I installed a JM GTR kit in my 48 a while back and have a question.Had over 100fps.drop in velosity and wondered if adding some spacers if I could increase the speed without hurting anything.The rifle is a lot smoother than with the stock spring but I feel that I have lost 20 to 30 yrds. of range for hunting.
Since you know about tuning I have to ask how tight your piston seal is and how much lube you might've used in the rifle? Also, how many shots have you fired through her since the rebuild/tune job? As you know sometimes these things take a little time to settle in.
Just throwing out some questions you probably already thought of.
If the seal is rubbing or the spring tar is pretty thick its easy to lose some velocity off that bat that comes back around after a nember of shots. I always looked at it as a second breaking in of the rifle.
Thanks for the response,I have less than 500 rounds since the kit was installed ,the seal was a snug fit but not real tight and the tar looked as if it had been spray painted on.I am going to keep shooting it for now but my try some spacing if the speed doesn't increase some.Was just throwing the spacing idea out to see if anyone had tried it.
Thanks,Gary
What does it chrony ? And What cal is it again ?(I know you've said before but I'm old and can't find it )
I just put a thin film over the spring with a q-tip (wound up tightly with finger before the first "dip" . Helps to hold down on cotton fiber sound deadener)
Mine is a .22 and is hitting about 785 - 790 with the GRT kit and 14.3g pellets . Don't know if that is good or bad but don't really care . I love the shot cycle (thud) and it is a true tack driver when the shooter is up to task .
that it seems loaded like a bullet for a 44 magnun
lets start; did you install the kit yourself?
what kind of kit from JM, 33 or 39 coils?
what was the gun shooting before and after, do not know bwcause you do not have a chrony, then how do you know, now without the chrony that the gun is underpowered? simple test for people with out a chrony, telephone directory. shoot OEM and count pages, shoot with a new spring and count penetration in pages. I told you simple, did I not
tar!!!! WOW, the gun comes dry from the factory, if you put a little bit with the kit it is more than what the factory put in it, don't worry about this.
next, seal, another big WOW. this is super important. it has to be pristine, nice fit and without any nicks in the edges, absolutely round and with no defects, did your seal look like this?
see Gary there are many questions about the difference in velocity without measures (chrony)
I am not being sarcastic but putting you in the spot beacause I have the 48 and treat it like royalty, just trying to open your eyes the hard way
hope no hard feelings
warren
and remember "it's 30% the gun and 70% the shooter"
Thanks for all replies guys,I guess I should have given a more detailed account of said rifle so here goes, 177 cal.shooting Beeman Ram Jet pellets(9.8grs.)before 935 fps.,after 840 fps.,new JM Apex seal installed with the JM GTR kit.new breech seal installed at same time.
I did the work myself,using JM lubes,velocity tar on spring,clear on comp.tube,and moly on pivot points etc.
I have no gripe with JM'S products they are the best! I was asking about spacing up the spring to get a little more out of the rifle. The rifle's firing cycle is very smooth now compared to when it was stock.
I am going to shoot it as is for at least two tins of pellets and check the velocity before I do anything more.
It sounds as though the rifle is a really smooth shooter right now. I can understand the difference in 100fps making you wish for more though. I wouldn't think that it would be that much of a difference from stock, but I can't tell you why.
When you cock the rifle, is there any point at all where the resistance seems different? This is starting to sound like a seal issue. Breech seal maybe? Is there something not tight there possibly? Because that's substantial it seems like, anyway.
Maybe its a breech seal, bud. I don't know but see if its ok and get back to us. I want you to be satisfied lol.
a normal drop in fps is OK with a new JM kit but normal is 25-50 FPS you are stating 100+ and this is not normal
Harv if he checked the seal and it was OK then it is what you say a smooth shooting cycle. about resistance I forgot about this. Gary answer the question from Harv about resistance, did you notice a difference, before and after? I mean cocking the side level. detente is tight when you close the breech?
now Gary this is your gun but when you post a problem with an RWS it becomes OUR problem because we might encounter this problems ourself's and we need to fix it for you, us and others that come along
just hope it settles down and pick's up some more FPS but if it's smooth and accurate, leave it like it is. it should not drop 1 more FPS after the kit installed, it should gain after more shooting,
If the breech seal is loose or missing, it won't affect how smoth the rifle shoots. The piston seal, the tar on the spring might all contribute but when he says the rifle shoots smoothly I have to wonder if the breech seal is the culprit. That would take away a lot of pressure to the pellet. The rifle's action would feel as it should but the final pressure to the pellet would be lacking. I would check the breech seal next.
I replaced the breech seal when the kit was installed and the cocking leaver has about two inches of tension before it snaps against the stock. I don't know of any other way to explain that. The cocking effort is about the same as before the kit was installed,maybe a little easier with no rough or hard spots. The gun may get better as it breaks in again I hope so. Gives me a reason to shoot more. I'll have to use that one for the little woman. She will like that. lol.
The side lever tension before lock up can be factory spec but there could still be a breech seal problem. If the recess that houses the seal was scored during removal, it could bleed high pressure air away from the breech. Also, when I replaced my factory installed breech seal, I observed a "burr" of material that had been scraped from the bottom edge and protruded along the side wall. Carefully starting the seal into the recess will alleviate this problem. I talked with a Diana authorized technician who said that once the seal is started square into the recess, simply close the action to seat it all the way.
Hope this makes sense...and helps...
100 fps is a lot. JM kits will generally be lower than the original gun unless there is some problem with it, then will gain a few fps as the kit breaks in. But thats a lot to make up. I'd tear it down, see where the tar is now, and re-do the tune kit. I had an HW77 that was overtarred, and it shot slow. Then got even slower. Tar ended up on the cylinder wall, and everywhere else too, what a mess.
If you can eliminate the tar, piston seal, and breech seal as the culprit, then you might as well start adding spacers. Go thin and test, add and test, etc., until it stops increasing in velocity. Then take one out, and if that is still smooth enough for you then its done.
JM kits are designed mainly for calm shooting, thats why most people like them so much. But feel free to diverge to see if it makes the gun more to your liking. You can always put it back to the kit.