in hole groupings at range, tested out some H&N and NSA slugs in it that did not shoot well, went back to JSB pellets and groupings are spread with some flyers. Did a few pull-through cleans on the barrel and back to hole on hole. 
• Ataman AP16 - very tight groups with JSB pellets, tried out some Crossman Premier that shot okayish, back to the JSB and spreading groups with flyers. Some pull-throughs to clean and groups are right back on point.

The things I have realized through this testing is that the pellets or slugs that fouled the barrel were made of a much harder alloy of lead than the JSB pellets. I'd be curious to know what causes this to happen more with certain ammo.

 

[sonny]

I could only guess diameter 

 

[nervoustrig]

I've heard that hard alloys are worse about it than soft ones, often citing hard Crosman pellets. However I don't know how much of it is attributable to the alloy vs other factors. For example, Crosman pellets have trended large according to my measurements over the course of several years so that may play a role. Conversely, JSBs have trended small. I'm speaking of my observations with .177 and .22 in particular.

For sure slugs will increase the rate of buildup compared to pellets because of the vastly larger bearing surface.

Throw in a choke as another aggravating factor. Rough bore (surface fretting from drawing through the rifling button)? Yep, another factor that makes it worse.

As a practical matter, the 3 things that can be to minimize it:
1. polish the bore
2. use a lubricant like FP-10, silicone oil, etc.
3. size the pellets/slugs for a proper fit (for pellets, the head only)