I've been shooting JSB jumbo's in my BSA Buccaneer for three years and found them to be extremely accurate in all weights. A couple of days ago I opened a new tin from a new batch of 18.13 Jumbo heavies and my 10 shot groups went from 1/4 inch to 1 1/2 inch. To make sure something else was not the problem, I shot another 10 with some rebranded Cometa 18.13 Jumbo heavies and those went under 1/4 at 30 yards. I rang the supplier and he said that he had sold 1/2 a pallet of them and not had any complaints. I measured a few with a micrometer and the heads measured 5.48 mm. I measured the Cometa branded ones at 5.50. They are supposed to be 5.52. The supplier is an FX dealer and said most of his customers were buying them for use in those guns and they must be working OK with them. Anyhow he suggested to try them in something else and if they were no better I could return them. I shot about a hundred through my Walther MaximaThor and I was getting roughly up to about 1/2 inch at 30 yards. Nothing special but acceptable. Has anybody else seen similar results with these, over there in the states? On the label at the bottom where it normally says CZ, it has a code. M 46/08 and the date on the sticker on the back is 28110018. 
The tins were not beat up and the pellets looked to be in perfect condition.
+1 Spry try several batchs and see but sure 5.48 want shoot well on most guns