I just bought a 54 in .22 and had a JM kit installed before I shot it. I have not had the chance to chrono it or measure proper groups yet (first thing tomorrow), but it is CLEARLY weaker than my old M48.

I recently had new factory seals and spring installed on the the 48 and it went from 620fps to 680 with 14.2grn Gamo Hunters. I figured a 54 with a JM kit would do much better, but a simple at-home penetration test revealed that the 54 is not punching its weight. I also think it is inconsistent (I know I should chrono it but it's late at night and I'm frustrated).

The gun is not dieseling and the firing cycle feels smooth.

A few questions:

Might velocity improve after more pellets?

Might something have gone wrong during the installation of the kit? Is there any way I can check for leaks etc.?

How exactly is the JM kit meant to improve performance? The spring's measurements were the same as the factory spring.

My JM spring in .177 54 was about 1" shorter than worn out stock spring. You can set a candle near the breach when you fire it to see if the breach seal is leaking. If your rifle doesn't straighten out after while, tear it down and tell us what you find.....who worked on your gun?

I would guess that what you have in your gun is the GSI kit? If so, the problem may not be with the spring installation. Did you also have the piston seal and breech seal replaced? Keep in mind also that tune kits don't neccessarily increase power. Most of JMs kits are intended to make the gun shoot smoother and more consistantly, not as a drop in power boost. Your gun should be shooting at least 820 or so in .22, about the same for the 48. Who did the work? If you are only seeing 680 after having the gun rebuilt with factory parts, either the parts installed were not the right ones or the repair was done incorrectly. Either way they aren't working right. I'd find someone else to look at both guns before you do any damage to them.

and altitude?

Unless you're way up in the Khyber pass, it seems to me too low MV's on both guns.

Keep us posted.



Un Abrazo!




Héctor

shoots 16 grain JSB Exacts(5.52) @ 806 fps for 23 FPE of Recoiless Airgunning Nirvana! I Love my 54(.22)!!!

My understanding is that a factory .22 will average around 830 fps with a 14.3 grain pellet like a Premier.

If you had a current GSI Kit installed you should not expect any velocity gains over stock. Expect 790 - 800 fps or so with the GSI Kit and Apex Seal and lighter cocking with smoother shooting characteristics.

If you had the Apex Seal installed as well, velocity should increase slightly as it wears in.

In the past Maccari used to make GSI Heavy and Monolith Kits for the RWS guns which boosted power over factory levels by 10 or more fps but was harder to cock.

Here's some pics of my 54(.22) and a group that was shot fairly quickly...















Regards,

Tony

1.Is your 54 an Export/full power version?
2.Jim's kits are all about no vibration,light pulse light recoil and far smoother operation than standard.If you achieve original performance,that's a bonus!
3.If your 54 is a "full steam " model like the ones I do over here,you should be seeing 19FP with FTS,maybe a little more.In my experience,the later guns don't shoot as hard as the early ones.I think strokes are different.
4.Generally,when a JM kit is installed by someone other than me and it doesn't "perform",it is never the fault of the kit.It's always a less than satisfactory installation.
Typical problem areas are the wrong lubrication or no lubrication,too much tar on the spring,tar on the piston,a crook piston seal and or breech seal.
Let us know.