Gamo Silent Cat .177 Project

[mechman]

I'm a new member and found this board after researching tuning for air guns. A family member asked me to look at his Gamo Silent Cat 0.177 after having accuracy issues and I agreed - not fully knowing what I was getting myself into. Here's where I'm at so far:
After checking all screws for tightness including scope mounts, test fired using the 7.9gn Crosman Premier Hollow Points. The 10 shot velocity range was 842-918 fps and the group size was about 1.5" at 20 yards. I also shot the 8.3gn RWS Superdome, 9.0gn Apolo Domed, 8.2gn Meisterkugeln Flat, and 8.2gn Norma SuperPointStrike. They also averaged about 1.5" groups but the 8.3gn RWS Superdome was the most uniform in group size appearance. The others had more vertical stringing, but all within about 2" total spread for 10 shots.

So, after reading everything I could find about tuning I took the gun apart. The spring was noticeably bent, the seal was pretty hard (not damaged), and the spring guide and top hat were a very loose fit. I ordered a new E3650 spring, Tesla Piston Seal and Heavy Tar from Air Rifle Headquarters. Made a new spring guide from Delrin for a snug fit on the inside diameter of the spring and trigger group, a new top hat from cold rolled steel (polished and case hardened) for a close fit with the inside diameter of the piston and the spring. Sized the seal on my lathe for a slight interference fit in the chamber. Reassembled everything with moly paste on the spring, top hat and piston contact points and a light coating of tar on the spring. Cleaned the barrel with Goo Gone until patches came out clean and white. I also installed the M2.5x0.45x8mm screw in the trigger to eliminate some of the trigger creep. Shot about 20-30 RWS Superdomes to set the seal and season the bore. Then I began accuracy testing.

Final results - worse accuracy! The gun now shoots 4" or greater groups at 25 yards with all previously tested pellets. I didn't record and analyze the velocity readings, but I think they were all in the 840-880 fps range. I did increase my range from 20 to 25 yards from my first test, since it seems most use 25 yards as the standard for accuracy testing. I have tried the artillery hold, a firm hold, and free recoil from sand bags with similar results.

I have shot about 150 rounds through the rifle now and have not seen any additional improvement.  What are some thoughts on where I might have taken a wrong turn? Do I need to just continue shooting more rounds to see if it will settle in or should I look at something specific? The groups are stringing more vertically than horizontal. I wish I could post some pictures, but I need to get my post count up to the required minimum.

[avator]

Welcome to the GTA..
This link will take to a thread I did some years ago. It's a lengthy read but you might find some useful information in it.

https://www.gatewaytoairguns.org/GTA/index.php?topic=98794.0

But if you just want to skip to the end result.....

https://www.gatewaytoairguns.org/GTA/index.php?topic=98794.msg949067#msg949067

[Roadworthy]

It should take three to five pounds of pressure to move the piston in the compression tube.  If you go much lower the piston will be moving too fast which will increase pellet velocity but also cause the seal to slam into the end of the compression tube.  If everything is solid and you have a good scope what mostly remains is technique.  This is not a firearm.  The projectile doesn't move when the gun goes off.  The recoil is felt, then the piston begins moving to compress some air.  This gets the pellet moving.  It is crucial to hold the gun exactly the same way for every shot.  The gun shoots where it's aimed at the instant the pellet leaves the barrel not where it's aimed when you pull the trigger.  This must be consistent from shot to shot.  A worthy goal is a five shot group at a half inch center to center at twenty yards but it may take you a long time to achieve that accuracy.  If you can get a three quarter inch group you're doing pretty well.

[mechman]

I read the tale of the "FireCat" previously which was awesome. Lots of great info in those posts. I'm curious about removing the muzzle piece like you did. I want to inspect the crown. Is it only removeable by cutting the plastic over-mold on the barrel? Also, I question just how well I cleaned the bore. I used the recommended pull-through technique with trimmer line balled on the end to drag a patch through the bore. It was a struggle feeding that trimmer line out the end of muzzle and interfering with the baffles. I'm much more familiar cleaning with rods and jags and own a couple 17 cal rods. My concern was keeping the rod guided well into the breach. I don't know if damage to the bore is likely with a carbon fiber cleaning rod, but didn't want to risk it. If the cleaning rod is no concern, I have lapped barrels with JB bore paste and might consider that for this barrel as well.

