What’s the difference between JSB MK1 and MKII?

[LSS152]

What’s the difference between the .25 JSB king Heavy 33.9 MKI and the MKII? Which would shoot better out of a 700mm STX superior liner?

 

[madeintheuk]

The mk2’s do well in superior liners. All I can see different is the skirt is shallower on the mk2.

 

[M0ist0ne]

Depends...old style mk1 OG's have a thinner skirt and deepest of cavities.

Mk2 has a very shallow cavity and a thick skirt edge.

Newer batches of mk1's seem to be a cross between the og mk1 and mk2's.

Superior liner I'd go mk2, personally

 

[nervoustrig]

What’s the difference between the .25 JSB king Heavy 33.9 MKI and the MKII?

The official differentiator between the MK1 and MK2 is the skirt diameter, the originals being larger. That caused incompatibility issues with some magazines so they introduced the MK2 to address it but kept the MK1 around because some people got better results with it.

6.68mm on MK1
6.60mm on MK2

With that said, JSB has also shipped MK2 labeled as MK1. At least that was the case for a time...see this lengthy thread for a more detailed history:
www.airgunnation.com/threads/is-jsb-heavy-mki-now-jsbheavy-mkii.836999

Other geometric differences like depth of the skirt cavity, head size, etc. are features that have always waxed and waned with JSB between batches purchased at different times. Even visual differences we see between JSB and the equivalent weight Air Arms-branded pellets flip flop over time. For example the skirts may be shallower on the JSB this month but shallower on the Air Arms next month. I have no idea why they do it unless it’s somehow related to the vetting process for a new mold but suffice it to say these types of geometric differences aren't particularly useful to uniquely identify JSB pellets over the course of time. And from batch to batch--meaning pellets from an aged mold or those from a new mold--there will be a difference in how they group in a given barrel.