LW .177-.25 is available choked or not choked, in standard or polygonal rifling from LW factory in either Georgia or Germany. $117.00 for a 23.83” blank that no Gunsmith should charge over $200.00 to turn soft AG steel.
I’m not getting into my gripes about dealer ep rifling, chokes & such. I’d just say I wouldn’t bother trying slugs from a 13.78” barrel choked or not; but if I did.
I wouldn’t expect huge power. I’d have inleade set up so you can actually thumb load it (as that design requires) and minimal blow by could be had with properly sized for common LW AG barrel or plan on using .218” NSA, VK, Griffin lightweight dish/cup base slugs. I see at least 0.003-4” light gap in LW barrels seated with .217” slugs. Not ideal at all.
I would personally not want a choke for slugs. I would want my barrels dedicated in slug or pellets only configuration. “Wylde hybrid” barrels are obviously possible as it’s pretty much on every factory AG. JSAR is first I’ve seen to offer a choice in semi mass produced guns.
That factory 350mm barrel in .22? I’d simply use Eun Jins as they’re way harder than JSB 33.95’s but they’d be a great option also. I don’t know if you want more penetration or retained long distance energy. A choked 13.78” barrel is not great for either but I’d try if I hadn’t already.
I’m going to assume you want power for pesting as you mention your gun not having a large air reservoir capacity. Since we don’t know what you’re hopping for it’s hard to help but you could always shoot tethered or get a custom dedicated slug gun that works for you.