I didn't measure the force required to move the piston in the compression tube, but with the new seal it definitely felt too tight. The new seal measured about 1.030" and the compression chamber measured about 0.982"- 0.983". I shot the initial 25-30 rounds with the un-sized seal to season the barrel and set the seal. When I started shooting groups and noticed they were worse, I immediately suspected the seal was too tight. My old seal measured about 0.985" and was pretty hard, so I gradually sized my new Tesla seal to about 0.987".  I took only a few thousandths off at a time until it took about 1-2 lbs of force to move it up and down the compression tube. This was with Super Lube synthetic grease lightly applied to the seal and compression tube. More force is needed when I block the transfer port with my finger.

I certainly agree with the advice on shooting technique and it is probably more of an issue than my ego is willing to accept. I did shoot the gun for groups before all the work was done, so if my shooting ability is consistent, it still seems something I did to the gun has had a negative impact on accuracy.

[avator]

as for the muzzle break removal... yes, the only way was to cut it.

[Roadworthy]

I go from the breech to the muzzle with a pull through cable.  I didn't care for fishing line or weed whacker string either.  I got an Otis Micro Kit but there are other pull cables on the market.  Mine is for .17 caliber so it works well for airguns.

[mechman]

Well, I thoroughly cleaned the barrel and took everything apart to inspect. I decided to use my cleaning rod and jag instead of a pull-through system. I figured I could probably be careful enough not to damage the breech where the pellet is loaded if I take my time. The first few patches came out with pretty heavy black fouling. Perhaps I was a little too generous with the Super Lube and moly paste on the outside of the piston tube during my previous reassembly? It must have been migrating through the transfer port little by little. Lots of patches, Hoppe's No.9 and Goo Gone for good measure and I felt I had a pretty spotless barrel. I decided to go ahead a lap the bore with JBs cleaning compound and Bore Bright following their instructions. Ended by pushing more patches, solvent, etc through the bore to clean all the compound out. Now I should have a completely clean and stripped bore ready for... seasoning? I'll have to do some reading on the preferred seasoning methods to see what I should do there. I'm open to suggestions.

All the internal components looked normal. Nothing unusual except maybe some excessive tar and moly that had been flung about.  I cleaned all Super Lube, Moly Paste and Heavy Tar off everything and decided to start over. I did re-lubricate the piston seal and piston (very sparingly this time) and tested how much force it took to push it down the compression tube. It took almost up to 3 pounds to get it moving and a little over 2 pounds to keep it moving the entire stroke. Hopefully this is about right. I have read where some like this number and others practically want it to drop under its own weight unless the transfer port is covered.

I used solvent to clean the spring and reapplied Heavy Tar sparingly. Its almost a little fuzzy in appearance with little stringy tar bits pointing from one coil to the next. Side note - when I received this E3650 spring it was longer than the original by at least 4". Now after being cycled 100 times or so, it is slightly shorter than the original spring. The original spring is about 10" long and this E3650 is now about 9.5". The power still feels the same, just a little less preload during reassembly.

I guess the next step is to season/foul the barrel and range test while recording some velocities.

[mechman]

Range testing resulted in better groups overall, but still not what I was hoping for. I shot 100 of the CPHP 7.9s as a means of fouling or seasoning the barrel. I don't think 100 was necessary after reviewing the velocity reading from my chronograph. The velocity stabilized after about 25-30 shots.
Pellet Caliber: 0.177
Crosman Premier Hollow Point (CPHP) 7.9 gr:
Average velocity: 843 fps
Standard Deviation: 5.6
Group Size at 20 yards: 2"

RWS Superdome 8.3 gr:
Average velocity: 834 fps
Standard Deviation: 10.9
Group Size at 20 yards: 1.7"

RWS Meisterkugeln Wad Cutter 8.2 gr:
Average velocity: 837 fps
Standard Deviation: N/A (chronograph issue)
Group Size at 20 yards: 1.5"

Apolo Domed 9.0 gr:
Average velocity: 730 fps
Standard Deviation: 9.7
Group Size at 20 yards: 1.0"

[lillysdad621]

the only thing i gather from your data is that it does better with heavy pellets. And im not surprised, i have a silent cat whisper also, and i did the trigger mod and cleaned my barrel also. My gun shoot 7.9 crosman poorly also, AND shoots them at around 14 fpe. But i tried barracudas 10.6 and the gun dropped in power to 11.3 fpe, but i was able to coerce .30 5 shot groups  at 15 yards, and .66 at 25 yards all day long. tried JSB10.3... similar results and 11.8 fpe. also crosman magnums 10.5 and again, like a laser. I tried everything from 7.0s to 8.44, and the heavier the pellet, the better the group. Not sure but maybe yours is the same way. My groups were not that big with the 7,9 thou... maybe 1.2 in at 15 yards.