Sumatra carbine .25 accuracy problems

[Birddog363]

Haven’t been shooting my Sumatra much lately but when I did it was all over the place at 40 yards. It used to be less than 1” with jsb heavies. Changed scopes but no improvement. Cleaned barrel with ballistol and brash brush careful to not damage the crown. Ran cleaning patches through until they were clean. Then ran patch through with ballistol on it. Groups have improved but not back to normal. Is it ballistol in the barrel causing this?  Thanks

Ben

[Nomadic Pirate]

Did you check if the dovetail rail is secured ? Once one of my Sumatras out the blue started to be inaccurate, it took me awhile to figure it out but it was the rail screws that come lose.

[Birddog363]

I’ll check it out. At 40 yards should I expect to see a drop with each shot starting with 3000 psi in the rifle?
Shooting barracudas

[Birddog363]

Scope rail is secure. I read somewhere you have to shoot 50-60 times after cleaning the barrel before it will shoot good. Could that be my problem?  I’ve tried every pellet that shot good before. I’m going to shoot over chronograph again and see if it’s consistent.
Thanks
Ben

[gendoc]

I’ll check it out. At 40 yards should I expect to see a drop with each shot starting with 3000 psi in the rifle?
Shooting barracudas

depends on your power setting.
even on mid-range setting it will drop a few fps with each shot until it falls out.
never adjusted setting any lower to check fps.
i only shoot EUNJIN's 43gr pellets.

[Ribbonstone]

Just a few ideas:

1. Same tin/lot of pellets?  Sometimes pellets from "back when" aren't as great a dimensional match to pellets of "now", even though they come in the same logo/color/printed lable  tin and are made by the same folks.

2. "Sneaky leak".  It is possible for the barrel to leak air out from the FRONT of the joint between barrel and reciever.  You'd not notice it unless specically looking in that spot and a varaible leak will play games with the accuracy.

3. The scope mounts screws onto the frame.  Tight as you want to make the rings/bases, if the mouting rail is slightly loose, accuracy is gonna not so good.

4. Reciever is a "clam shell" (the hollow clam-shell part fits over a center section attached to the stock)...if the two screws that hold it togeter are loose, accuracy sucky.

5. The system that rotates and locksthe cylinder/magazine to the action is a little complex...but generally works great for alignement. LOAD the pellets base forst from the front of the mag. (as if you were loadig a cap-n-ball revolver).

6. Scopes can go bad and not lock-donw to their zero.  Swap scopes for whatever other scope you have on another rifle that you know runs right.

[Nomadic Pirate]

Yep, the scope would be the next thing I would check.

[steveoh]

My Sumatra .25 Carbine was super accurate, then one day I was getting pellets all over the place. So I chrony’d the thing and FPS was all over the place. Took it apart and found the valve stem poppet in two pieces. Ordered up two new ones from Pyramyd, got them, installed one, and rifle is back to being happy.

Now if only the rats would cooperate.

I also ordered an improved valve stem poppet thingy from Will Piatt. Hope to get that soon as I no longer trust the stock ones from Eun Jin.

[Froggy.]

Sounds like it's the poppet that failed. Check for barrel and TP o-ring leaks... Can also cause vertical stringing. Also check the grub screw that holds the barrel down on the TP in the receiver, but I would bet on the poppet.

[Birddog363]

Shot the eun Jin 43 grains over chronograph. First shot was 890 and each one after a little slower. Seems like when I got the rifle it was shooting the eun Jin’s over 900. Put a brand new vortex diamondback 4-12 scope on it.  Checked every screw I could find on the rifle. I’ve never taken a pcp apart so I don’t know what a poppet is or tp o-rings.  Seems like it’s starting to group better but I only shot one group with Benjamin domes. Wish there was someone local I could take it to. May have to send it to Will P. Thanks

Ben

[Nomadic Pirate]

A trip to Will Piatt is extremely worth it anyway, you'll get back a real sweet gun overall.

[Habanero69er]

A trip to Will Piatt is extremely worth it anyway, you'll get back a real sweet gun overall.

+1

[Ribbonstone]

Does seen like some inside attention might be called for.  Aren't a lot of folks who seem to work on these (or if they do, are pretty shy about it).  Can understand why, although it's not really difficult to get them apart and together, they are more complicated than many...and so long as you have it apart, better to change all the seals.

[A.Z. in SC]

Shot the eun Jin 43 grains over chronograph. First shot was 890 and each one after a little slower. Seems like when I got the rifle it was shooting the eun Jin’s over 900. Put a brand new vortex diamondback 4-12 scope on it.  Checked every screw I could find on the rifle. I’ve never taken a pcp apart so I don’t know what a poppet is or tp o-rings.  Seems like it’s starting to group better but I only shot one group with Benjamin domes. Wish there was someone local I could take it to. May have to send it to Will P. Thanks

Ben

If you want to take it apart all you need is a spanner wrench and a screwdriver. Its not as hard as u might think

[Birddog363]

Are there any videos or articles that explains how to do it in detail?  Work has been slow lately so I don’t have any extra money to send rifle right now. Thanks

Ben

[Lion]

Have you checked if the scope mounts are secured on the 11mm rail and are the ringnuts secured?

[Ribbonstone]

Ill look...been about 2 computers ago, and I lost a lot of the photos/text....but I've been into an old one, resealed it, and got out again alive and running.

It's not rocket sicience, but is about 2-3X more complex than taking totally apart a simple PCP (like a Discovery or an M-Rod), but if you reason it out (and take a bunch of phone=photos to get it back together right) it is something you can do with hand tools.

[Lion]

If you copy in your search engine e.g. Google  ' Sumatra част '   you will find 2 videos.
No idea where he is taking about